WO1996028578A1

WO1996028578A1 - Formation and use of intermetallics

Info

Publication number: WO1996028578A1
Application number: PCT/GB1996/000596
Authority: WO
Inventors: Tianyi Cheng; Malcom Mclean
Original assignee: Imperial College Of Science, Technology & Medicine
Priority date: 1995-03-14
Filing date: 1996-03-14
Publication date: 1996-09-19
Also published as: GB9505114D0

Abstract

A method of producing an intermetallic compound or composite which comprises forming a mixture which contains elemental metal powders in the required atomic ratio and optionally an additive such as a ceramic, compacting the powdered mixture into a preform, heating the preform and subjecting the heated preform to hot extrusion to obtain a solid extruded intermetallic-containing product.

Description

FORMATION AND USE OF INTERMETALLICS

This invention is concerned with the production of the group of metallic compounds which are known as intermetallics. Intermetallics are potential candidates for advanced structural materials. Usually, intermetallics consist of two or more combined elemental metals, for example:

Nickel-aluminium Ni,

Titanium-aluminium Ti,_χ,-Al,_v

Silicon-molybdenum Si,_χ^-Mo, v Nickel-aluminium-chromium Ni ,_χX -Al _y\ -Cr _,x , and Nickel-aluminium-iron Ni ,_χ -Al _*. -Fe _,_* wherein x + y = 100 Atomic % or x + y + z (if present) = 100 Atomic %

Specific example of known Nickel-aluminium intermetallics include, for example the following nickel aluminides:

^A175 ^Ni25'^{" Al}50 ^Ni50' ^A125 ^Ni75'^'

Traditional processing to produce bulk products made of structural materials includes at least the following three steps:

(a) Synthesis to obtain billet or block materials mainly through casting or powder metallurgy routes at a temperature higher than (in casting) or close to (in powder metallurgy) the melting temperature ( T_m) of the materials for a long time. Coarse microstructures can result, particularly during the casting process. Expensive equipment is normally needed.

(b) Hot working to produce semifinished rods, sheets or tubes. The temperature at this stage should be higher than the recrystallization temperature (T_R T_R~0.4T_m). Repeated annealing is however necessary to reduce the internal stress and hardness of the material. The overall working time also is long. In addition, working, such as hot extrusion, only deforms and consolidates the materials after their synthesis and much mechanical energy expended in their production has been wasted.

(c) Machining to form final products. Although this stage is normally carried out at room temperature, repeated annealing at relatively high temperatures is still needed. Consequently, traditional processing for production of bulk products needs separate, multi-step processing with repeated heating to a high temperature and expensive equipment. The situation is even more demanding for products made of advanced materials.

The following intermetallic materials are known. TiAl and NiAl-B which were manufactured through hot extrusion directly from elemental powders and TiAl was produced by cold extrusion or cold rolling followed by heat treatments. The applied hot extrusion processing was still complex and the operating temperature was very high. Cold extrusion or rolling only played a role to mix the elemental powders and no reaction or sintering occurred in that stage so the processing was similar to normal hot pressure reaction sintering.

The intermetallic compounds (herein "intermetallics") have been likened to being compounds which, in atomic bonding, fall part way between ceramics and alloys. In addition to having metallic chemical bonds, the intermetallics are partially covalent bonded. They tend to be difficult and costly to produce and to work by deformation. In particular it has been difficult to extrude intermetallics into products with a useful shape e.g., bars, rods, tubes, sheets and the like.

Intermetallics, such as those in the Ni-Al system noted above have, as a generality, low densities compared to 'super- alloys' of 9-10 elements, but the high temperature strength is good. Being brittle and hard at ambient temperature, they are notoriously difficult and expensive to machine or otherwise process by deformation processing. Most intermetallics are stable compounds and involve an exothermic reaction in their production. There have been attempts to extrude some intermetallics by hot extrusion processes but these have not apparently been successful.

In addition to the casting process, intermetallics conventionally have been produced by a powder metallurgy technique known as 'reaction sintering' wherein the elemental metal powders in the desired atomic ratios are intimately mixed then heated to an appropriate temperature. When produced by reaction sintering, the density of the material is usually inadequate without still further and costly processing steps. For example heating temperatures of the order 1200-1600°c and a pressure greater than 30MPa can be required for the production of dense Ni-Al intermetallics by reaction sintering. It can also be necessary to densify such products by hot isostatic pressing (HIP) involving higher pressures. The other similar process is reaction hot isostatic pressing (RHIP) which combines reaction sintering and HIP.

Moreover the process time even at these elevated temperature levels can be of the order 2-3 hrs, with an additional period allowed for the preheating and subsequent cooling before the intermetallic can be handled or further processed. Overall, it may take some 5-7 hrs to produce dense Ni-Al intermetallics, and temperatures up to 1600°.

Usually the intermetallic compound is formed as a block or billet. Once the product has been formed and cooled it is almost invariably necessary to re-heat the formed product before attempting any extrusion or other working steps.

In addition to reaction synthesis (alternatively known as reaction sintering) mentioned above there are other known techniques for producing intermetallics. These other powder metallurgy techniques include shock induced reaction synthesis (SIRS) and self-propagating high temperature synthesis (SHS) but it is difficult to apply them to produce dense intermetallics in an industrial scale.

The technique of hot extrusion is normally used as a hot working process to extrude previously synthesised (cast or sintered) alloys into an intermediate product having a longer dimension e.g. rods, tubes, bars and the like. It can also be used to densify pre-alloyed powders by consolidation and deformation. The actual extrusion step is generally of a short duration.

The high temperature intermetallics, such as those of the Ni-Al system, normally have exceptionally high heats of formation and are difficult to produce using conventional casting techniques because of their high melting points.

There is considerable commercial potential to use the intermetallic compounds as high temperature-stable structural materials in diverse applications. NiAl has the highest melting temperature (1633°c for stoichiometric NiAl) amongst the more commonly investigated intermetallics. The problem is that NiAl and similar intermetallics are difficult and very costly both to produce and to subsequently machine or otherwise deform into useful commercial products or even into semi-finished goods such as extruded bars, rods or tubes.

NiAl intermetallics are known to have potentially much wider commercial application than presently is the case.

We have now found that intermetallics and especially intermetallics containing Ni and Al or Ti and Al can be chemically formed and extruded simultaneously and the present invention is based on this discovery. Moreover composites also containing an alloy and/or a ceramic component such as SiC can also be formed and simultaneously extruded into semi-finished products.

This invention provides a method of producing an intermetallic compound or composite which comprises forming a mixture which contains elemental metal powders in the required atomic ratio and optionally an additive such as a ceramic, compacting the powdered mixture into a preform, heating the preform and subjecting the heated preform to hot extrusion to obtain a solid extruded intermetallic- containing product.

The invention will now be described with reference to the following non limiting examples.

It is preferred to preheat the preform, e.g. a billet made of elemental powders e.g. nickel and aluminium powders of a certain composition to an appropriate temperature ^τl ^τl ^< ^a' ^{wnere τ} _a ^^{s t}"^{e react}i°ⁿ temperature between the elemental powders) before hot extrusion starts. Because of temperature rise in the billet during hot extrusion from transition of mechanical energy and the increasingly intimate contact between elemental powders as the powders deform, the reaction for the formation of intermetallics can be initiated with simultaneous consolidation in a very short time (few seconds) . Hence, it is possible that the reaction synthesis proceeds actually during the hot extrusion step (normally a few seconds duration) . The extrusion stress can promote gas release produced from the reaction and promote densification of the preform 'compacts. ' The heat released from the reaction of the powders synergistically benefits the extrusion deformation of the synthesised intermetallics. The simultaneously combined reaction sintering (RS) and hot extrusion (HE) can not only save energy and time in producing materials in relative bulk in comparison with the separate, previously described RS - HE or RHIP + HE steps. It also should enhance the density and homogeneity of the product so as to further improve its mechanical properties.

Initial examples using the present HERS (hot extrusion reaction synthesis) process

(1) The intermetallic Al^Ni was synthesised in an Al-10wt%Ni compact while a mixture of (2) Al-,Ni and (3) Al₃Ni2 were synthesised in an Al-30wt%Ni compact. A mixture of (4) Ni₃Al and (5) NiAl were synthesised respectively in Al-50wt% Ni and Al-70wt%Ni compacts, all of which were made of corresponding mixtures of pure elemental powders. At the same time as the reaction sintering the preform 'compacts' were hot extruded into a long rod with a diameter of 14mm - 19.3mm and a length of about 60cm-l00cm.

Technique

(1) Weigh portions of pure elemental Ni + Al or Ti + Al powders then intimately mix for hr. If a composite is to be made, mix in the material for the eventual composite at this stage e.g. adding silicon carbide if an intermetallic/ceramic composite material is required.

(2) Subject the mixture to a cold compaction to form a compact with about 75-80% theoretical maximum density.

(3) Preheat the compact using a simple furnace in air or an inert gas such as Argon to an appropriate temperature, about 400°c, for about 20 minutes.

(4) Place the still hot compact into a conventional hot extrusion container and hot extrude also at a temperature of about 400°c. Using an extrusion ratio of about 10:1 push the compact by ram through the extrusion orifice to form a solid, extruded rod.

(5) Allow the produced rod to cool before further treatments.

(6) Heat treat the extruded rods at relatively low temperatures e.g. 400°C in a normal furnace. (7) Ni-Al proportions in weight can be e.g. from 10-70 Ni : 90-30 Al.

Other embodiments of the invention will now be described with reference to the table 1 herein and the accompanying drawings, by way of non-limiting example only and wherein:

Figure 1 shows appearance of pieces of HERS materials (a) Ni-50wt%Al (top) and Ni-30wt%Al (bottom) rods (b) Al- 10wt%Ti-10wt%SiC rod (c) Al-50wt%Ni sheet.

Figure 2 shows microstructures of HERS Al-Ni alloys (a) Al-70wt%Ni optical micrograph of transverse section and (b) and (c) SEM micrographs of transverse section and longitudinal section respectively of Al-50wt% Ni (the length of mark is lOOμm) .

Figure 3 shows microstructures of HERS Al-70wt%Ni (a) α-Al and -Ni (black) (b) extensive dislocations and subgrain boundaries in α-Al (c) dislocations in _f-Ni (TEM micrographs) .

Figure 4 shows intermetallics formed during hot extrusion in Al-70wt%Ni (TEM micrograph, three EDS patterns correspond to three spots respectively) .

Figure 5 shows XRD patterns of HERS Ni-50wt%Al (NA-52) and the alloys after gradual heat treatments with increasing temperatures and holding time (NA-521-NA-524) In the NA-524 there only are Al-,Ni and Al₃Ni2 intermetallics.

Figure 6 shows microstructures of HERS Al-50wt%Ni after preliminary heat treatments (a) at smaller magnification (b) at bigger magnification.

The major feature of HERS includes hot extrusion of elemental powders at a rather low temperature (e.g. T<l/3T-_ for some products) for a very short time to get sound rods containing some intermetallics (-5vol%); hot rolling into sheets and/or machining to get near finished shape and size of materials; heat treatments also at a relatively low temperature (.e.g < 1/2T__ for some intermetallics) to complete the reaction; minor machining to obtain final semi¬ finished products. Full details of the processing conditions are given in the technique above. These other embodiments are based on: Al-xwt%Ni (x= 10, 30,50 and 70), Al-10wt%Ni-10wt%SiC and Al-10wt%Ti-10wt%SiC. The elemental nickel powders (average size 3 μ ) were provided by Inco Ltd. Aluminium powders (average size 25μm) were produced by atomisation. Titanium powders (averages size 45μm) and SiC powders (average size 5μm) were commercial powders.

The microstructures in cross-section of the extruded and heat treated rods were studied by means of Philips x-ray diffractometer (XRD) , Nikon Epiphot metallographic microscopy, JEOL T-220A scanning electron microscopy (SEM) and JEOL 2000FX transmission electron microscopy (TEM) at 200kV. The microstructures in the section parallel to the extruded direction of some materials were also investigated by means of SEM. The microco positions of different phases in the alloy have been examined using the electron probe in JSM-35CF SEM with minimum probe size about 2μm and energy dispersive spectroscopy (EDS) in TEM. Co is chosen as the calibration element so the analysis error can be less than 5at% for most elements. Co pressive testing and three points bend testing of some HERS materials were performed at room temperature and 350°C respectively by means of Nene 100 Universal Mechanical Testing Machine at a strain rate of 1.5X10^-3s~¹. The diameter and the length of the cylindrical specimen in compression were 5mm and 10mm respectively. The specimen size for bend testing was 2mmx4mmxl2mm and the span distance between two support points was 10mm. The cylindrical axis or the longitudinal direction of the specimens was parallel to the hot extrusion direction.

Microstructures after hot extrusion

The diameters of rod of Al-Ni, Al-Ni-SiC and Al-Ti-SiC products produced by HERS were in the range of 16-19mm and the length about lm. Figure 1 shows parts of typical extruded rods for Al-50wt%Ni, Al-70wt%Ni and Al-Ti-SiC and a sheet for Al-50wt%Ni after further rolling, all exhibiting a smooth uncracked surface.

Basically the HERS materials are dense, homogeneous and fully sintered, which can be seen from metallography micrographs (Fig.2). The porosity was estimated to be less than 5%. The materials examined by SEM also showed homogeneous and dense microstructures in both transverse and longitudinal sections. There was no evidence of elongated microstructures parallel to the extrusion direction (Fig.2(c)) .

All of the hot extruded materials consisted of several phases. Examination by means of electron probe in SEM and EDS in TEM indicated that the matrices in Al-Ni products were α-Al solutions for the 10 to 50wt%Ni products and 2f-Ni for 70 wt%Ni products containing various intermetallics, including Al₃Ni, Al₃Ni₂, Ni-_jAl and NiAl (Table 1), generated during hot extrusion process. The block phases (2f—Ni in Al- 10-50wt%Ni and α-Al in Al-70wt%Ni) were in the range of 10- 70μm in diameter and were dispersed throughout the matrices.

The microstructures of Al-70wt%Ni (about Ni-50at%Al) examined by means of TEH are shown in Fig.3. The α-Al solutions exhibited a polygon-like grain structure and the grain size is small, about 5μm, due to the low extrusion temperature. There is a high density of dislocations or dislocation networks. Dislocation subgrain boundaries can also be found in some grains. The facts indicate that a sintering process, extensive plastic deformation after sintering and recrystallization to some extent occurred among aluminium powders during hot extrusion and cooling. On the other hand, many tf-Ni solutions (black contrast) showed a stretched block-like or band-like appearance and the high density of dislocations can be found in some grains, suggesting that plastic deformation after sintering in "Ni solutions was minor and recrystallization might not occur due to its higher hardness and recrystallization temperature compared with that of α-Al. Further investigation by means of TEM proved that various intermetallics produced from hot extrusion distributed mainly in an area near to the interface between α-Al and X-Ni (Fig.4) . Since the resolution of XRD analysis is normally about 5 vol% and some weak peaks contributed from intermetallics just can be seen in the low angle area of the XRD pattern of hot extruded Al-50wt%Ni (NA-52) in Fig.5(a) the relative content of intermetallics formed during hot extrusion is about 5vol%. In addition, the interface bonding between resolutions and between intermetallics and solutions, or between reinforcements (such as of the ceramic SiC) and the matrix all are very good and no apparent microcracks were observed.

Microstructures after heat treatments

The most apparent change of the microstructures of HERS rods after preliminary heat treatments also at relatively low temperatures is the appearance of radial bands in the matrix e.g. α-Al in Al-50wt%Ni product (Fig.6) . The radial bands were located close to block-like complementary solution phase i.e. -Ni. The band normally consisted of many very small grains, only about 0.5μm, and some of the grains being equi-axed and others being longitudinal. The analysis by means of EDS showed that the grains were intermetallics e.g. Al₃Ni and Al-.Ni₂ formed during heat treatments in the Al-50wt%Ni product. The examination from XRD analysis revealed a similar result. The gradual appearance of many new and split peaks contributed from intermetallics and enhancement of their relative intensities with increase of temperatures and hold times indicate a progressive generation of intermetallic phases (Fig.5(a)) . The complete reaction and full intermetallic production can be achieved at the end. The same results were obtained in Al-70wt%Ni and Al-Ti-SiC products. A single β-NiAl phase existed in the heat treated Al-70wt%Ni product based on limitations of XRD analysis (Fig.5(b)). Considering the very high melting temperatures of this alloy it is promising to produce the full β-NiAl by hot extrusion and heat treatments all at relatively low temperatures.

Mechanical properties

Compressive yield strengths of the HERS rods measured on the Al-50wt%Ni alloy of 219MPa at room temperature, 120MPa at 350°C and bend yield strength on 10%wtTi-10wt%SiC of lOlMPa at room temperature were reasonable. The major reason for these strength values is believed to be due to the hot extrusion and heat treatments being effected in air so there was some oxidation although the operating temperatures were low. In addition, the purity of the Sic powders used is not very good and some inclusions around the powders were observed.

There was considerable heat generation during hot extrusion due to internal friction within the billet and surface friction between the billet and the die. This heat is not normally a positive feature of conventional extrusion processing. However, this sort of heat in the present HERS assists effectively short-range diffusion to initiate the reaction between elemental powders mainly in an area near to their interface at a low temperature. Once the reaction starts locally the very high formation heat of intermetallics apparently accelerates the diffusion in powders to sinter them together and to promote further reaction. In addition, the large extruding stress can considerably increase the contact surface between elemental powders in the billet and reduce diffusion distances. The extruding stress and high temperature resulting from the local reaction also is helpful for further consolidation of the reacting materials and removal of gas produced from the reaction. Hence, the interaction between mechanical work produced from hot extrusion and chemical reaction energy from synthesis of intermetallics appears to play a key role in HERS. However, the reaction in HERS is not complete and there appears to be internal stress in the extruded materials, hence further heat treatment to complete the reaction has been found necessary.

As shown in Fig.1(c) , the HERS rod can be further rolled to manufacture sheet materials. HERS Ni-Al rod can reduce thickness by 70% by rolling without cross cracks. HERS is an efficient and economic way to produce materials with a shape of a rod, possibly tube, and sheet.

The hot extrusion reaction synthesis (HERS) process has been successfully developed in Al-Ni alloys and Al-Ni-SiC, Al-Ti-SiC composites to produce rods or sheets containing various intermetallics. The processing combines synthesis, sintering and deformation together so there are commercial prospects for reducing the manufacturing cost of various semifinished materials.

Table 1 Microco position (at%Ni) of different phases found in HERS Ni-50wt%Al (NA-5) and HERS Ni-50wt%Al heat treated at 385°C for lOh (NA-51)

Al ₃Ni Al ₃N i₂ AIN i AlN i₃ tf-Ni α-Al

NA-5 23 . 3 38 . 4 56 . 5 72 . 3 98 . 9 1 . 0

NA-51 24 . 9 37 . 5 51 . 8 75 . 1 98 . 4 0 . 8

The technique can be used to produce alloys strengthened by intermetallics, pure intermetallic compounds or alloys, intermetallics reinforced with e.g. ceramic additives. Intermetallics other than the Ni-Al system can be used in the simultaneous reaction synthesis and hot extrusion process.

Without in any way wishing to be bound by theoretical considerations, it is postulated that the dense and deformed intermetallic materials could be actually produced during, and not before, the hot extrusion step which may last only a few seconds. This represents significant time and cost savings compared to other fabrication techniques and to the separate, subsequent extrusion processes which are difficult even under ideal conditions. It is also believed that the products obtained by the present method can have improved mechanical properties due to high density and homogeneity in the extruded products produced by this process.

It is not necessary to deploy a separate high pressure furnace or HIP furnace, and conventional hot extrusion (HE) presses can be used even within the ambient air atmosphere for production of most intermetallic materials. The required extruded shape can be made relatively easily and more economically. The number of processing steps can be significantly reduced and the heating temperatures/times significantly reduced as well. Using the method, the traditionally poor workability of the intermetallics could be overcome or mitigated. Intermetallic composites comprising at least one intermetallic and at least one ceramic and/or at least one alloy can be formed using this method.

Claims

1. A method of producing an intermetallic compound or composite which comprises forming a mixture which contains elemental metal powders in the required atomic ratio and optionally an additive such as a ceramic, compacting the powdered mixture into a preform, heating the preform and subjecting the heated preform to hot extrusion to obtain a solid extruded intermetallic-containing product.

2. A method as claimed in claim 1 in which the preform is a compacted billet which is formed before extrusion.

3. A method as claimed in either preceding claim wherein the elemental powders comprise nickel and aluminium.

4. A method as claimed in claim 1 or 2 wherein the elemental powders comprise titanium and aluminium.

5. A method of producing an intermetallic composite as claimed in any preceding claim wherein the mixture further comprises at least one ceramic component.

6. A method as claimed in claim 5 wherein the ceramic component comprises silicon carbide.

7. A method as claimed in any preceding claim wherein the extruded product is a rod, tube or sheet.

8. A method as claimed in any preceding claim which is carried out in an inert, non-oxidising atmosphere.

9. A method as claimed in any preceding claim followed by one or more heat treatments to increase the quantity of intermetallic formation.

10. A method as claimed in any preceding claim wherein the preform is heated to a temperature which is less than about one third of the melting temperature of the intermetallic to be produced.

11. A method as claimed in claim 9 or 10 wherein the heat treatment(s) is(are) effected at a temperature which is less than about one half of the melting temperature of the said intermetallic.

12. A method as claimed in any preceding claim wherein the extrusion temperature is about the same as the temperature to which the preform is heated.

13. A method as claimed in any preceding claim wherein the extrusion pressure is about 200 to 500 MPa.

14. A method as claimed in any preceding claim wherein the preform is heated for a period of about 30 minutes before extrusion.