JP7403144B2 - Method for producing titanium alloy with suppressed twin deformation and titanium alloy - Google Patents

Description

The present disclosure relates to a method for producing a titanium alloy and a titanium alloy in which twin deformation is suppressed.

Titanium alloys, for example, are promising as biological materials.

Japanese Patent Application Publication No. 2015-048500

Some titanium alloys exhibit twin deformation. Titanium alloys that cause twin deformation include β-type titanium alloys. β-type titanium alloy has a Young's modulus that is not significantly different from that of human bone, and is therefore particularly promising as a biological material.

However, titanium alloys that undergo twin deformation, such as β-type titanium alloys, tend to undergo plastic deformation and may not have sufficient strength. This is because the deformation stress of twin deformation is smaller than that of slip deformation. Therefore, a titanium alloy that undergoes twin deformation has lower strength than a titanium alloy that undergoes sliding deformation.

Therefore, in a titanium alloy that causes twin deformation, it is desirable to suppress the twin deformation.

An aspect of the present disclosure is a method of manufacturing a titanium alloy in which twin deformation is suppressed. The disclosed manufacturing method includes incorporating a β-phase stabilizing element into a titanium alloy that undergoes twinning.

Another aspect of the present disclosure is titanium alloys. The disclosed titanium alloy has a first region where twin deformation occurs and a second region where shear deformation occurs due to the inclusion of a β-phase stabilizing element.

In still another aspect of the present disclosure, the disclosed titanium alloy is a titanium alloy having a harmonic structure in which a fine crystal grain region formed around a coarse crystal grain region constitutes a three-dimensional network, wherein the coarse crystal grain region The grain region has a first region where twinning deformation occurs and a second region where sliding deformation occurs due to the inclusion of a β-phase stabilizing element.

Further details are described in the embodiments below.

FIG. 1 is a flowchart showing a procedure for manufacturing a titanium alloy in which twinning deformation is suppressed. FIG. 2A is an EBSD image of a titanium alloy with harmonic texture. FIG. 2B is a SEM image of a titanium alloy with a harmonic texture. FIG. 3A is an enlarged image of FIG. 3B. FIG. 3B is an EBSD image of the titanium alloy after rolling. FIG. 4A is a strain distribution image. FIG. 4B is a SEM image of the same site as FIG. 4A. FIG. 5 is a SEM image showing plot positions. FIG. 6 shows the measurement results of iron content at plot positions. FIG. 7 is an image showing the presence or absence of iron element at the plot position. FIG. 8A is an IPF map of harmonic tissue. FIG. 8B is an iron element EDS map.

<1. Production method of titanium alloy with suppressed twin deformation and overview of titanium alloy>

(1) The manufacturing method according to the embodiment includes adding a β-phase stabilizing element to a titanium alloy that undergoes twin deformation. If the stability of the β phase is low, twinning deformation is likely to occur, but by incorporating a β phase stabilizing element into the titanium alloy that causes twinning deformation, the deformation mode of the titanium alloy can be changed from twinning deformation to slip deformation. Can be converted. Examples of the β-phase stabilizing element include iron (Fe), hydrogen (H), molybdenum (Mo), niobium (Nb), tantalum (Ta), and vanadium (V). By this manufacturing method, a titanium alloy in which twinning deformation is suppressed is manufactured from a titanium alloy that causes twinning deformation. By subsequently including a β-phase stabilizing element in a titanium alloy that causes twin deformation, the deformation mode can be easily controlled. Here, the "titanium alloy with suppressed twin deformation" may be a titanium alloy in which twin deformation remains even if it is suppressed, or a titanium alloy with no twin deformation due to suppression of twin deformation. It may be an alloy.

(2) The β-phase stabilizing element is preferably iron. Since iron has excellent biocompatibility, it is advantageous when titanium alloys are used as biomaterials.

(3) Including the β-phase stabilizing element may mean diffusing the β-phase stabilizing element into the titanium alloy that causes twin deformation. By controlling the concentration of the β-phase stabilizing element generated by diffusion, it is possible to control the deformation mode. It is preferable to diffuse iron as the β-phase stabilizing element. Since iron diffuses relatively slowly, it is easy to control the amount of twinning.

(4) Diffusion of the β-phase stabilizing element may be performed by heat treatment. In this case, by controlling the heat treatment temperature or heat treatment time, the concentration of the β-phase stabilizing element can be controlled, and as a result, the deformation mode can be controlled.

(5) The titanium alloy that causes twin deformation is, for example, a β-type titanium alloy.

(6) The titanium alloy that causes twin deformation preferably has a structure in which a fine grain region is formed around a coarse grain region. Such a structure makes it possible to achieve both high strength and high ductility. Therefore, by controlling the deformation mode in such a structure, it becomes easier to achieve both strength and ductility.

(7) Including the β-phase stabilizing element may include diffusing the β-phase stabilizing element from the fine crystal grain region to the coarse crystal grain region. By diffusing the β-phase stabilizing element into the coarse grain region, twinning deformation in the coarse grain region can be suppressed.

(8) The titanium alloy in which twinning deformation is suppressed preferably has a region in which the content of iron as the β-phase stabilizing element is greater than 0.1% by mass. When the iron content is greater than 0.1% by mass, twinning deformation is sufficiently suppressed. The iron content is more preferably greater than 0.5% by mass, and even more preferably greater than 1% by mass. The iron content is preferably less than 15% by mass, more preferably less than 10% by mass, even more preferably less than 5% by mass, and even more preferably less than 3% by mass.

(9) The titanium alloy according to the embodiment has a first region where twin deformation occurs and a second region where sliding deformation occurs due to the inclusion of a β-phase stabilizing element. In the second region, by containing the β-phase stabilizing element, twinning deformation is suppressed and slip deformation becomes dominant. The second region where sliding deformation occurs has higher strength than the first region where twin deformation occurs. Therefore, a titanium alloy having the first region and the second region has higher strength than a titanium alloy in which only twinning deformation is dominant. Moreover, since the first region causes twin deformation, it is also possible to maintain ductility. By controlling the ratio of the first region and the second region, it is possible to achieve both strength and ductility while controlling the balance between the two.

(10) The titanium alloy according to the embodiment is a titanium alloy having a harmonic structure in which a fine grain region formed around a coarse grain region constitutes a three-dimensional network, wherein the coarse grain region is It has a first region where twinning deformation occurs and a second region where sliding deformation occurs due to the inclusion of a β-phase stabilizing element. Harmonic structure enables both high strength and high ductility. Moreover, since twinning deformation in the coarse grain region is suppressed in the second region, it is possible to increase the strength of the coarse grain region mainly for ensuring ductility in the harmonic structure.

(11) The β-phase stabilizing element is preferably iron. Since iron has excellent biocompatibility, it is advantageous when titanium alloys are used as biomaterials.

(12) Preferably, the second region exists around the first region. In this case, the second region will be located near the fine grain region. As a result, the dispersed first region (responsible for ductility) is surrounded by high-strength regions (fine grain region and second region), but it becomes a three-dimensional network structure, and the basic structure of the harmonic structure is maintained. be done.

(13) Preferably, the second region includes a region in which the content of iron, which is the β-phase stabilizing element, is greater than 0.1% by mass. In this case, twinning deformation in the second region is sufficiently suppressed. The iron content in the second region is more preferably greater than 0.5% by mass, and even more preferably greater than 1% by mass. The iron content in the second region is preferably less than 15% by mass, more preferably less than 10% by mass, even more preferably less than 5% by mass, and even more preferably less than 3% by mass. preferable.

(13) Preferably, the content of iron, which is the β-phase stabilizing element, in the first region is 0.1% by mass or less. When the iron content is 0.1% by mass or less, twin deformation is likely to be maintained.

<2. Manufacturing method of titanium alloy with suppressed twin deformation and details of titanium alloy>

<2.1 Manufacturing method>

FIG. 1 shows a manufacturing method according to an embodiment. In the embodiment, first, a raw material powder is produced (step S11). The raw material powder is titanium alloy powder. The titanium alloy powder is produced, for example, from a titanium alloy as a base material by a plasma rotating electrode method (PREP). The titanium alloy serving as the base material is, for example, a Ti--Nb--Zr alloy. Ti-Nb-Zr alloy is a β-type titanium alloy. β-type titanium alloys undergo twinning deformation. Each particle constituting the titanium alloy powder is composed of a plurality of relatively coarse crystal grains (coarse crystal grains). The average particle size of the titanium alloy powder is not particularly limited, but is, for example, 100 μm to 300 μm.

Subsequently, iron powder is added to the titanium alloy powder produced in step S11 to produce a mixed raw material powder. It is preferable that the iron powder has a smaller average particle diameter than the titanium alloy powder. The average particle size of the iron powder is not particularly limited, but is, for example, 1 μm to 50 μm. The average particle diameter of the iron powder is preferably from 1 μm to 30 μm, more preferably from 1 μm to 20 μm. The average particle size of the iron powder may be about 1 to 10 μm.

By performing mechanical milling on the mixed raw material powder created in step S12, the crystal grains in the surface layer of each particle constituting the titanium alloy powder are refined, and processed powder is obtained (step S13). ).

Mechanical milling forms a fine grain region in the surface layer of the titanium alloy powder. When titanium alloy powder is subjected to strong processing such as mechanical milling, the crystal grains become flattened due to plastic deformation, and lattice effects (dislocations) are introduced one after another, causing the crystal grains to become fragmented and finely divided (for example, into nano-sized particles). It will be done.

In mechanical milling, the region inside the surface layer (inner region) is not refined or is not sufficiently processed. Therefore, the inner region is a coarse grain region. The coarse grain region is a region where coarse grains in the titanium alloy contained in the mixed raw material powder are generally maintained. In this manner, mechanical milling yields a titanium alloy powder having a shell/core structure in which the surface layer is a fine grain region (shell) and the inside of the surface layer is a coarse grain region (core). Here, the fine crystal grains are crystal grains smaller than coarse crystal grains.

The treatment for refining the crystal grains in the surface layer of the powder is not limited to mechanical milling, and may be performed by other strong processing such as jet milling.

Note that fine crystal grains can be said to be crystal grains having an average crystal grain size smaller than the average crystal grain size of coarse crystal grains. The average grain size is determined by, for example, processing image data of a cross-sectional structure of a harmonic structure or a grain boundary map using a scanning electron microscope and determining the area of the target crystal grain using image analysis software. The diameter of a circle with the same area as the area is taken as the particle size, and the particle size of a predetermined number of samples can be calculated and averaged.

In the processed powder after mechanical milling, the iron powder contained in the mixed raw material powder exists on the surface of the fine crystal grain region or has penetrated into the inside of the fine grain region.

Subsequently, the processed powder obtained in step S13 is sintered by heat treatment to obtain a sintered material having a harmonic structure (step S14). Sintering is performed, for example, by plasma discharge sintering (SPS). The sintering temperature is, for example, 800°C to 1000°C. The harmonic structure is a three-dimensional network structure in which dispersed coarse crystal grain regions are surrounded by fine crystal grain regions. That is, a fine crystal grain region is formed around a coarse crystal grain region. In the fine grain region, the hardness is increased due to grain refinement.

Generally, a material having a harmonic structure (harmonic structure material) can have both high strength and high ductility (see Patent Document 1). The harmonic textured material has high strength in the fine grain region, and thus has higher strength than the homogeneous textured material having uniformly sized grains. Moreover, the harmonic structure material can maintain ductility comparable to that of the uniform structure material due to the coarse grain region.

However, in the case of titanium alloys that undergo twin deformation, even if the strength is increased simply by forming a harmonic structure, sufficient strength may not be obtained because the material is inherently susceptible to plastic deformation. That is, even if the strength of the fine grain region is increased, sufficient strength cannot be obtained from the harmonic structure as a whole due to the occurrence of twin deformation in the coarse grain region.

However, in this embodiment, iron is contained in the titanium alloy that causes twin deformation, and iron suppresses twin deformation, so that the strength of the titanium alloy that causes twin deformation can be increased. That is, in this embodiment, the iron element diffuses from the fine grain region to the coarse grain region in the harmonic structure by heat treatment to obtain the sintered material. That is, at the initial stage of sintering, iron exists only in the fine grain region, but as sintering (heat treatment) occurs, iron is thermally diffused into the coarse grain region. Therefore, in the sintered harmonically textured material, iron is present not only in the fine-grained regions, but also in the coarse-grained regions.

In the embodiment, the harmonically textured material is cold rolled at room temperature (step S15).

<2.2 Examples>
A metastable β-type Ti-25Nb-25Zr alloy was used as a base material for producing a titanium alloy powder serving as a raw material powder. In the examples, the chemical composition of the base material is [Zr: 25:22, Nb: 24.92, Fe: 0.08, C: 0.03, N: 0.01, H: 0.01, O: 0.08, Ti:bal (mass%)]. Twin deformation occurs in the metastable β-type Ti-25Nb-25Zr alloy. Titanium alloy powder was produced from the base material by a plasma rotating electrode method (step S11). The average powder particle size of the titanium alloy powder is 163.1 μm.

Subsequently, iron powder was added to the titanium alloy powder (step S12). SUJ2 powder was used as the iron powder. The average powder particle size of the iron powder is 30 μm. Here, iron powder was added to titanium alloy powder at a ratio of 20% by weight to obtain mixed raw material powder.

The mixed raw material powder was mechanically milled for 72 ks (20 hrs) using a planetary ball mill in an Ar atmosphere (step S13).

Thereafter, the mixed raw material powder was plasma discharge sintered under conditions of 1073 K and 50 MPa to produce a harmonically textured material (step S14). For sintering, the temperature was raised to 1073K and then lowered immediately without holding the temperature. That is, the retention time at 1073K is substantially 0 seconds.

Further, this harmonically textured material was cold rolled at room temperature with a rolling reduction of 20% (step S15).

FIG. 2A shows the results of observing the harmonic structure material before cold rolling by electron backscatter diffraction (EBSD). Moreover, FIG. 2B is a SEM image of the same observation site as FIG. 2A. As shown in FIGS. 2A and 2B, it can be seen that a harmonic structure, which is a three-dimensional network structure in which a coarse crystal grain region is surrounded by a fine crystal grain region, is obtained.

FIGS. 3A and 3B show the results of observing a harmonic structure material plastically deformed by cold rolling using EBSD. The observation was performed from the TD direction perpendicular to the rolling direction. In FIG. 3A, black streaks indicating twin deformation appear near the center of the coarse grain region (inner region; first region). In other words, twin deformation is dominant in the inner region (first region) of the coarse grain region.

On the other hand, twin deformation hardly occurs near the fine grain region side (outer region; second region) of the coarse grain region, and slip deformation is dominant. In the coarse grain region, twinning deformation is suppressed and slip deformation occurs in the outer region (second region) existing around the inner region (first region).

FIG. 4A shows the observation results of strain distribution by EBSD for the same observation site as FIG. 3A. In FIG. 4A, bright parts shown in white indicate large distortions, and dark parts shown in gray indicate small distortions. As is clear from FIG. 4A, in the inner region (first region) of the coarse grain region, twinning deformation in which plastic deformation occurs with relatively small deformation stress is dominant, so that strain due to rolling is concentrated. On the other hand, in the outer region (second region) of the coarse grain region, sliding deformation in which plastic deformation occurs with a relatively large deformation stress is dominant, so that the strain caused by rolling is relatively small. Note that FIG. 4B shows a SEM image of the same observed site as FIGS. 3A and 4A.

FIG. 5, FIG. 6, and FIG. 7 show the results of evaluating the distribution of iron elements in the observed portions shown in FIGS. 3A and 4A. In FIG. 5, a circle plot (EDS plot) indicating the position where the iron element content was evaluated is provided on the same SEM image as in FIG. 4B. There are a total of 28 plots from the left end to the right end of the coarse grain region in FIG. Each plot is given a number from 1 to 28. FIG. 6 shows the amount of iron element at plot positions 1 to 28 shown in FIG. The amount of iron element at the plot location was measured by energy dispersive X-ray spectrometry (EDS).

The content of iron element at each plot position from 1 to 28 is as follows.
1: 3.75% by mass
2: 1.96% by mass
3: 0.37% by mass
4: 0.18% by mass
5 to 22: 0% by mass
23: 0.09% by mass
24: 0.38% by mass
25: 1.17% by mass
26: 2.23% by mass
27: 4.28% by mass
28:12.98% by mass

The base material contains 0.08% by mass of iron, but 0.08% by mass is close to the measurement limit by EDS, and the amount of iron contained in the base material is extremely small. It is thought that iron is not necessarily present at the plotted position, and the measurement result is 0% by mass.

In FIG. 7, the plots shown in FIG. 5 are added to the image in FIG. 3A, plots in which iron elements are not present (5 to 22) are shown in gray, and plots in which iron elements are present (1 to 4, 23 to 28) are shown in gray. ) is shown in white. As shown in Figure 7, iron elements exist in the inner region (first region) where twin deformation occurs, and iron elements exist in the outer region (second region) where twin deformation does not occur. I understand that. Therefore, it can be seen that in titanium alloys, the deformation mode (twinning deformation or sliding deformation) depends on the presence or absence of iron. Furthermore, since twinning deformation is suppressed depending on the iron concentration, the deformation mode depends on the iron concentration. That is, the higher the iron concentration, the higher the degree of suppression of twinning deformation, and the lower the iron concentration, the lower the degree of suppression of twinning deformation.

FIG. 8A shows an inverse pole figure (IPF) map of the harmonic structure, and FIG. 8B shows an iron element EDS map at the same observation site as FIG. 8A. In FIG. 8B, the white portion indicates the presence of iron element. In FIG. 8B, it can be seen that iron element is present in the outer region (second region) of the fine crystal grain region and the coarse grain region in FIG. 8A. The iron element present in the outer region (second region) of the coarse grain region is diffused from the fine grain region by heat treatment.

According to the example (harmonically textured material in which iron is added to the metastable β-type Ti-25Nb-25Zr alloy), the material maintains the same level of ductility as the uniformly structured material of the metastable β-type Ti-25Nb-25Zr alloy. , the strength can be increased. Ti-Nb-Zr alloys (β-type titanium alloys) such as metastable β-type Ti-25Nb-25Zr alloys are promising as biomaterials, but conventional homogeneous texture materials are not suitable for biomedical applications due to twin deformation. It was difficult to obtain both sufficient strength and ductility as a material. On the other hand, in the examples, it is possible to achieve both strength and ductility due to the synergistic effect of suppressing twinning deformation by adding iron. That is, when iron diffuses from the surface of the titanium alloy powder, twinning deformation is suppressed in the layer where iron is diffused (iron diffusion layer), and the strength increases. On the other hand, inside the particles where iron diffusion has not occurred, twinning deformation occurs and ductility is ensured. In this way, the diffusion of iron provides characteristics similar to a harmonic structure in which the strength near the particle surface is high and the ductility is ensured inside the particle. Moreover, since the examples also have a harmonic structure, it is possible to achieve both strength and ductility with both the addition of iron and the harmonic structure.

Furthermore, in the embodiment, the transformation from twin deformation to slip deformation can be controlled by the iron concentration, making it easy to obtain a titanium alloy with desired strength and ductility.

Note that the addition of iron is not limited to the addition of iron powder. Iron may be added by forming a thin film of iron on the surface of the titanium alloy by sputtering or plating, and then diffusing iron from the thin film into the interior of the titanium alloy by heat treatment. Further, in the manufacturing method of the embodiment, it is not necessary to perform mechanical milling, and it is sufficient to simply sinter a mixed material in which iron powder is mixed with titanium alloy powder that has not been mechanically milled. In this case as well, iron diffuses into the titanium alloy powder due to sintering (heat treatment). Note that when the milling container and ball for mechanical milling are made of an iron alloy, the iron powder added may include iron powder produced by wear of the milling container and ball.

<3. Additional notes＞
The present invention is not limited to the above embodiments, and various modifications are possible.

Claims (12)

Adding a β-phase stabilizing element to a titanium alloy powder that causes twin deformation to produce a mixed raw material powder of the titanium alloy powder and the β-phase stabilizing element,
A method for producing a titanium alloy, comprising: obtaining a sintered material by sintering the mixed raw material powder. Adding a β-phase stabilizing element to a titanium alloy powder that causes twin deformation to produce a mixed raw material powder of the titanium alloy powder and the β-phase stabilizing element,
A method for producing a titanium alloy, comprising obtaining a sintered material by sintering processed powder obtained by subjecting the mixed raw material powder to strong processing. By subjecting titanium alloy powder that causes twin deformation to strong processing, titanium alloy powder having a structure in which a fine crystal grain region is formed around a coarse crystal grain region is obtained,
A method for producing a titanium alloy, comprising: sintering a processed powder containing the titanium alloy powder having a structure in which the fine crystal grain region is formed around the coarse crystal grain region and a β phase stabilizing element. The method for producing a titanium alloy according to any one of claims 1 to 3, wherein the β-phase stabilizing element is iron. The method for manufacturing a titanium alloy according to any one of claims 1 to 4, wherein the titanium alloy that causes twin deformation is a β-type titanium alloy. A titanium alloy that is a sintered material,
A titanium alloy in which particles constituting the sintered material have a first region where twin deformation occurs and a second region where sliding deformation occurs due to the inclusion of a β-phase stabilizing element. A titanium alloy which is a sintered material made by sintering a titanium alloy powder having a structure in which fine crystal grains are formed around coarse crystal grains and a β-phase stabilizing element,
The sintered material is a titanium alloy having a harmonic structure composed of the coarse crystal grains and the fine crystal grains formed around the coarse crystal grains. A titanium alloy having a harmonic structure in which a fine grain region formed around a coarse grain region constitutes a three-dimensional network,
The coarse grain region has a first region where twinning deformation occurs and a second region where sliding deformation occurs due to the inclusion of a β-phase stabilizing element.Titanium alloy. The titanium alloy according to any one of claims 6 to 8 , wherein the β-phase stabilizing element is iron. The titanium alloy according to claim 6 or 8 , wherein the second region exists around the first region. The second region includes a region in which the content of iron , which is the β-phase stabilizing element, is greater than 0.1% by mass. Titanium alloy. The content of iron, which is the β-phase stabilizing element , in the first region is 0.1 % by mass or less . titanium alloy.

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