RU2771150C1 - Method for obtaining porous material based on titanium nickelide by self-distributing high-temperature synthesis - Google Patents

Abstract

FIELD: powder metallurgy.

SUBSTANCE: invention relates to powder metallurgy, in particular to the production of porous material based on titanium nickelide by self-distributing high-temperature synthesis. It can be used in medicine, in particular for surgical implants. A quartz tube with a mixture of nickel and titanium powders is placed in a reactor, the volume of which exceeds the volume of a reaction mixture by 15-20%. The reactor is filled with a gas mixture consisting of 20-30% of nitrogen and 70-80% of argon, with an excess pressure of 0.15 ± 0.05 MPa, preheating is carried out, SHS reaction is initiated, and titanium nickelide synthesis, cooling and unloading of the target product is carried out.

EFFECT: fatigue strength of a porous alloy is increased in corrosive tissue fluids under cyclic loads from a living organism.

1 cl, 8 dwg

Description

The invention relates to the production of products from titanium and titanium alloys by self-propagating high-temperature synthesis and can be used in medicine, specifically in the field of surgical implants and other industries.

Titanium and titanium alloys are successfully used in medicine due to good biocompatibility and relatively high strength at low weight compared to steel products. Along with monolithic products such as shape memory clamps, implants made of porous titanium nickelide obtained by the reaction of self-propagating high-temperature synthesis (SHS) are widely used. One of the disadvantages that hinder the use of nickel-titanium alloys is the high chemical activity, which manifests itself in a tendency to surface corrosion. A common technique for increasing the corrosion resistance of titanium alloys is surface nitriding. Traditionally, ion-plasma nitriding of parts made of titanium and titanium alloys is carried out in industry using a glow discharge at operating pressures (0.5-10) mbar, as indicated in the sources: [V.A. Belous, G.I. Nosov, I.O. Klimenko Hardening of titanium alloys by ion-plasma nitriding. ISSN 1562-6016. VANT. 2017. No. 5(111); Ilyin A.A., Skvortsova S.V., Lukina E.A., Karpov V.N., Polyakov O.A. Low-temperature ion nitriding of implants from titanium alloy VT20 in various structural states // Metals. No. 2. 2005, p. 38-44].

As a result of nitriding, the surface layer of titanium is enriched with titanium nitrides, which increases its hardness and wear resistance. However, this method, which is effective for products with an open surface, turns out to be ineffective for porous products due to the inaccessibility of most of the surface for ion bombardment. Thus, the problem of nitrogen saturation of the surface of porous products made of titanium nickelide alloys needs to be solved.

As an analogue, consider a method for producing porous titanium nickelide by self-propagating high-temperature synthesis (SHS) [Itin V.I., Bratchikov A.D. SHS alloys with shape memory. // Self-propagating high-temperature synthesis. Tomsk, 1991, pp. 124-132]. Possessing the main features of all the methods developed later on its basis, it can be taken as a prototype of the invention.

A well-known method for producing a porous material based on titanium nickelide by the SHS method includes placing a quartz tube with a mixture of nickel and titanium powders into a reactor purged with an inert gas, preheating, initiating the SHS reaction, subsequent cooling and unloading the target product (Fig. 1).

When implementing the known method, the reactor is, as a rule, a steel pipe. The quartz tube protects the powder mixture from contamination when in contact with the reactor. As a rule, a nitrogen-argon mixture is used to purge the reactor, preheating is carried out to a temperature of 350°C ÷ 850°C, the SHS reaction is initiated using an electric spiral, in the process of synthesis, the inert gas pressure is maintained within 0.1 ÷ 0.2 MPa. It can be seen from the figure that during the course of the reaction, inert gas 1 enters the reactor, and the mixture of reaction gases 2 flows out of it.

In accordance with this method, a porous titanium nickelide widely used for endoprosthetics is obtained, which has a shape memory effect, with a total porosity of 30-70% and an open porosity of up to 90%, giving it a high degree of permeability.

The Ti2Ni phase, which has the lowest melting temperature compared to other phases, plays an important role in the SHS process. A distinctive feature of the Ti2Ni compound is the increased chemisorption of gaseous impurities such as oxygen, nitrogen, and carbon. It is their presence that contributes to the formation of a protective oxycarbonitride layer on the surface of the Ti2Ni phase [News of higher educational institutions, physics. No. 10, p. 98–107, 2017].

The disadvantage of this method is the low fatigue strength of the porous alloy in corrosive tissue fluids under cyclic loads from a living organism. This disadvantage is due to the structure of the peritectic layer near the surface of the porous alloy. The zone of peritectic crystallization is shown in Fig. 2: a – in the near-surface layer, b – around the pore. This zone is characterized by the fact that in it the dendrites formed by plastic grains of the TiNi intermetallic compound are surrounded by a brittle intergranular peritectic phase of the Ti2Ni intermetallic compound. The continuous brittle phase occupying the intergranular space is the source of a large number of cracks at the TiNi–Ti2Ni interphase boundary under the action of an external load. A typical crack pattern is shown in Fig. 3.

To identify the features and ways to overcome this drawback, an experimental study of the corrosion fatigue of the porous SHS TiNi alloy was carried out by cyclic bending to failure in various media, after which its relationship with the structural features of the surface layers of the specified alloy was established. The studies were carried out on the setup shown in Fig. 4.

During cyclic bending in air of samples obtained in a flow reactor, 72% of them withstand more than 1 million loading cycles. The remaining 28% of samples fail after 0.1–0.3 million cycles. When tested in distilled water, the number of samples that withstood more than 1 million loading cycles decreased to 43%. The remaining 57% of the samples failed after 0.1–0.5 million bending cycles. This result is interpreted as a decrease in the activation energy of the process of destruction of the alloy under the influence of water. When tested in 1% HCl solution, no sample reached 1 million bending cycles. Of all samples, 14% failed at 0.6 million cycles, 43% failed at 0.1–0.3 million cycles, and 43% did not reach 0.1 million cycles. Thus, a 1% hydrochloric acid solution significantly lowered the activation energy of the destruction process and the corrosion endurance limit of the alloy.

Using electron and optical microscopy, it was found that the mechanism of destruction of the porous TiNi alloy obtained by the SHS method is associated with the destruction of surface layers and the formation of fatigue cracks on nonmetallic inclusions. In FIG. Figure 5 shows photographs of the fatigue fracture surface of a porous TiNi alloy plate: a – general view of the fracture; b - non-metallic inclusion in the upper corner of the destroyed bridge; (c) fracture crack in the surface layer.

Non-metallic inclusions, as mentioned above, perform the main protective function against corrosion. Their origin is due to the composition of the flowing gaseous medium, which is a mixture of blown inert gases and reaction gases. Both those and others contain impurities O, N, C, which are usually called technological impurities. Due to the exothermic nature of the reaction, gaseous impurities acquire a high temperature and participate in the formation of a gradient surface layer of intermetallic oxycarbonitrides at the stage of cooling and structuring of the porous alloy. Due to the high corrosion resistance of intermetallic oxycarbonitrides, it is this surface layer, which is formed as a result of chemisorption of O, N, C impurities by the molten peritectic layer, that performs the main protective function.

The protective layer is thin and may be torn. For higher corrosion resistance, it is desirable that it has a greater thickness and continuity. This can be achieved by increasing the proportion of the reaction component, nitrogen, in the inert gas mixture. At the same time, a simple increase in the thickness of the corrosion-resistant layer does not solve the problem of creating a reliable protective layer, since the Ti2Ni interdendritic phase located in the peritectic layer is brittle compared to the TiNi plastic phase and is prone to cracking along the Ti2Ni–TiNi, Ti2Ni–TiNi3 interfaces. , which is reflected in the sources: [Yuri Yasenchuk, Ekaterina Marchenko, Gulsharat Baigonakova et al. Study on tensile, bending, fatigue, and in vivo behavior of porous SHS–TiNi alloy used as a bone substitute / Biomed. mater. 16 (2021) 021001. https://doi.org/10.1088/1748-605X/aba327; Ekaterina Marchenko, Yuri Yasenchuk, Sergey Gunther et al. Structural-phase surface composition of porous TiNi produced by SHS / Mater. Res. Express 6 (2019) 1165b1 https://doi.org/10.1088/2053-1591/ab4e32].

In order to increase the endurance of the alloy, it is necessary to disperse the continuous phase of Ti2Ni while maintaining the corrosion resistance of the surface. This can be achieved, for example, by vacuum annealing of the porous alloy, which leads to recrystallization of the peritectic layer, the formation of an accumulation of secondary Ti2Ni grains among TiNi grains [see. source: Influence of annealing on the phase composition and physical and mechanical properties of porous SHS-titanium nickelide: master's thesis in the field of study: 03.04.02–Physics. 2017// Electronic resource

http://vital.lib.tsu.ru/vital/access/manager/Repository/vital:4209/SOURCE01].

In FIG. Figure 6 shows photographs of the area of peritectic crystallization: a – before annealing; b – after vacuum annealing. It is noticeable that the grain size of the material is significantly reduced as a result of annealing. Unfortunately, this method makes it possible to achieve good results only on small samples of a porous alloy. In larger samples suitable for practical use, it is impossible to avoid local overheating, which leads to melting and degradation of the porous structure.

Thus, using the known method for producing porous titanium nickelide, it is possible to form a protective anti-corrosion layer, but the resistance of the resulting alloy to cyclic deformations becomes unacceptably low.

The objective of the invention is to create a continuous layer of titanium nitrides on the surface of a porous titanium nickelide alloy obtained by the SHS method in an inert gas mixture and recrystallize a brittle peritectic layer in order to increase its plasticity.

The technical result of the invention is to increase the fatigue strength of the porous alloy in corrosive tissue fluids under conditions of cyclic loads from a living organism.

The technical result is achieved by the fact that when implementing a method for producing a porous material based on titanium nickelide by the method of self-propagating high-temperature synthesis, including placing a quartz tube with a mixture of nickel and titanium powders in a reactor purged with an inert gas, preheating, initiating the SHS reaction, subsequent cooling and unloading of the target product, the difference is that the reactor is filled with a gas mixture consisting of (20-30)% nitrogen and (70-80)% argon with an overpressure of 0.15 ± 0.05 MPa and the reactor is isolated from the external atmosphere for the duration of the reaction, while the volume of the reactor is chosen to exceed the volume of the reaction mixture by 15-20%.

The achievability of the claimed technical result is due to the following.

1. Insulation of the reactor prevents the loss of heat generated by the reaction and ensures the circulation of an inert gas mixture inside it along with the released reaction gases. Hot reaction gases mixed with a heated inert gas mixture remain in the reactor volume and heat the surface layers of the porous alloy, providing recrystallization of the peritectic two-phase layer. Recrystallization of the massive Ti2Ni interdendritic phase, which is much more brittle than the viscous TiNi phase, into an accumulation of small secondary crystals eliminates the brittleness of the surface layer of the porous alloy. It has been experimentally established that during synthesis in a closed reactor, the surface layer of the porous alloy becomes more uniform and acquires a more ordered crystal structure and a smaller thickness, as shown in Fig. 7.

2. To ensure the circulation of the heated gas mixture, the volume of the reactor must exceed the volume of the reaction mixture, but not more than 15-20%. A reactor with smaller dimensions does not allow placing a quartz glass containing the reaction mixture, the necessary technological equipment for loading and unloading the reaction mixture, controlling the temperature of heating and ignition of the mixture. A larger reactor does not allow the necessary heating of the porous ingot by the reaction gases.

3. The gas mixture containing the declared ratio of inert (Ar) and reactive (N2) components provides the necessary and sufficient degree of saturation of the surface with nitrogen. Formed on the surface of porous titanium nickelide, a continuous but thin layer of titanium nitrides increases the fatigue strength of the alloy. The small thickness allows the corrosion-resistant layer to deform together with the base without cracking, which increases the corrosion resistance of the alloy. The proportions of the mixture are set experimentally. A smaller amount of nitrogen does not provide sufficient thickness and uniformity of the surface protective layer and the required corrosion resistance. More nitrogen leads to the formation of a thicker than necessary, nitride-hardened surface layer, which makes the alloy irreversibly brittle. The ratio of components in the range of 20-30% nitrogen and 70-80% argon is recognized as optimal for combining advantages, both in the degree of anti-corrosion protection and in the degree of endurance to cyclic mechanical loads.

4. An excess pressure of 0.15 ± 0.05 MPa is necessary to compensate for small gas losses at the stages of heating, synthesis and cooling of the reactor. Unlike the prototype method, excess pressure is not associated with a constant purge of the reactor, but with a single filling and maintaining excess pressure to create circulation of a heated gas mixture in a closed volume. The overpressure range was set experimentally. In the specified pressure range, a sufficient amount of gas components is present in the reactor, and at the same time, it is not required to ensure high strength of the reactor structure.

The difference of the proposed method is illustrated in Fig. eight.

The method for producing a porous material based on titanium nickelide by the method of self-propagating high-temperature synthesis includes placing a quartz tube with a mixture of nickel powders into a reactor filled with gas mixture 1 at a pressure of 0.15 ± 0.05 MPa, preheating, initiating the SHS reaction with an electric coil, cooling and unloading target product. As can be seen from the figure, the gas mixture during the reaction does not have the possibility of outflow from the reactor, retaining the heat necessary for recrystallization of the peritectic layer.

The difference is that the synthesis is carried out in a closed reactor filled with a mixture consisting of (20-30)% nitrogen and (70-80)% argon. The volume of the reactor should not significantly exceed the volume of the reaction mixture in order to minimize the loss of released heat. As a rule, the volume excess is 15-20%, which makes it possible to place inside a quartz tube (glass) with the reaction mixture, as well as means for loading, unloading, temperature control and ignition of the mixture.

The method is carried out as follows. To obtain the desired result in the form of massive ingots of porous titanium nickelide, synthesis is carried out in a closed reactor filled with a gaseous medium, the composition of which (20-30)% N2 + (70-80)% Ar was selected experimentally. In this case, argon and nitrogen of technical purity, standard gas reducer, mixer and rotameters are used. The reactor is a metal cylinder closed at the ends. Filling the reactor with a gas mixture is carried out before immersion in the furnace. The heating of the reactor with the workpiece is carried out by an electric resistance furnace to a temperature in the range of 350°C÷850°C, depending on the weight, chemical and granulometric composition of the reaction mixture. The temperature of the start of synthesis for workpieces of various masses is selected experimentally, controlling the quality of the porous alloy by metallographic methods. At the same time, it is achieved to obtain the most suitable isotropic porous alloy for medical implants with an average pore size of 80–120 μm, with the presence of secondary Ti2Ni crystals and without a peritectic two-phase Ti2Ni–TiNi layer at the pore surface. The temperatures thus refined are fixed as operating parameters for a particular particle size distribution and weight of the powder mixture and a particular reactor design. The beginning of the synthesis is initiated by an electric coil, immediately before turning off the heating of the furnace. Cooling of the resulting product to room temperature after synthesis is carried out together with the reactor and furnace in order to increase the annealing time and its homogenizing effect. The cooling time, depending on the mass of the ingot, is from 30 to 60 minutes.

The results of a microscopic study of the structure of the surface layers of the porous alloy obtained by the known method and the proposed method are shown in Fig. 2 and 7.

Samples obtained in an open flow reactor (Fig. 2) have two-phase areas of peritectic crystallization along the boundaries of open pores, which consist of dendrites and an interdendritic phase. The dendrites are formed by grains of the TiNi intermetallic compound, and the interdendritic phase is formed by the Ti2Ni intermetallic compound. The reason for the formation of a near-surface zone of peritectic crystallization is the heat of flowing reaction gases, which are filtered through the zone of alloy structuring. Under the action of additional heat during the peritectic reaction, intragranular segregation develops and a two-phase dendritic zone TiNi–Ti2Ni is formed, where plastic TiNi grains are surrounded by a brittle Ti2Ni phase. Near the peritectic zone there is a zone of crystallization of a plastic TiNi solid solution, in the grains of which there are a large number of small inclusions of secondary phases.

Samples obtained in an isolated reactor (Fig. 7) demonstrate the advantage of the claimed method. During SHS in an isolated reactor, a zone of secondary crystals is formed along the surface of large open pores under the action of the heat of the reaction gases circulating in the heated protective atmosphere of the isolated reactor. Secondary Ti2Ni crystals are formed by recrystallization of the Ti2Ni interdendritic phase of the peritectic layer. Dendrites from TiNi grains recrystallize into densely packed secondary TiNi grains, which are visually indistinguishable from grains in the solid solution zone formed by primary crystallization.

The amount of heat retained in an isolated reactor is much greater than in a flow reactor, and it is sufficient for the recrystallization of peritectic zones. When comparing the micrographs of Fig. 2 and FIG. It can be seen from Fig. 7 that, during open synthesis, the Ti2Ni interdendritic phase is continuous and occupies a region with a width of more than 20 μm. During closed synthesis, clusters of secondary crystals of the Ti2Ni phase are located in the elastic matrix phase of TiNi, are isolated, and have a maximum size of no more than 5 μm.

Since the Ti2Ni phase in the Ti–Ni system is the most stressed, brittle, and prone to cracking, due to its recrystallization into clusters of dispersed crystals, the stresses in the surface zone of porous alloys after closed synthesis become lower than in the dendritic zone after open synthesis. Thus, alloys produced in an insulated reactor are less prone to cracking and more resistant to cyclic loads.

Claims (1)

A method for producing a porous material based on titanium nickelide by the method of self-propagating high-temperature synthesis, including the placement of a quartz tube with a mixture of nickel and titanium powders into a reactor purged with an inert gas, preheating, initiation of the SHS reaction, subsequent cooling and unloading of the target product, characterized in that the reactor is filled with a gas mixture consisting of 20-30% nitrogen and 70- 80% argon, with an excess pressure of 0.15 ± 0.05 MPa, and isolate the reactor from the external atmosphere for the duration of the reaction, while the volume of the reactor is chosen to exceed the volume of the reaction mixture by 15-20%.

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