WO2021243398A1 - Zinc-based alloys for bioresorbable medical implants - Google Patents

Abstract

A zinc-based alloy comprising 0.1-6 wt% aluminium and 0.1-2 wt% lithium with the remainder being zinc and incidental impurities.

Description

Zinc-Based Alloys for bioresorbable medical implants Technical Field

[0001] The present application relates to a biodegradable metallic alloy that has application particularly in relation to bioabsorbable implants. The invention relates to high strength zinc/aluminium/lithium alloys, optionally also including copper, that have been found to have high strength and high elongation (ductility), but also favourable biodegradation and bioabsorbable rates when applied to medical devices. The present application also relates to methods of forming such biodegradable metallic alloys and to medical devices incorporating such alloys.

Background of Invention

[0002] Biodegradable metallic materials have provided suitable alternatives to the permanent orthopaedic implants, cardiovascular devices and tissue engineering scaffolds that are currently used in medical devices and implants. Biodegradable materials maintain the expected mechanical integrity during the healing process, before gradually dissolving within the body and releasing harmless corrosion products. Therefore, this eliminates the need of secondary surgery to remove the device. Predominantly, research has concentrated on iron and magnesium alloys for potential medical applications. However, the limited mechanical strength, elongation and the high corrosion rate, particularly of magnesium alloys, or the low corrosion rate of iron alloys have limited their broad use in medical applications.

[0003] The moderate degradation rate and significant physiological biocompatibility of zinc and its alloys has led to interest in its potential capability for replacing magnesium and iron alloys for biomedical applications. Zinc has a standard corrosion potential between iron and magnesium and is also regarded as a vital element of human nutrition, that is, it plays an important role in the body metabolism, immune and nervous system and promotes bone formation and strengthening. However, the mechanical properties of pure zinc are too low and, therefore, the clinical use of zinc in bioabsorbable implants is mainly restricted to low load- bearing applications. The addition of alloying elements is known to be an effective way of manipulating the microstructure and thus, enhancing the mechanical properties of pure zinc alloys. In previous studies, binary and ternary alloys with magnesium, lithium, manganese, iron and copper additions have shown promising results for enhancing the mechanical strength while preserving a suitable degradation rate. In particular, the binary alloys such as zinc/magnesium and zinc/lithium have shown remarkable mechanical strength, for example

Substitue Sheets (Rule 26) RO/AU 450 MPa and 560 MPa, respectively. However, these alloys exhibited low ductility, which limits their use in medical devices, such as stents.

[0004] Zinc alloys have recently attracted the attention of researchers because their degradation rate matches the healing process better than magnesium or iron alloys. On the other hand, the poor mechanical properties of zinc alloys have restricted the miniaturisation of the medical devices. Therefore, the research efforts have focused on improving the strength, while retaining the elongation and the corrosion rate of these alloys.

[0005] US patent 20090069884 describes an example of a stent made from an alloy that includes at least in part arsenic or selenium containing alloys with at least one of magnesium, iron, tungsten, zinc or molybdenum. The inventors found that the presence of arsenic and/or selenium leads to beneficial apoptosis within the human body. Such studies demonstrate the benefits of careful selection of alloying elements.

[0006] US patent 20040131700 describes implantable medical devices such as stents and grafts wherein the medical device includes a zinc containing component which has been found to inhibit plaque formation. However, there was no control of the alloying elements to optimize both the mechanical properties of strength and ductility, together with biocompatibility.

[0007] US patent 9522219 describes an example of a stent comprising a biodegradable magnesium alloy which may include up to 25 wt% dysprosium and up to 3 wt% zinc amongst other alloying elements. The inventors found that such alloys were suitable to form bioabsorbable stents that could be combined with polymeric coatings to control the degradation of the stents.

[0008] Previous studies with alloys for use in medical devices and implants have endeavoured to maximise certain aspects that are required for biocompatible devices and implants. Such studies may lead to optimisation of particular characteristics such as strength, ductility or degradation rate. The optimum balance among these variables depends on each particular application, and there remains a need to continuously improve and optimise these characteristics.

[0009] The present invention aims to overcome the difficulties associated with previously developed biocompatible alloys and to develop new alloys with enhanced mechanical properties and acceptable bio-corrosion and biocompatibility.

[0010] The present invention further aims to provide a biocompatible alloy having both improved strength and ductility while maintaining a level of bio-corrosion performance that

Substitue Sheets (Rule 26) RO/AU makes it suitable for the manufacture of medical devices and implants such as orthopaedic plates, screws and pins, cardiovascular devices such as stents, and tissue engineering scaffolds.

[0011] The present invention further aims to provide a biodegradable metallic alloy that is suitable for bioabsorbable implant applications.

[0012] The present invention also aims to provide a process for producing a zinc-based alloy where control of the manufacturing process can lead to improved mechanical properties.

[0013] The discussion of documents, acts, materials, devices, articles and the like are included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.

[0014] Where the terms "comprise", "comprises", "comprised" or "comprising" are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof.

Summary of Invention

[0015] The present application relates to new zinc-based alloys that include aluminium, lithium and optionally copper as alloying elements. Such zinc-based alloys have application in the manufacture of medical devices and implants such as orthopaedic plates, screws and pins, cardiovascular devices such as stents, as well as tissue engineering scaffolds. The inventors have found that lithium is a particularly important strengthening element for the alloy such that the new biodegradable zinc-based alloy is provided with enhanced mechanical properties while maintaining an acceptable bio-corrosion performance. It has also been found that the addition of aluminium leads to an increase in the strength and ductility of the alloy. Moreover, the addition of copper may lead to further increases in the ultimate tensile strength (UTS) and creep resistance (i.e. deformation of the material over time at constant load).

[0016] It has been found that the alloy microstructure is sensitive to the lithium, aluminium and copper contents and the formation of hard intermetallic phases, such as UZn , LiZn₃AI, CuZn_4-5 and AI₃Li. The volume fraction, size and distribution of these intermetallic phases in the microstructure are largely responsible for the strengthening mechanism of the alloy.

Substitue Sheets (Rule 26) RO/AU [0017] The present application relates to a new zinc/aluminium/lithium alloy, that optionally includes copper, aimed to provide improved strength and ductility while maintaining acceptable corrosion rates and biocompatibility, making them suitable materials for bioabsorbable implants.

[0018] Accordingly, in a first embodiment of the invention, there is provided a zinc-based alloy comprising 0.1 -6wt% aluminium, 0.1-2 wt% lithium with the remainder being zinc and incidental impurities.

[0019] In a preferred embodiment, the invention relates to zinc-based alloys comprising 2- 6wt% aluminium and 0.4-0.8 wt% lithium with the remainder being zinc and incidental impurities.

[0020] In a further preferred embodiment, the invention relates to zinc-based alloys comprising 3.5-4.5wt% aluminium and 0.5-0.7wt% lithium with the remainder being zinc and incidental impurities.

[0021] In a further preferred embodiment, the zinc-based alloys of the invention include 2- 6wt% copper

[0022] In a further preferred embodiment, the zinc-based alloys of the invention relates to zinc-based alloys comprising 2-4wt% aluminium, 3.5-5.5wt% copper and 0.4-0.8wt% lithium with the remainder being zinc and incidental impurities.

[0023] In a further preferred embodiment, the invention relates to a zinc-based alloy comprising about 2wt% aluminium, about 4wt% copper and about 0.6wt% lithium with the remainder being zinc and incidental impurities.

[0024] In a further embodiment of the invention, there is provided a method of producing a zinc-based alloy, said method essentially being a hot extrusion process. In a preferred embodiment, the method includes the steps of: i) providing high purity blocks of zinc and aluminium and adding to a furnace, preferably the furnace is an induction furnace and the zinc and aluminium are placed within a crucible; ii) maintaining the melt at a temperature in excess of the melting point of zinc (preferably 490-550°C) for a period of 10-60 minutes (preferably 15-35 minutes); iii) optionally adding copper together with the aluminium and allowing all elements to dissolve in the melt; and iv) subsequently adding pieces of lithium to the melt, preferably under a protective inert gas atmosphere to avoid oxidation, and holding the melt for an additional 5-30 minutes (preferably 7-15 minutes) until the alloy elements are homogenously mixed. Preferably, before the lithium is added, the chamber of the furnace is

Substitue Sheets (Rule 26) RO/AU evacuated to a base pressure of from 90-110 Pa, preferably 100 Pa and purged with an inert gas, preferably argon, and a positive pressure is maintained to avoid oxidation of the lithium.

[0025] Preferably, the method includes the further steps of allowing the mix to cool and forming the mix into billets. The cooled billets may then be preheated to a temperature between 200°-310°C (preferably 270°-290°C) for a period of 3-30 minutes, preferably 5-15 minutes and hot-extruded, preferably into rods or wires. Optionally, a homogenisation heat treatment step may be applied before extrusion. The homogenisation step may include maintaining the billet at a temperature of from 200°-300°C for a period of up to 5 hours.

[0026] During extrusion, the extrusion has a ratio within the range of from 10-200 which will allow the extrusion of billets through to wires, but preferably from 20-100 and more preferably about 32.

[0027] In a further embodiment of the invention, there is provided a zinc-based alloy having mechanical strength greater than 400 MPa, preferably from 450-700 MPa, and more preferably 530-650 MPa.

[0028] In a further embodiment of the invention, there is provided a zinc-based alloy according to anyone of the preceding claims having a ductility of greater than 20% elongation, preferably 22-100% elongation, and more preferably about 26-45% elongation in a tensile test.

[0029] In a further embodiment of the invention, the extrusion process achieves a small grain size of from 0.5-20 pm, preferably 1-10 pm but more preferably less than 2 pm.

[0030] In a further embodiment, the alloy has a corrosion rate of from 5-80 pm/year, preferably from 10-60 pm/year

[0031] In the preferred embodiment of present invention, the alloy comprises lithium, aluminium and optionally copper additions to zinc in order to design new biodegradable zinc alloys with enhanced mechanical properties and acceptable bio-corrosion performance. Lithium and copper additions provide mechanical strength through the precipitation of LiZn and CuZn_4-5 intermetallic phases, while aluminium forms LiZn₃AI and AI₃Li precipitates as well as a fine Zn-AI eutectic structure, which has been found to improve ductility and may lead to superplasticity during creep tests. It has been found that the ternary zinc/aluminium/lithium alloy has improved ductility over binary zinc/lithium alloys while still maintaining the high strength and acceptable corrosion performance.

Brief outline of the Figures

Substitue Sheets (Rule 26) RO/AU Figure 1 illustrates the XRD results of the extruded Zn-AI-Li alloys.

Figure 2 illustrates quality (IQ) maps obtained from EBSD analysis on the extruded (a) Pure zinc (b) Zn-4AI alloy (the figure is divided into two), (c) Zn-0.4Li, (d) Zn-4AI-0.2Li, (e) Zn-4AI- 0.6U and (f) Zn-6AI-0.4Li alloys.

Figure 3 illustrates a graph for the calculated average grain size distribution and the secondary phase area fraction using the EBSD measurements for developed Zn-AI-LI alloys.

Figures 4 illustrates the mechanical properties of developed Zn-AI-Li alloys including true stress against true strain (a) and stress against elongation (b).

Figure 5 illustrates (a) the potentiodynamic polarization curves of the Zn-AI-Li alloys in the Flank’s solution, (b) the weight loss and corrosion rate obtained after 7 and 40 days of immersion in the modified Flank’s solution and (c) the calculated corrosion rate corresponding to the curves.

Figure 6 illustrates the SEM and EDS analysis of the corrosion surfaces of alloys after immersion for (a) Low and high magnification of Zn-0.4Li corrosion products, (d) Zn-4AI-0.6Li after 7 days and (e) Zn-4AI-0.6Li after 40 days.

Figure 7 illustrates the optical morphologies of developed alloys after 4 days incubation in stem cells and L929 cell in the presence of 21 -day ion leached media (a and d) Zn-0.4Li, (b and e) Zn-4AI-0.6Li, and (c and f) Zn-6AI-0.4Li alloy extracts.

Figure 8 illustrates the cell viability of the Zn-AI-Li alloys for (a) 4 days of cell growth in 21 -day ion leached media and (b) 3-hour cell viability in the presence of the alloy.

Figure 9 illustrates the XRD results obtained from the series of extruded Zn-AI-Li alloys with Cu additions.

Figures 10 illustrates the elemental mapping of extruded zinc alloys and magnified SEM image of zinc alloys.

Figure 11 illustrates EBSD image quality map, orientation and phase map corresponding to zinc alloys with copper additions.

Figure 12 illustrates a summary of the grain size and h-phase fraction for different Zn-AI-Li alloys with Cu additions.

Substitue Sheets (Rule 26) RO/AU Figure 13 illustrates the tensile stress-strain curves of the extruded Zn-AI-Li alloys with Cu additions.

Figures 14-15 show the SEM images and EDS composition maps of the corroded surfaces corresponding to different Zn-AI-Li alloys with Cu additions.

Figure 16 provides the morphologies of L929 cells and HuMSCs after 4 days in the presence of 21 -day ion leached media (Zn-AI-Li alloys with Cu additions) after 4 days of incubation.

Figure 17 provides the cell viability of Zn-AI-Li alloys with Cu additions for (a) 4 days of cell growth in 21 -day ion leached media and (b) 3-hour cell viability in the presence of the alloy.

Detailed description of the invention.

Microstructure and mechanical properties (strength and ductility)

[0032] The microstructure characteristics of binary zinc/lithium alloys, ternary zinc/ /lithium/aluminium alloys and quaternary zinc/lithium/aluminium/copper alloys after hot extrusion were studied. The influence of lithium, aluminium and copper contents on the microstructure characteristics were examined, particularly the grain size, phase composition, distribution of intermetallic phases and crystallographic orientation. The effect of the microstructure on the mechanical properties, corrosion rate in simulated physiological environments and in-vitro biocompatibility tests was also investigated.

[0033] The inventors have found that lithium, aluminium and copper can lead to the formation of intermetallic phases (UZn , LiZn₃AI, CuZn _.5 and AI₃Li) and a significant grain refinement. This, in turn, results in higher mechanical properties, particularly ultimate tensile strength (preferably greater than 400 MPa and up to 700 MPa) and ductility (elongation in tensile tests greater than 20% and up to 50% and more). However, an increase in the volume fraction of intermetallic phases tends to increase strength at the expense of ductility of these zinc alloys.

[0034] It has further been found that thermomechanical processing plays an important role in refining the microstructure and creating ductile and high strength zinc alloys. The intermetallic phases (UZn , LiZn₃AI, CuZn _.5, AI₃Li), which are responsible for the precipitation strengthening mechanisms, also promote dynamic recrystallisation and grain refinement during thermomechanical processing. With this regard, hot extrusion may lead to a superior level of grain refinement and enhancement of mechanical properties compared to other conventional

Substitue Sheets (Rule 26) RO/AU processing methods, such as hot/cold rolling and drawing. The extreme grain refinement produced by hot extrusion may alter the dominant deformation mechanisms, from twinning and basal plane slip to non-basal slip and/or grain boundary sliding. In this way, the small grain size and increased density of grain boundaries enhance the ductility of the alloy and may lead to superplasticity under certain loading conditions. Therefore, the processing of the alloy will preferably achieve a grain size of 0.5-20 pm, preferably 1 -10 pm but more preferably less than 2 pm.

[0035] Extrusion of zinc-based alloys with a wide range of lithium, aluminium and copper contents may result in significantly different microstructures, particularly regarding the grain size distribution and volume fraction of secondary phases, namely UZn , LiZn₃AI, CuZn _.5 and AhLi. Therefore, changes in alloy composition and extrusion conditions provide opportunities to manipulate the microstructure and obtain a suitable balance of mechanical properties, particularly increasing strength while maintaining the ductility of the alloy.

[0036] While the volume fraction of intermetallic phases increases with the addition of lithium, it seems that the microstructure breaks into severely refined grains. This can be closely attributed to two major factors; first, the secondary intermetallic phases act as nucleation sites for dynamic recrystallization; and secondly the growth of re-crystallized grains is constrained by its neighbors. Similar observations were made in the microstructure of Zn-4AI-0.6Li (4wt% Al and 0.6wt% Li) and Zn-6AI-0.4Li (6wt% Al and 0.4wt% Li).

[0037] These regions of the microstructure possess higher strain energy and, as a result, they become preferred locations for dynamic recrystallization during the hot extrusion process. As the content lithium and aluminum increase, so does the volume fraction of intermetallic phases, and the extent of dynamic recrystallization and grain refinement.

[0038] In the case of ternary alloys that include zinc, aluminium and lithium, the presence of the higher fraction of secondary phases leads to intense dynamic recrystallization of refined grains with random orientations. Therefore, a higher fraction of dynamically recrystallized grains associated with particle stimulated nucleation (PSN) is to be expected leading to a weaker crystallographic texture intensity.

[0039] Among the many alloying elements, lithium has shown to successfully improve the strength and corrosion performance of pure zinc alloy. The addition of lithium up to 2wt% may significantly improve the ultimate tensile strength while reducing the corrosion rate compared to pure zinc. However, the increase in the lithium content of the zinc alloy forms a higher fraction of hard LiZn intermetallic phases, and this may decrease the ductility of these alloys

Substitue Sheets (Rule 26) RO/AU relative to pure zinc. With the addition of aluminium up to 1 to 6wt% on the other hand, the mechanical strength can be enhanced while maintaining the ductility.

[0040] Overall, the increase in the lithium content along with the additions of aluminium both contributed to the improved elongation and UTS of the developed alloys. The corrosion measurements also revealed that the high addition on lithium and aluminium did not aggravate the corrosion properties significantly compared to the extruded zinc counterpart.

[0041] Significant grain refinement in the Zn5AI (5wt% Al) alloys can enhance the grain boundary sliding leading to superplastic deformation at room temperature. However, the content of aluminium needs to be kept at a minimal as presence in the body has been linked to the development of Alzheimer's disease. Therefore, considering the weight of the designed implant and its yearly degradation rate, a restricted weight percent of the aluminium addition is preferred.

[0042] The analysis of the mechanical properties in the developed alloys indicates that lithium is a major factor in enhancing the strength of the developed alloys. Due to the low solubility of all elements in zinc, the solid solution strengthening in zinc alloys is insufficient to attain the observed enhancement in properties without the contribution of other strengthening mechanisms. Therefore, significant changes in the microstructure of developed alloys such as grain refinement and increased volume fraction of secondary phases can play a decisive role in the observed mechanical properties. In fact, the addition of lithium in the developed alloy increases the fraction of harder UZn phase, which is found to be a harder brittle phase in binary Zn-Li alloy systems.

[0043] The grain refinement introduces a higher fraction of high angle grain boundaries to the system which act as barriers for dislocation movement and propagation into the adjacent grains. As a result, high angle grain boundaries along with the dislocation strengthening (that is Bailey-Hirsch relationship) can be regarded as a major contributor to the higher yield strength (YS) and ultimate tensile strength (UTS) values for the developed alloy. In addition, the extrusion of the material results in a textured microstructure where the h-phase crystals have their c-axis perpendicular to the loading axis. This means a low Schmidt factor for the basal slip and higher tension for deforming the material. In other words, the grain boundary strengthening along with the texture strengthening are the major contributing factors on the enhanced mechanical properties, that is YS and UTS of the developed alloys.

[0044] However, the grain refinement can determine the main deformation mechanism in hep alloys. It is known that the deformation of Zn and Mg alloys is governed by twinning.

Substitue Sheets (Rule 26) RO/AU However, below a critical grain size (usually 1-10gm) a change from twinning to slip and/or grain boundary sliding at room temperature may occur.

[0045] However, the mechanical properties obtained do not suggest a pure superplastic deformation by the grain boundary sliding. Therefore, a mixed contribution of non-basal slip and grain boundary sliding activated by the refined grain structure may contribute to the high ductility and strength of the developed alloys.

Corrosion properties

[0046] During the immersion and electrochemical corrosion tests of the zinc alloys in a simulated physiological environment (Hank’s solution), a stable and protective ZnO passive layer formed during the early stages of the corrosion process. This layer was subsequently converted into more stable corrosion products (zinc chloride hydroxide, zinc carbonate or calcium phosphate), which further reduced the degradation rate of the material over time.

[0047] One of the major factors in controlling the corrosion rate of a material is the presence of crystalline defects, such as grain and/or twin boundaries in the microstructure. Although a significant increase in grain boundary fraction can be observed for the developed alloys, the occurrence of dynamic recrystallization and material relaxation during the slow cooling procedure leads to the formation of strain free grain structure and low energy grain boundaries. Therefore, the reduction in the corrosion resistance of the ternary alloys is not associated with the microstructural defects. In fact, it can be associated with the different equilibrium potential of alloying elements and with the microstructural features of each alloy such as the alloy content, secondary phase component, precipitates, segregation of alloying elements and impurities

[0048] The higher standard potential of the alloying elements means that the intermetallic phases (LiZ , LiZnsAI, LiZ -s and AhLi) achieved a more negative potential than Zn. In this way, the intermetallic phases can become active sites and lead to micro galvanic cells and localised corrosion (micro-pitting). Therefore, an increase in the content of intermetallic phases can lead to a decrease in the corrosion resistance of the alloys and an accelerated degradation rate.

[0049] The corrosion measurements revealed that additions of lithium, aluminium and copper did not aggravate the corrosion properties significantly compared to pure zinc (i.e. degradation rates in the range of 10-20 pm/year). With higher content of alloying elements and volume fraction of secondary phases, the corrosion rate increased to 30-70 pm/year, which is still within the acceptable degradation rate for bioresorbable medical devices.

Substitue Sheets (Rule 26) RO/AU Biocompatibility

[0050] The cell proliferation was measured after 4 days of culture in 21 -day ion leached extracts (100%) from different alloys and compared to cells grown in media only. Morphology of the cells in the leached extracts showed that the cells were viable but there was reduced cell proliferation for all of the ion leached extracts for both L929 and HuMSC cells.

[0051] The zinc/aluminium/lithium alloy with the least toxicity (Zn-6AI-0.4Li) maintained -80% viability of cell over the 3 hours period compared to the controls, and other ternary alloys (Zn-4AI-0.6Li) showed promising results for L929 cells, with a viability of -75%. For reference, the ISO 10993-5: 2009 indicates that cell viability higher than 75% can be regarded as acceptable cytotoxicity. It should be also mentioned that the standards using 100% ion extracts were developed for biostable materials. Diluting the media 6-10 times has been reported to better simulate the in-vivo environment for biodegradable materials, and this practice would improve the cell viability.

[0052] The results suggest that while the alloys are non-toxic they may interfere with the proliferation of cells. Maintenance of cell viability suggests the pH of the leached material was kept at a level that was compatible with cells. The rounded morphology of the cells when incubated with leached ions suggests the cells cannot establish contact with the substratum, but maintain viability. This may be due to the presence of ions in the media which interfere with the cells ability to attach to the charged plate surface.

Example 1. Zn-AI-Li alloys Methods

[0053] Zinc alloys with different contents of aluminium and lithium were cast in Arcast IND500 induction furnace. The blocks of high purity zinc (99.99%) and aluminium (99.95%) were placed in an alumina crucible inside a stainless-steel chamber. Small pieces of lithium were cut inside a glove box, wrapped in high purity aluminium foil and quickly transferred to the feeding chamber of the furnace, to minimise oxidation. The chamber was evacuated to a base pressure of 100 Pa and purged with argon gas; this procedure was repeated 3 times. A positive pressure of argon was kept throughout the casting process. The zinc and aluminium blocks were melted using the induction coil. The melt was held at 500-510°C for 20 minutes until the small cuts of lithium wrapped in high purity aluminium foils were added to the melt from the feeding chamber. Then the melt was held for an extra 10 minutes so that the lithium mixed with the melt and avoid its evaporation. The melt was poured into a copper mould to produce ingots of 100 x 25 mm in size.

Substitue Sheets (Rule 26) RO/AU [0054] Table 1 lists the chemical composition of the alloys. The relative density was measured using the Archimedes method. The samples were cleaned and immersed in distilled water utilising an electron densimeter (model SD-200L) with 0.0001 g/cm³ resolution. Three measurements for each alloy were made and the average values are also reported in Table 1. [0055] The as-cast ingots were machined into extrusion billets, 30 mm in diameter. The billets were preheated to 280°C held at that temperature for 5 minutes and extruded into rods, using a modified Instron 500 klM (8803) machined fitted with a 5.3 mm die (extrusion ratio = 32).

Table 1. The nominal composition (wt.%) of the investigated Zn-AI-Li alloys and their measured density.

[0056] The phase analyses of the zinc alloys were performed on a plane parallel to the extrusion axis using a PANalytical X-ray diffractometer (X-pert Pro MRD XL). The instrument was equipped with Cu-Ka radiation source, operated at 40 kV and 30 mA in point focus mode and with a spot size of 3 mm x 2 mm. The scans were collected in a q - 2Q configuration, over a range of 20° to 90° with a step size of 0.2° and an integration time per step of 5 seconds. The Rietveld refinement method was used to obtain the volume fraction of the constituent phases in the developed alloys. The method was implemented with the FullProf suite software integrated with PCR Editor (Version 2.00). The statistical data of the fitting process showed a goodness of fit between 1.5 and 3.13 for all the developed alloys. In this regard, the crystallographic information for each constituent (i.e. , h (Zn), a (Al), LiZn4 and AI3L1) was obtained from their corresponding crystallographic card sheets and from the international crystallographic tables.

Substitue Sheets (Rule 26) RO/AU [0057] The overall microstructure of the developed alloys was investigated on the plane parallel to the extrusion direction. The as-extruded samples were mounted in epoxy resin, ground to 4000 grit SiC paper then mechanically polished with 3 pm, 1 pm diamond compounds and 0.4 pm OPS suspension. Finally, the samples were silver painted and attached to a conductive stage with a carbon tape for electron microscopy observations. The electron backscattered diffraction (EBSD) analyses were conducted in a FEG Quanta 3-D FEI scanning electron microscope (SEM) under a 20 kV and 8 nA probe condition. Areas of 25 x 25 pm were mapped with step size of 0.1 pm. The post-processing of the EBSD maps was accomplished using the TexSEM Laboratories Inc. software (TSL OIM analysis 6 x 64) and the ATEX software

[0058] Dog-bone shape tensile specimens were machined from the extruded rods according to the ASTM E8-16a standard having a gauge length and diameter of 16 mm and 4 mm, respectively. The tensile tests were performed at room temperature and at a strain rate of 0.001 s ¹, using an Instron 5567 testing machine equipped with a 30 klM load cell. The displacement was monitored by an Instron AVE2 optical extensometer, with a 0.5 pm resolution and ±1 pm accuracy. Three replicates were performed to provide a statistically reliable results for each developed alloy.

[0059] Samples of each alloy were cold mounted in epoxy resin. The exposed area (25 mm²) was mechanically polished to a 4000 grit SiC paper finish, ultrasonically cleaned in ethanol and dried with warm air. The tests were conducted using a conventional three-electrode set up, according to the ASTM G5-14: an Ag/AgCI glass reference electrode, a titanium mesh counter electrode (150 c 20 mm), and the sample as the working electrode. The mounted samples were electrically connected by insulated copper wires.

[0060] The polarization tests were performed in naturally aerated modified Hank's solution (Sigma Aldrich H4891 balanced salt) at 37 ± 0.5 °C. The samples were immersed in the solution for 30 minutes to attain a steady open circuit potential (OCP) value (approximately -1 mV). The test commenced by sweeping the potential from -0.2 to 2 mV relative to the OCP at a scanning rate of 1 mV/s. Five tests were conducted for every sample, and the resulting corrosion current was measured using the Tafel extrapolation technique. Finally, the corrosion rate in pm/year was calculated according to ASTM-G102 standard.

CR=3.72 x 10⁶ (I_con EW/pA)

[0061 ] Where I_¥p· is the corrosion current, p is the density of corroding materials (g.crrr

³), £W is the corresponding equivalent weight (g) and A is the exposed surface area (cm^-2). The

Substitue Sheets (Rule 26) RO/AU relative density was measured by the Archimedes method using an electronic densimeter (Qualitest Densimeter-SD-200L, with 0.0001 g/cm³ resolution). The value used for the calculations was the average of three measurements

[0062] Small discs having a diameter of 5.2 mm were prepared and covered with lacquer paint (Lacomit Varnish). Only the samples’ surface was exposed to the modified Hank’s solution (Sigma Aldrich H4891 balanced salt) kept at a constant temperature of 37 ± 0.5 °C. The ratio of exposed metal surface area (-21.22 mm²) to volume of solution (30 ml) was maintained constant according to the ASTM-G31 -12a. The beakers were covered with parafilm to minimise the loss of solution through evaporation, and the level of solution was checked daily and topped up if required. Four samples of each developed alloy were exposed to the solution for 1 , 7, 21 and 40 days. After immersion, the samples were first cleaned with ultrapure water (Type 1), then ethanol and finally dried with warm air before weighing.

[0063] The corroded surfaces were examined using an Oxford Instruments Aztec EDS detector linked with a Jeol JSM 7800F SEM operated at 12 kV. The corrosion products were also analysed with a Bruker-Vertex Fourier transform infrared (FTIR) spectrometer in the wavenumber range of 4000 cnr¹ to 600 cm^-1.

[0064] The corrosion products were subsequently removed through immersing in an aqueous solution of 100 g ammonium chloride (NH CI) at 70°C for 5 minutes (ISO8407). Then, they were washed with deionized water and lightly brushed to remove the corrosion products and finally dried with warm air. The weight loss (g.cm ²) after the immersion tests was calculated through the following equation:

Weight loss = (m,·- m_f)/A

[0065] Here, the m, is the initial weight of the sample before the immersion tests and rr is the final weight of the sample after the corrosion product removal. Finally, the corrosion rates (pm/year) were calculated based on the ASTM G31-12a:

Corrosion rate = (Kx A W)/(A p t)

[0066] Where, K is a constant (8.76x107 pm/year), AW, t and p are the weight loss

(mg), immersion time (hours) and the density of the alloys (Table 1), respectively.

Substitue Sheets (Rule 26) RO/AU [0067] The in vitro biocompatibility of the prepared alloys was evaluated for human mesenchymal stem cells (FluMSCs) and mouse fibroblast (L929) cells (CellBank Australia). Before cell culture experiments all ribbons of similar size were cleaned with 70% ethanol for 30 minutes and washed with Phosphate Buffered Saline (PBS, pH 7.4) and autoclaved to maintain sterility. Typically, the cells were maintained in DMEM:F12 (hMSCs) or DMEM (L929) medium supplemented with 2 mM glutamax (Gibco) (10 % (v/v) fetal bovine serum (FBS), and incubated in a humidified atmosphere at 37°C with 5% CO2. The ion leached extracts were made from each alloy by incubating 3 mm x 5 mm ribbons in 2 ml media for 21 days in an incubator a humidified atmosphere at 37°C with 5% CO2. After 21 days the ion leached media was decanted and stored at -20°C for analysis. For testing cell viability and proliferation, 1 x104 cells were seeded in 100 pL of culture medium in 96 well plates and 100 pL of ion leached media was added. For repeatability, triplicate wells were used for each ion leached media. Cells were incubated for incubated for 4 days and cell viability was determined by the MTS assay according to manufacturer’s instructions. Morphology of cells was observed prior to MTS assay and was achieved by use of a Nikon light microscope at x100 magnification. Images were captured using live imaging and Infinity Analyze and Capture software.

[0068] Cell toxicity was measured by seeding 1x105 cells in 200 pL a 48 well plate and allowed to settle for 1 hour at 37 °C with 5% C02. Alloys of 3mm x 5mm ribbons were added and further incubated for 3 hours. Alloys were removed and cell viability was measured by using the MTS assay according to manufacturer’s instructions. For both MTS assays the viable cells were indicated by the optical absorbance at 450 nm determined by a microplate spectrophotometer (GloMax Discover, Promega). The cell viability was calculated by the formula: cell viability (%) = (mean of Abs. value of treatment group/mean Abs. value of control) x 100%.

Results

Microstructure of Zn-AI-Li alloys

[0069] The XRD results illustrated in Figure 1 , revealed a dual-phase structure in the extruded binary zinc-lithium and zinc-aluminium alloys. The hexagonal h-phase was identified as the main constituent, along with smaller fractions of LiZn₄ and a-AI rich phases (Figures 1 a and b). The extruded ternary Zn-AI-Li alloys exhibited the presence of LiZn₄ intermetallic phase while the a-AI rich phase peaks were observed for alloys with higher Al content (Figures 1c-d). It should be noted that two weak peaks corresponding to the 2Q values of 22° and 32° can be observed for the ternary alloys and their intensity increases with the addition of Al and Li alloying elements (highlighted box in Fig. 1). These peaks correspond well with the previously

Substitue Sheets (Rule 26) RO/AU characterized AI₃Li intermetallic phase with a cubic crystal structure (i.e., P m 3 m space group), however, the peaks can also be attributed to the LiZn₃AI intermetallic phase, of unknown crystal structure.

[0070] The microstructures of the extruded samples are presented in Figure 2. The extruded pure zinc revealed a coarse polygonal microstructure having an average grain size of ~49 pm (Fig. 2a). On the other hand, the extrusion of the developed alloys resulted into a significantly refined microstructure (Fig. 2b-f). For the Zn-4AI alloy, a fine eutectic structure and a bimodal distribution of h grains are present (Fig. 2b), while the Zn-0.4Li alloy and ternary alloys had a homogeneous distribution of fine h grains and no signs of eutectic structure (Fig. 2c). It is also evident that along the matrix h phase, the secondary phases were located at the h-phase grain boundaries and arranged in rows along the extrusion direction (dark regions in Figs. 2b-f). It should be noted that the etching of these secondary phases during the sample preparation reduced the quality of the EBSD patterns and made it challenging to acquire detailed information. According to the XRD results, these regions can be attributed to either a- Al rich phases, UZn and/or AI₃Li/LiZn₃AI intermetallic phases

[0071] The area fraction of secondary phases (dark regions) showed an increase with the addition of Li and, to a lesser extent, Al (Fig. 2). The area fraction of secondary phases for Zn-4AI was ~9%, while for alloys containing Li the fraction of secondary phases was significantly higher, reaching ~29 and -33% for the Zn-6AI-0.4Li and Zn-4AI-0.6Li alloys, respectively. To have a meaningful analysis on the fraction of formed intermetallic phases and their role on the material properties, the Rietveld refinement method was utilized to analyse the volume fraction of the phases of the developed alloys Table 2. It is important to note that the crystallographic information related to the ternary LiZn₃AI intermetallic phase is still unknown. As the AI₃Li characteristic peaks were also considered for the LiZn₃AI phase in previous studies, the refinement for the AI₃Li phase fraction was reflected as a representation of both phases’ volume fraction in this study. Overall, the calculated values in Table 2 were close to the fractions considered by the image analysis (i.e., ± 10%). It is also inferred that the AI₃Li+LiZn₃AI phases can be formed in all of the ternary alloys. Finally, the grain size measurements also revealed that the h matrix experienced a significant grain size reduction with the addition of alloying elements (Fig. 3). Moreover, the microstructure was further refined with increasing fraction of secondary phases in the extruded material (Fig. 3 and Table 2). Therefore, the increasing the content of alloying elements both influenced the secondary phase formation and the degree of grain refinement during the hot extrusion process.

Substitue Sheets (Rule 26) RO/AU Table 2 The volume fraction of different constituents obtained by the Rietveld refinement method

[0072] The change of the as extruded sample grain size from approximately 4 to 1 .75 pm resulted in a lower fraction of deformation twins. Also, a lower aspect ratio of the grains for Zn- 4AI-0.6U compared to the Zn-4AI-0.2Li may indicate that grain boundary sliding is also contributing to the total deformation leading to the higher plasticity of the alloy.

Mechanical properties of Zn-AI-Li alloys

[0073] An overview of the mechanical properties of the developed alloys is illustrated in Figure 4 including the mechanical response during tensile tests. Figure 4 (a) illustrates the stress strain curve for the developed alloys, and Figure 4 (b) illustrates their corresponding yield strength, ultimate tensile strength and elongation. For (b) the yield strength and ultimate tensile strength are depicted based on the primary axis (left) and the elongation values are illustrated based on the secondary axis (right).

[0074] The samples of pure zinc strain-hardened up to a maximum load of ~111 MPa and a limited elongation of -13% before failing. The addition of both Al and Li significantly changed the flow curve characteristics of the Zn-alloys, compared to the pure zinc counterpart. The binary Zn-0.4Li alloy showed an increase in the yield strength (325 MPa), UTS (406 MPa) and elongation (27%), while retaining the work-hardening stage and uniform elongation, typically observed in pure zinc. It should be mentioned that the employment of the extrusion process has significantly improved the strength and elongation compared to the previously reported binary Zn-Li alloys.

[0075] On the other hand, the addition of aluminium (Zn-4AI alloy) resulted in an extreme increase in the total elongation (-60%), mainly post-uniform, and a relatively small increase in the YS and UTS (164 MPa). For the ternary alloys, Al and Li additions resulted in further improvements in tensile strength and total elongation (Fig. 4b). However, the work-hardening stage and uniform elongation, typically observed in pure zinc and in Zn-0.4Li, were largely absent in all the Al-containing alloys (Fig. 4a). Overall, the enhancement in UTS for ternary

Substitue Sheets (Rule 26) RO/AU alloys is governed by the addition of lithium: the UTS values for alloys containing 0.2 to 0.4 wt% Li were -380 MPa and -400 MPa, respectively, which increased to 451 MPa for the Zn-4AI- 0.6Li alloy (Fig. 4b). Moreover, the addition of Al for a given Li content (0.4 wt%) resulted in a small increase in UTS, from 405 MPa to 432 MPa, and a significant increase in ductility, from 31% to 42% for the Zn-2AI-0.4Li and Zn-6AI-0.4Li alloys, respectively. High contents of both Li and Al led to a maximum total elongation of 47%, observed in the Zn-4AI-0.6Li ternary alloy (Fig. 4b)

[0076] The mechanical properties of the developed alloys may be demonstrated by looking at the yield strength (YS), ultimate tensile strength (UTS) and elongation to failure values presented in Figure 4 (b). Both the addition of lithium and aluminium significantly increased the elongation of the alloys, although the impact of aluminium was much higher, that is 60% elongation in the alloy with 4 wt% Al alloy compared the 27% elongation with 0.4 wt% Li in the alloy.

[0077] On the other hand, the aluminium addition increased the UTS to 164 MPa, compared to the 111 MPa of the pure zinc. While lithium enhanced UTS up to 406 MPa with 0.4 wt% lithium in the alloy (Figure 4 (b)).

[0078] Moreover, the simultaneous increase in the lithium and aluminium content had a synergistic effect and resulted in higher UTS values (for 300-450 MPa) along with the mechanical characteristics, the corrosion behaviour of the developed alloys were examined using the electrochemical and immersion testing.

Corrosion properties of Zn-AI-Li alloys

[0079] Figure 5 shows the results of the potentiodynamic polarization tests for the developed alloys and for the zinc reference. The cathodic side of the curves showed an increase in the current density for higher contents of lithium and lower contents of aluminium. The Ec_orr was in the range of -1.1 V to -0.9 V vs. AgCI for all the alloys, having the highest values for the Zn-4AI and Zn-4AI-0.6Li alloys and lowest for Zn-2AI-0.4Li. The measurements showed that for lowest Al content the curves are shifted to electronegative direction indicating a higher surface reactivity.

[0080] Using the Tafel extrapolation technique, the corrosion current densities were measured (Fig. 5c) and utilized to calculate the corrosion rates for all the developed alloys (Fig 5b). It can be observed that the corrosion current densities are within the range of 1 -4.5 pA.cnr ², with the Zn-4AI and Zn-4AI-0.6Li alloys having the lowest and the highest values, respectively. The addition of Li and Al led to an increase in the current density to ~4 pA.cnr²

Substitue Sheets (Rule 26) RO/AU for both Zn-4AI-0.4Li and Zn-4AI-0.6Li alloys. The calculated corrosion rates were the results obtained for the pure zinc over 40 days (20 pm/year, Fig. 5b) had a good correlation with the content of alloying elements as the lowest rates were associated with the binary alloys (Zn-4AI and Zn-0.4Li alloys, ~10 pm/year, Fig. 5b) and the highest rates were related to the highly alloyed ternary ones (Zn-4AI-0.6Li with a rate of 50 pm/year over 40 days, Fig. 5b). It is observed that the addition of Li results in a reduction of the corrosion resistance as the corrosion rates for Zn-4AI-0.2Li, Zn-4AI-0.4Li and Zn-4AI-0.6Li changes from ~35 pm/year to ~55 and 60 pm/year, respectively.

[0081] Immersion corrosion. Figure 5 (b) displays the corrosion rates obtained from the mass loss after static immersion test for 7 and 40 days in the modified Hanks solution. It appears that the dissolution of the alloy took place consistently as the time prolonged. The corrosion rates calculated from the immersion corrosion tests were in line with the corrosion rates obtained from the electrochemical corrosion tests, with the exception of sample Zn-2AI- 0.4Li. The corrosion rates for this alloy over 40 days was around 20 pm/year (Fig.5 (b) while the corrosion rates calculated by using the electrochemical parameters was ~60 pm/year (Fig.5 (c)). Overall, at the early stages of the immersion the corrosion rates were higher compared to the 40-day immersion results (Fig. 5 (b)). Moreover, the increase in the alloying elements decreased the corrosion resistance of ternary alloys, while binary Zn-4AI and Zn-0.4Li alloys had the lowest corrosion rates (i.e. , ~10 pm/year) in comparison to the pure zinc (Fig. 5b).

[0082] The differences in the corrosion rates can be rooted in the corrosion kinetics. The low values of corrosion rates can indicate the possible formation of a passive layer that makes the alloy resistant to the corrosion process. On the other hand, it is expected that the different electronegativity of the intermetallic phases can lead to the formation of micro galvanic corrosion cells. The surfaces of the corroded samples were studied to obtain a better understanding of the chemical interaction during the immersion tests.

[0083] Figure 5. (a) Potentiodynamic curves of the developed alloys after 30-minute immersion in the Hank’s solution, (b) The weight loss and corrosion rate obtained after 7 and 40 days of immersion in the modified Hank’s solution (c) the calculated corrosion rate corresponding to the curves.

[0084] The addition of lithium and aluminium decreases the corrosion resistance of the pure zinc which can be associated with the different equilibrium potential of alloying elements and with the microstructural features of each alloy such as the content of alloying elements in solution, the presence of secondary phases and intermetallic precipitates, segregation of alloying elements and impurities. The higher standard potential of the alloying elements such

Substitue Sheets (Rule 26) RO/AU as lithium and aluminium that is, -3.04 and -1 .67 for Li and Al, respectively, means that the secondary phases achieved a more negative potential than that of Zn.

[0085] LiZn can work as an active phase which is prone to corrosion when exposed to the Hank’s solution. Therefore, the increased corrosion rates of zinc alloys can be influenced by the formation of micro galvanic cells of h-phase matrix and the secondary phases. In this regard, an increase in the content of secondary phases can lead to decrease in the corrosion resistance of the alloys. Similar observations were made on the initial degradation of zinc alloys in the simulated body fluids showing localized corrosion with thick products.

[0086] On the other hand, it seems that this is not the case for the binary Zn-0.4Li and Zn- 4AI alloys as they both performed better in the Hanks solution compared to pure zinc (see Fig 5b). Such observation can be associated with lower volume fraction of secondary phase and an even distribution of protective layer on the surface of binary alloys.

[0087] The microstructure observations of the surfaces exposed to the Hank’s solution for 7 and 40 days are illustrated in Figure 6. Overall, the corroded surface can be separated into two regions: the corroded region and the outer region (i.e. main alloy). The corroded region had a thin needle like morphology as the major characteristic feature of the corrosion product for all the examined samples. The corroded surface of the pure zinc revealed a large pit within the centre of these needle-like features indicating that a localized corrosion is taking place (Fig. 6 (a). While, the large pit formation was largely absent for the other developed alloys (Fig.6(b)- (e)), the alloy immersed for 40 days showed shallow cracks around the corroded and the outer region (Fig. 6 (e)). This clearly indicated that the outer region had also a thin layer formed on its surface. It is known that zinc undergoes passivation in neutral and alkaline environments. Finally, elemental analysis of the examined samples revealed that the corrosion products were rich in phosphorous and depleted from the alloying elements (Fig. 6 (b), (d) and (e)) which can be originated from chemical interaction of the zinc substrate and the modified Hanks solution during the immersion process.

[0088] The EDS analysis on the corroded surfaces of the developed alloys shows mainly a zinc matrix element, oxygen and phosphorus, which is consistent with the formation of zinc phosphate, zinc carbonate and zinc hydroxide (Figure 6 (a)). The corrosion products located on the surface of the corrosion pits had a needle like morphology (Figure 6(a)).

[0089] The corrosion reaction for the zinc alloys can occur by an anodic and cathodic reaction which lead to the formation of Zn(OH)2 and ZnO. The presence of the Chloride ions in the modified Hanks solution produces cracking and dissolution of the passive layer and

Substitue Sheets (Rule 26) RO/AU accelerates the corrosion process through pitting. Along with the chloride, the presence of phosphate ions is likely to form thermodynamically stable corrosion products such as Zh₃(R0₄)2·2H₂0 and ZnHP0₄.

[0090] The high fraction of phosphorus detected around the pitted areas can strongly suggest the occurrence of these reactions (Figure 6 (b)). The corrosion products along with the calcium phosphate (CaP), zinc carbonate (ZnC0₃) and zinc chloride hydroxide were also reported. These corrosion compounds can significantly change the characteristic of the corrosion behaviour during the exposure to the Hanks solution such as the formed passive ZnHP0₄ films acting as a protective layer and delaying the degradation of the underlying zinc alloy.

Biocompatibility of Zn-AI-Li alloys

[0091] According to the above studies the binary Zn-0.4Li, Zn-4AI-0.6Li and Zn-6AI-

0.4Li revealed promising mechanical properties along with acceptable corrosion behaviour and therefore, it was selected for biocompatibility studies. In vitro biocompatibility studies of the alloys were carried out by using the L929 mouse fibroblast cell line and Human Umbilical Mesenchymal Stem Cells (HuMSCs). Cell proliferation was measured after 4 days of culture in 21 -day ion leached extracts from different alloys and compared to cells grown in media only (Fig. 7 and 8a). The results show reduced cell proliferation for all the ion leached extracts for both L929 and HuMSC cells (Fig. 8a). Morphology of the cells in the leached extracts showed the cells were viable (Fig. 7). Morphology of L929 cells grown in Zn-4AI-0.6Li leached extracts showed both rounded and typical flattened morphology of healthy cells similar to L929 cells grown in media only, while the Zn-0.4Li (Fig. 7a and d) and Zn-6AI-0.4Li (Fig. 7c and f) ion leached extracts showed mostly rounded cell morphology. For HuMSCs all ion leached extracts caused the cells to adopt a rounded morphology compared to the controls (Fig. 7d-f). The HuMSC cells appeared not to proliferate in the presence of the ion leached extracts compared to the controls.

[0092] The toxicity evaluation results with 100% extracts of Zn-0.4Li, Zn-4AI-0.6Li and

Zn-6AI-0.4Li alloys are presented in Fig. 7. The ISO 10993-5: 2009 indicates that cell viability higher than 75% can be regarded as an acceptable cytotoxicity. The toxicity was also measured by adding the 100% extracts of the mentioned alloy material to cells directly for a 3-hour period (Fig. 8b). These results showed that Zn-6AI-0.4Li has the least toxicity and maintained -80% viability of cell over the 3 hours period compared to the controls. Moreover, the Zn-4AI-0.6Li alloy showed promising results in the L929 with a viability of -75%.

Substitue Sheets (Rule 26) RO/AU Example 2. Zn-AI-Cu-Li alloys

[0093] Zinc alloys with different contents of aluminium, lithium and copper were cast in an induction furnace (IND500, Arcast Inc.) under a 2 kPa positive pressure of inert gas. The high purity Zn (99.99%), Al (99.95%) and Cu (99.95%) blocks were melted in an alumina crucible. Small blocks of Li (99.99%) were wrapped in high purity Zn foils and added to the melt. The melt was held at 500-510°C for 20-25 minutes and poured into a copper mould, 55 mm in diameter and 130 mm in height (-1.5 kg). Table 3 describes the chemical composition of the alloys. The ingots were cut and machined into 30 mm extrusion billets. The billets were preheated to the extrusion temperature (280°C), held for 5 minutes and extruded with an Instron 500 klM (8803) machine fitted with an extrusion die and a heating system. The diameter of the die was 5.3 mm, leading to an extrusion ratio of 32.

Table 3. Chemical composition of the investigated Zn-AI-Cu-Li alloys.

[0094] The relative density of the developed alloys was measured using the

Archimedes method. The samples were cleaned and immersed in distilled water utilizing an electronic densimeter (Qualitest Model SD-200L) with 0.0001 g/cm³ resolution.

[0095] The phase composition analysis of the developed zinc alloys was performed on a plane perpendicular to the extrusion axis using a PANalytical X-ray diffractometer (XRD) equipped with Cu-Ka radiation source. The radiation source was operated at an accelerating voltage of 40 kV and a current of 30 mA in a point focus mode with spot size of 3 mm c 2 mm. The scans were collected in a continuous scan mode over a 2Q range of 20° to 90° with a step size of 0.2° and time per step of 5 seconds.

Substitue Sheets (Rule 26) RO/AU [0096] The microstructure of the developed alloys was investigated on the plane perpendicular to the extrusion direction by scanning electron microscopy (SEM), electron back- scattered diffraction (EBSD) and energy-dispersive spectroscopy (EDS). The samples were first ground to 4000 grit SiC paper, mechanically polished with 3 pm, 1 pm diamond compounds and 0.4 pm OPS suspension. The EBSD analyses were conducted using a FEG Quanta 3-D FEI SEM instrument under 20 kV and 8 nA probe conditions. Two different areas were mapped: 120 x 120 pm and 25 x 25 pm, with step sizes of 100 nm and 50 nm, respectively. The EBSD post-processing was conducted using the TexSEM Laboratories Inc. software (OIM analysis 6.1 ). The chemical composition of the developed alloys and the corrosion products were analysed by energy dispersive spectroscopy (EDS) using a Jeol JSM 7800F SEM instrument operated at 12 kV.

[0097] Dog bone shape tensile specimens were machined from the extruded rods according to the ASTM E8-16 [50] with a gauge length and diameter of 16 mm and 4 mm, respectively. The room temperature tensile tests were carried out at a strain rate of 0.1 s ¹ using an Instron 5567 testing machine equipped with 30 klM load cell. The displacement was monitored by an Instron AVE2 optical extensometer, with a 0.5 pm resolution and ±1 pm accuracy. Three replicates were performed to provide a statistically reliable results for each developed alloy.

[0098] The samples for electrochemical corrosion testing were cold mounted in epoxy resin, leaving an exposed area of 5 mm². The surface was ground up to 4000 grit SiC paper, ultrasonically cleaned with ethanol and dried in hot air. The system included three-electrodes, namely: the reference Ag/AgCI reference electrode, a titanium mesh (150 x 20 mm) counter electrode, and the sample surface as the working electrode according to the ASTM G5-14 standard. The mounted samples were electrically connected by sealed copper wires.

[0099] The polarization tests were performed in a modified Hank's balanced salt solution (Sigma Aldrich H4891 ) in a naturally aerated condition at 37±0.5°C. The samples were immersed in the solution for 30 min to reach a steady open circuit potential (OCP) value, approximately -1 V. The test commenced by sweeping a potential range from -0.2 V to 2 V versus the OCP, at a scanning rate of 1 mV/s according to G102-89(2015) standard. The current density was measured using the Tafel extrapolation technique. Each measurement was

Substitue Sheets (Rule 26) RO/AU repeated five times for each of the alloys. Finally, the corrosion rate in pm/year was calculated according to ASTM G102-89(2015) standard as below:

CR = 3.72 X 10⁶ Q_corrEW / p A) (1 ) where I_corr is the corrosion current, p is the density of corroding materials ( gcm^{~ 3}), EW is the corresponding equivalent weight (g). The surface morphology of corroded samples was analysed using a Jeol JSM 7800F SEM operated at 12 kV.

[0100] Small discs having a diameter of 5.2 mm were covered with lacquer paint

(Lacomit Varnish), leaving one side of the disc unmasked. The sample surface was polished up to 1200 grit SiC sand paper, then exposed to the modified Hank’s solution (Sigma Aldrich H4891 balanced salt), kept at a constant temperature of 37 ± 0.5 °C. The ratio of exposed metal surface area (~21 .22 mm²) to volume of solution (30 ml) was maintained constant according to the ASTM G31 -12a. The beakers were covered with parafilm to minimise the loss of solution through evaporation, and the level of solution was checked daily and topped up if required. Samples of the developed alloy were exposed to the solution for 7 and 21 days. After immersion, the samples were first cleaned with ultrapure water (Type 1 ), then ethanol and finally dried with warm air before undergoing the EIS testing. Finally, the corroded surfaces were examined using an Oxford Instruments Aztec EDS detector linked with a Jeol JSM 7800F SEM operated at 12 kV.

[0101 ] The biocompatibility of the newly developed Zn-AI-Cu-Li alloys was analysed in- vitro using human mesenchymal stem cells (HuMSCs) and mouse fibroblast (L929) cells (CellBank Australia). Prior to the experiments samples of each alloy were cut into similar sized ribbons, cleaned with 70% ethanol for 30 minutes and washed with Phosphate Buffered Saline (PBS, pH 7.4) and autoclaved to maintain sterility. Typically, the cells were maintained in DMEM:F12 (hMSCs) or DMEM (L929) medium supplemented with 2 mM glutamax (Gibco) (10 % (v/v) fetal bovine serum (FBS), and incubated in a humidified atmosphere at 37°C with 5% CO2. The ion leached extracts were made from each alloy by incubating 3 mm x 5 mm ribbons in 2 ml media for 21 days in an incubator with humidified atmosphere at 37°C and 5% CO2.

[0102] After the incubating period the ion leached media was decanted and stored at -

20°C for analysis. For testing cell viability and proliferation, 1 x10⁴ cells were seeded in 100 pl_¬ ot culture medium in 96 well plates and 100 mI_ of ion leached media was added. For repeatability, triplicate wells were used for each ion leached media. Cells were incubated for 4

Substitue Sheets (Rule 26) RO/AU days and cell viability was determined by the MTS assay (CellTiter96 Aqueous One Solution Proliferation Assay, Promega) according to manufacturer’s instructions. Morphology of cells was observed prior to MTS assay and was achieved by use of a Nikon light microscope at x100 magnification. Images were captured using live imaging and Infinity Analyze and Capture software.

[0103] Cell toxicity was measured by seeding 1 x10⁵ cells in 200 mI_ a 48 well plate and allowed to settle for 1 hour at 37°C with 5% CO2. Alloy ribbons of 3 mm x 5 mm were added and further incubated for 3 hours. Alloys were removed and cell viability was measured by using the MTS assay (CellTiter96 Aqueous One Solution Proliferation Assay, Promega) according to manufacturer’s instructions. For both MTS assays the viable cells were indicated by the optical absorbance at 450 nm determined by a microplate spectrophotometer (GloMax Discover, Promega). The cell viability was calculated by the formula: cell viability (%) = (mean of Abs. value of treatment group/mean Abs. value of control) c 100%.

Results

Microstructure of Zn-AI-Li alloys with Cu additions

[0104] Figure 9 illustrates the XRD results obtained from the series of extruded Zn-AI-

Cu-Li alloys. The characteristic peaks were similar for both the as-cast and the extruded conditions. The main peaks were associated with the hexagonal h phase (zinc rich phase) along with smaller fractions of UZn₄ and CuZn_x phases. The £-CuZn_x phase peaks became more prominent as the Cu content increased. A weaker peak at -45° was attributed to the fee Al phase; its low intensity in all the alloys indicates a limited volume fraction of this phase. The addition of Li strongly increased the volume fraction of LiZn phase. The copper addition resulted in a relatively smaller increase in the fraction of CuZn₄ phase. A similar phase composition was reported in previous studies on Zn-Li and Zn-Cu-Li alloys. Due to the similar crystal structure of these phases, the peaks overlap making it difficult to quantify the phase fractions.

[0105] Figures 10-11 show the microstructure analysis of the extruded Zn-AI-Cu-Li alloys. EDS maps show the distribution of three major alloying elements, namely Zn, Cu and Al, in the microstructure, whereas the detection of Li is beyond the capabilities of the EDS detector, preventing the identification of the LiZn₄ phase. Overall, the microstructures have a bimodal grain size, containing both equiaxed grains alternated with bands of darker secondary phases elongated along the extrusion direction. The equiaxed grains had a higher concentration of Cu, while the darker elongated grains exhibited a greater Al concentration. The Cu-rich grains can result from solid solution of Cu in the Zn h-phase and/or the formation

Substitue Sheets (Rule 26) RO/AU of £-CuZri4-5 phases. It should be mentioned that the maximum solubility of Cu in Zn at room temperature is approximately 2 wt%. Therefore, the intermetallic £-CuZn_4.5 phases are likely to form in alloys with higher Cu content. The EDS line scans in Figure 10 also reveal an approximate ratio of 3 to 1 between Zn and Cu, indicating the possible formation of the CuZn phase. The second line scan along the small elongated grains shows a higher concentration of Al and Zn. These regions correspond to a non-lamellar eutectic structure, according to the SEM observations, and are composed of h-phase and fee Al phase (Fig. 10b).

[0106] The EBSD maps in Figure 11 provide additional information on the constituent phases, grain structure and the texture characteristics of the Zn-AI-Cu-Li alloys. The intermetallic phases were identified as UZn₄, CuZn_4.5 and eutectic Al structures. The XRD peaks corresponding to CuZn₅ or CuZn₄ phases overlap with the UZn₄ and h-phase, but the EDS analysis revealed a relative atomic ratio of Cu and Zn close to the CuZn₄ phase (Fig. 10b). Therefore, the crystallographic information corresponding to the s-CuZn₄ phase was used to analyse the crystallographic texture. It should be mentioned that the crystal structures and lattice parameters for the CuZn₄ and UZn₄ are very close and, therefore, the corresponding orientation can be related with both phases. Flereafter, the CuZn₄ and UZn₄ phases will be treated as one and referred to as CuZn₄+LiZn₄ phase. A multiphase microstructure with h-zinc and CuZn₄+UZn₄ phases can be observed in Figure 11. The small elongated grains rich in Al were identified as eutectic Zn-fcc Al regions by EDS, but the fee Al lamellas were too small to be detected by EBSD. Both the h and the CuZn₄+UZn₄ phases seemed to undergo dynamic recrystallization during the extrusion process. Overall, the grain size of h-phase is relatively smaller than the CuZn₄+LiZn₄ phases. Interestingly, the grain sizes attributed to the CuZn₄+LiZn₄ phases did not change with the addition of alloying elements and remained around ~5.5 pm in all cases (Fig. 12). The addition of Li decreased the h-phase average grain size from ~2.5 pm for the Zn-2AI-2Cu alloy to -1.2 pm in Zn-2AI-4Cu-0.8Li alloy. On the other hand, the average area fractions of the Zn rich h-phase and the CuZn₄+LiZn₄ phases were strongly affected by the addition of Li, while the phase composition was less sensitive to the addition of Cu. The addition of Li resulted in an increase in the CuZn₄+LiZn₄ phase fraction from -23.3 vol% in Zn-2AI-2Cu alloy to 69.3 vol% for Zn-2AI-2Cu-0.8Li. This indicates that the Li addition is in fact providing higher fraction of LiZn₄ intermetallic phases in the microstructure.

[0107] In contrast, the change in the Cu content increased the CuZn₄+LiZn₄ phase fraction from -23.3 vol% to 44.6 vol% for Zn-2AI-2Cu and Zn-2AI-5Cu alloys, respectively. The underlying mechanisms for the evolution of CuZn₄+LiZn₄ volume fraction and Zn h-phase grain size in the developed alloys, as well as their effects on the deformation behaviour, will be discussed in the following sections.

Substitue Sheets (Rule 26) RO/AU Mechanical properties of Zn-AI-Li alloys with Cu additions

[0108] Figures 13a and b illustrate the tensile stress-strain curves of the extruded Zn-

Al-Cu-Li alloys and their corresponding yield strength, ultimate tensile strength (YS and UTS) and elongation to fracture values. For the ternary Zn-2AI-xCu alloys, the addition of Cu did not have a significant influence on the tensile strength, although the elongation to fracture improved from -27% in the Zn-2AI-2Cu to 45% in the Zn-2AI-5Cu alloy. On the other hand, the Li addition and the associated increase in the volume fraction of LiZn intermetallic phases, resulted in higher UTS values. In parallel, the h phase grain size reduction correlates well with the increase in the UTS values for the developed alloys. Flowever, it should be mentioned that the h phase fraction is less than 10% in high strength alloys, namely Zn-2AI-4Cu-0.8Li and Zn-2AI-5Cu- 0.8Li, and the grain size of the dominant CuZn₄/LiZn₄ phases does not change with increasing alloying content. Consequently, the increase in tensile strength seems to be primarily associated with the volume fraction of brittle CuZn₄+LiZn₄ phases, while the h-phase grain size refinement makes only a secondary contribution to the UTS. The increase in Li concentration from 0 to 0.8 wt.%, in the Zn-2AI-4Cu-xLi alloy series, enhanced the tensile strength from 395 MPa to -535 MPa, although the elongation was nearly halved from 37% to 16% for Li contents higher than 0.6 wt.%. Moreover, the increase in Cu and Li content led to further increase in UTS and reduction in the ductility, with alloy Zn-2AI-5Cu-0.8Li showing a remarkable UTS of -631 MPa but only 2.5% elongation. This can be attributed to the low volume fraction of h-phase, which seems to have a positive impact on the ductility of the alloys at room temperature.

[0109] The alloying of Zn with Cu, Li and Al has effectively enhanced the tensile strength and ductility. The improvements in mechanical properties are strongly linked to the volume fraction of constituent phases. Cu additions increased the volume fraction of secondary phases, mainly CuZn₄, from 23.3 vol.% in Zn-2AI-2Cu to 45.1 vol.% in Zn-2AI-5Cu, which improved both UTS and elongation values up to -450 MPa and 34%, respectively. On the other hand, the addition of Li led to the formation of higher CuZn₄+LiZn₄ phase fractions, which resulted in remarkable UTS values of -535 MPa and elongation of 30% for Zn-2AI-4Cu-0.6Li and 15% for Zn-2AI-4Cu-0.8Li alloy.

[0110] These results indicate that the ductile h phase and the hard-secondary phases play a vital role in the mechanical properties. A further reduction in the volume fraction of the h phase for the Zn-2AI-5Cu-0.8Li alloy increased the UTS up to 635 MPa but reduced the elongation to ~2%. Overall, the harder intermetallic phases, such as eutectic AI+h and CuZn +LiZn phases, resist the dislocation movement and act as preferential sites for dynamic recrystallization, leading to increased mechanical properties. The mechanical properties are

Substitue Sheets (Rule 26) RO/AU comparable to 316L stainless steel and titanium-based alloys (UTS > 500 MPa), making these bioresorbable zinc alloys promising candidates for biomedical implant applications.

Corrosion behaviour of Zn-AI-Li alloys with Cu additions [0111] The electrochemical parameters and the calculated corrosion rates of the developed alloys from potentiodynamic polarization tests in Hanks solution at 37°C are summarized in Table 4. The change in the content of alloying elements had different effects on the corrosion behaviour of the alloys. The corrosion potential shifted slightly to more positive values with the increase in Cu content from 2 wt% to 5 wt%. Moreover, the addition of Cu increases the corrosion current for the Zn-alloys containing only Al and Cu. The addition of Li also shifted the corrosion potential (E_COrr) in the noble direction, except for the Zn-2AI-4Cu-0.8Li alloy, and the current density of the Li containing alloys increased with the Li content. In general, the current density values were between 1.5 x 10⁴ - 7.5 x 10⁴ pA.cnr² and the results were consistently higher for the alloys containing Li. These changes were also reflected in the calculated corrosion rates, which showed an increasing trend with the addition of Li. The variations in current density and in the calculated corrosion rates were very small for the Zn- 2AI-2Cu alloy series, remaining around the 10-15 pm/year. However, with the increase in Cu and Li content, the corrosion rate increased from -16.5 pm/year to -42.1 pm/year for the Zn- 2AI-4Cu and Zn-2AI-4Cu-0.8Li, respectively. Interestingly, the highly alloyed Zn-2AI-5Cu and Zn-2AI-5Cu-0.8Li alloys had the highest corrosion rates of -73.4 pm/year, for alloys with no Li, and 90.7 pm/year, for Li containing alloys, respectively.

Table 4. Calculated corrosion current and potential using the intersection of anodic and cathodic lines and the corresponding corrosion rates.

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[0112] The standard potentials of Li and Al (i.e. , -3.04 and -1.67 for Li and Al, respectively), are higher than those of zinc and copper, making the LiZn and AI₂Cu secondary phases prone to localised corrosion. The formation of micro galvanic cells between the h phase and the secondary phases leads to increased corrosion rate at the early stages of immersion. Figure 14 and 15 reveal the microstructure of the corrosion products developed on Zn-2AI-5Cu and Zn-2AI-5Cu-0.8Li alloys after 1 , 7 and 21 days of immersion in modified Hank’s solution. At the early stages of immersion, a smooth homogenous layer with higher concentration of phosphorous is formed on the surface of the samples. In addition, localized regions with high concentration of Cl and low concentrations of Cu can be observed for the Zn-2AI-5Cu alloy (Fig. 14a). This suggests that localized corrosion around the CuZn₄ and AICu₂ phases had already commenced. However, the difference in the corrosion rate of Zn-AI-xCu alloys was not significant, which indicates that localized or microgalvanic corrosion are not dominant, leading to the formation of a uniform corrosion product on the surface of these sample.

[1013] In the case of the Zn-2AI-5Cu-0.8Li alloy, the corrosion products were rich in phosphorous with a needle-like morphology and signs of crack formation (Fig. 15a), which limit their protective effect. After 7 and 21 days of immersion, the surface morphologies of the Zn- 2AI-5Cu and Zn-2AI-5Cu-0.8Li alloys showed a thick layer of corrosion products with large cracks (Fig. 14c and 15c). Interestingly, the cracked regions have significantly higher concentration of Cl and C, which may result from localized corrosion (Fig. 15c). The absence of cracks in the early stages of corrosion for Cu containing alloys indicates the formation of a stable protective layer (Fig. 14a and b). This is in accordance with the EIS results, where the diameter of the impedance loops was significantly larger for Cu containing alloys compared with the Li rich alloys.

[0114] The corrosion of zinc alloys in modified Hanks’ solution often produces zinc oxide and hydroxides, phosphates, and carbonates. The high content of P indicates that samples are prone to form thick and porous phosphate layers. Moreover, the presence of chloride ions has been associated with localized corrosion and formation of ( Zn₅(OH)₈Cl₂ H₂0 ) around pits. The analysis on the corrosion properties of the developed alloys reveals that the Zn-Cu-AI alloys present lower corrosion rates and more uniform corrosion. Additionally, the immersion of the alloys in Hanks’ solution for long periods of time was associated with the formation of a dense protective layer on the surface of the samples, which resulted in steady corrosion rates for these alloys. On the contrary, the addition of Li produced a significant rise

Substitue Sheets (Rule 26) RO/AU in the corrosion rate, with evidence of localized corrosion. Nevertheless, the corrosion rate of Zn-AI-Cu-Li was lower than other bioresorbable metals (Mg alloys), which implies that they can retain their mechanical integrity in the human body for longer periods of time. Therefore, Zn-AI- Cu-Li alloys are promising candidate materials, with mechanical properties and degradation rates close to the benchmark values.

Biocompatibility of Zn-AI-Li alloys with Cu additions

[0115] The alloys with the most prominent mechanical properties, namely Zn-2AI-4Cu-

0.8Li and Zn-2AI-5Cu-0.8Li, were considered for biocompatibility studies. Figure 16 and 17 reveals the measured cell proliferation after 4 days of culture in 21 -day ion leached extracts of the alloys and its comparison to the cells grown in media only. The results show reduced cell proliferation for the entire ion leached extracts of both L929 and HuMSC cells. Morphology of the cells in the leached extracts showed the cells were viable. Morphology of L929 cells and HuMSCs grown in Zn-2AI-4Cu-0.8Li and Zn-2AI-5Cu-0.8Li leached extracts showed mostly rounded morphology of healthy cells similar to L929 and HuMSCs cells grown in media only (Fig. 16a-f). The HuMSC cells appeared proliferate more slowly in the presence of the ion leached extracts compared to the controls.

[0116] The toxicity evaluation results with 50% extracts of Zn-2AI-4Cu-0.8Li and Zn-

2AI-5Cu-06Li alloys are presented in Fig. 17. The ISO 10993-5: 2009 indicates that cell viability higher than 75% can be regarded as an acceptable cytotoxicity. The toxicity was also measured by adding the 100% extracts of the mentioned alloy material to cells directly for a 3-hour period (Fig. 17b). These results showed that the alloys have the least toxicity in the HuMSC and maintained -45% viability of cell over the 3 hours period compared to the controls.

[0117] The morphology and cell viability assays performed on the different alloys suggest that while the alloys are non-toxic, they may interfere to some degree with proliferation of cells. This may be due to the nature of the corrosion product, but maintenance of cell viability suggests the pH of the leached material was maintained at a level that was compatible with cells. The rounded cell morphology of the cells when incubated with leached ions suggest the cells cannot establish contact with the substratum but maintain viability. This may be due to the presence of ions in the media which interfere with the cells ability to attach to the charged plate surface. It should be mentioned that these standard tests are designed for the non-degradable materials, while it has been mentioned that diluting the media about 6-10 times to may better simulate the in vivo environment for bioresorbable materials. Similarly, in-vivo experiments have also shown that the Zn-alloys can have an acceptable biocompatibility. Therefore, considering the in-vitro tests, it can be concluded that the Zinc alloys containing Cu and Al can

Substitue Sheets (Rule 26) RO/AU present adequate biocompatibility, making them potentially suitable materials for biomedical implant applications. The biocompatibility of these alloys will be further investigated with different cell types specially using osteoblastic cells to investigate its possible application for bio-implants and scaffolds materials.

Conclusion

[0118] The inventors have developed a new series of biodegradable zinc alloys having different contents of lithium, aluminium and optionally copper. The alloys were cast, hot extruded and the microstructure evolution, the mechanical, corrosion properties and biocompatibility were examined through conventional SEM and EBSD analysis, tensile testing, polarization and immersion corrosion testing, and in-vitro cell proliferation studies, respectively.

[0119] The alloying elements played a major role in the microstructure characteristics of the extruded alloys. The addition of Li and Al led to increased volume fractions of intermetallic phases, which were characterized as LiZn and AI₃Li, LiZn₃AI in the ternary alloys.

[0120] The hot extrusion led to recrystallized microstructure in the matrix h-phase. It was shown that the hard-intermetallic phases were the preferred locations for dynamic recrystallization of the matrix h-phase through the particle stimulated nucleation mechanism.

In addition, the intermetallic phases around the h-phase inhibited the grain growth, resulting in significant grain refinement and texture weakening of the matrix h-phase in the extruded alloys.

[0121] Alloys with higher Li content exhibited higher fraction of intermetallic phase and had the smallest grain size among the developed ternary alloys (up to 1.75 pm for Zn-4AI- 0.6Li alloy). The decrease in the grain size promoted the grain boundary strengthening and homogeneous plastic deformation through non-basal slip and grain boundary sliding. This ultimately led to the increase in both strength and ductility of the alloys, with the ternary Zn- 4AI-0.6Li alloy exhibiting the highest strength of 453 MPa and an elongation of 46.3%.

[0122] Both immersion and polarization corrosion tests resulted in similar corrosion rates for the ternary alloy. The binary Zn-0.4Li and Zn-4AI had the lowest corrosion rates of 10 pm/year, compared to pure zinc, ~20 pm/year. The Zn-4AI-0.6Li, which had the most promising mechanical properties, exhibited higher corrosion rates, of approximately 60 pm/year.

[0123] The formation of stable corrosion products, mainly zinc phosphates and zinc carbonates, protected the underlying zinc alloy and reduced the corrosion rates over time.

Substitue Sheets (Rule 26) RO/AU This protective layer also reduced the pitting through micro galvanic corrosion between the Zn matrix and the Li-rich precipitate.

[0124] The addition of Al with Li and the thermomechanical process significantly improved the mechanical properties of zinc, while retaining a suitable corrosion rate for bioresorbable medical implants for the ternary alloy. The developed Zn-4AI-0.6Li and Zn-6AI- 0.4Li alloys appeared to be cytocompatible to the mouse fibroblast cell line and human umbilical mesenchymal stem cells becoming a promising candidate for biodegradable implant applications. Further investigation is required to assess the biocompatibility and to reduce or eliminate the Al content of these alloys.

[0125] The quaternary Zn-AI-Cu-Li alloys also produced a high strength bioresorbable

Zn alloy. The cast alloys were thermomechanically processed by hot extrusion to eliminate the cast structure and induce grain refinement. The mechanical properties, corrosion properties and biocompatibility of the alloys were investigated, and the role of microstructure constituents was analysed through SEM, EBSD and APT analyses. The following summarises the most important findings of this investigation for the quaternary alloys:

• The microstructure was highly sensitive to the Cu and Li contents. With increasing Cu and Li content, the harder CuZn +LiZn phases became dominant, reaching a phase fraction of -95% for the Zn-2AI-5Cu-0.8Li alloy.

• Recrystallization led to a significant grain refinement of the h-phase during hot extrusion, while the grain size of CuZn₄+LiZn₄ phases remained unchanged, resulting in a bimodal grain size distribution. The fine h-phases accommodated high strain levels, while the hard- intermetallic phases were largely responsible for the strengthening mechanism. Segregation of Cu at the grain boundaries introduced lattice distortions, increased the strength and reduced the ductility further.

• The Zn-2AI-4Cu-0.6Li alloy exhibited the best combination of mechanical properties, with a UTS value of -535 MPa and an elongation of 37%. The maximum strength of -635 MPa was obtained for alloy Zn-2AI-5Cu-0.8Li, at the expense of ductility. This alloy exhibited appreciable creep resistance at 37°C, even for stresses as high as 0.8 of the yield stress.

• The immersion and polarization corrosion tests showed similar corrosion rates to pure Zn for all the alloys. The Zn-2AI-4Cu-0.6Li the corrosion rate as low as 38.5 pm/year, compared to pure zinc, ~20 pm/year. The formation of stable corrosion products, at later stages of immersion resulted higher corrosion resistance of the material. This protective layer is more pronounced for Cu containing alloys which reducing the pitting through micro galvanic corrosion between the Zn matrix and secondary precipitates.

Substitue Sheets (Rule 26) RO/AU • The developed Zn-2AI-4Cu-0.6Li alloy appeared to be cytocompatible to the mouse fibroblast cell line and human umbilical mesenchymal stem cells becoming a promising candidate for bioresorbable implant applications.

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Claims

Claims:

1. A zinc-based alloy comprising 0.1-6 wt% aluminium and 0.1-2 wt% lithium with the remainder being zinc and incidental impurities.

2. zinc-based alloy according to claim 1 comprising 2-6 wt% aluminium and 0.4-0.8 wt% lithium with the remainder being zinc and incidental impurities.

3. A zinc-based alloy according to claim 1 comprising 3.5-4.5 wt% aluminium and 0.5- 0.7wt% lithium with the remainder being zinc and incidental impurities.

4. A zinc-based alloy according to any one of claims 1 -3 further including 2-6wt% copper.

5. A zinc-based alloy according to claim 4 comprising 2-4wt% aluminium; 3.5-5.5 wt% copper and 0.4-0.8 wt% lithium with the remainder being zinc and incidental impurities.

6. A zinc-based alloy according to anyone of claims 1 to 4 comprising about 2 wt% aluminium; about 4 wt% copper and about 0.6wt% lithium with the remainder being zinc and incidental impurities.

7. A zinc-based alloy according to anyone of the preceding claims wherein the alloy is formed by a hot extrusion process.

8. A zinc-based alloy according to anyone of the preceding claims wherein fine AI₃Li, LiZn₃AI, UZn₄ and/or CuZn_4-5intermetallic phases form.

9. A zinc-based alloy according to claim 8 comprising 1 5-2.5wt% aluminium; 4.5-5.5wt% copper; 05-0.8wt% lithium, with the remainder being zinc and incidental impurities, wherein the CuZn _.5, LiZn₃AI and UZn₄ phases are dominant with a phase fraction of 90 to 95%.

10. A zinc-based alloy according to anyone of the preceding claims having mechanical strength UTS value of greater than 400 MPa, preferably from 450-650 MPa, and more preferably 530-570 MPa.

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11. A zinc-based alloy according to anyone of the preceding claims having a ductility of greater than 20% elongation, preferably 22-100% elongation, and more preferably about 26-45% elongation.

12. A zinc-based alloy according to anyone of the preceding claims wherein the grain size is between 0.5-20 pm, preferably from 1-10 pm, more preferably less than 2 pm.

13. A zinc-based alloy according to anyone of the preceding claims wherein the alloy has a corrosion rate of 5-80 pm/year, preferably 10-60 pm/year.

14. A method of producing a zinc-based alloy according to claim 1 , said method including the steps of i) providing high purity blocks of zinc and aluminium and adding to a furnace; ii) maintaining the melt at a temperature above the melting point of the zinc (preferably 490-550°C) for a period of from 10-60 minutes (preferably 15-35 minutes); iii) adding pieces of lithium to the melt in the furnace and holding the melt for an additional 5-30 minutes (preferably 7-15 minutes) until the alloy elements are mixed.

15. A method according to claim 14 wherein the chamber of the furnace is evacuated to a base pressure of from 90-110 Pa, preferably 100 Pa and purged with an inert gas, preferably argon, and a positive pressure is maintained to avoid oxidation of the lithium.

16. A method according to claim 14 wherein the furnace is an induction furnace, and the zinc and aluminium are placed in a crucible.

17. A method according to any one of claims 14-16, including the further steps of allowing the mix to cool, and formed into billets, preheating the billets to a temperature of from 250°-310°C (preferably 270°-290°C) for a period of 3-30 minutes, preferably 5-15 minutes and hot-extruding into rods.

18. A method according to claim 17 wherein the extrusion has a ratio of from 10-200, preferably from 20-100 and more preferably about 32.

19. A method according to any one of claims 14-18 further including a homogenous step prior to the extrusion step wherein the mix is maintained at a temperature of from 200°- 300°C for a period of up to 5 hours.

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20. A method according to anyone of claims 17-19 wherein the mix is recrystallized during the hot-extrusion leading to grain refinement of the h-phase and bimodal grain size distribution.

21. A method according to any one of claims 14 to 20 wherein copper is added to the induction furnace together with the zinc and the aluminium.

22. Use of a zinc-based alloy according to any one of claims 1 to 13 in the manufacture of medical devices and implants such as orthopaedic implants such as plates, screws and pins, cardiovascular devices such as stents, and tissue engineering scaffolds.

23. Medical devices and implants such as orthopaedic implants such as plates, screws and pins, cardiovascular devices such as stents and tissue engineering scaffolds manufactured with a zinc-based alloy according to any one of claims 1 to 13.

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