JP5512145B2 - Shape memory alloy - Google Patents

Description

  The present invention relates to a shape memory alloy, and more particularly to a shape memory alloy having good cold workability.

The shape memory alloy exhibits a shape memory effect and a superelastic function due to the inherent thermoelastic martensitic transformation. In the former, even if the shape memory alloy is arbitrarily deformed at a temperature below the A _S point (the martensite reverse transformation start temperature), it is restored to its original shape when heated to a temperature above the A _f point (the martensite reverse transformation end temperature). The latter is a phenomenon in which the stress-induced martensite generated when stress is applied to the shape memory alloy at a temperature higher than the A _f point reversely transforms during the unloading process and the strain recovers. Since shape memory alloys have unique functions not found in conventional metals, they are used in various industrial and medical fields.

  Various alloys have been proposed as shape memory alloys. Among them, Ti-based Ti alloys represented by Ti-Ni alloys have been put to practical use because they have excellent mechanical properties such as tensile strength of 1000 MPa and elongation at break of 60%. . The alloying elements in Ti alloys are α-stabilizing elements that increase the α-β transformation point by expanding the α region of the low temperature phase to the high temperature side, and lower the α-β transformation point by expanding the β region of the high temperature phase to the low temperature side The Ti-stabilizing elements are roughly classified into three types, ie, α-type Ti alloys, α + β-type Ti alloys, and β-type Ti alloys, depending on the normal temperature structure.

  The crystal structure of α-type Ti alloy is a close-packed hexagonal structure, the crystal structure of α + β-type Ti alloy is a combination of close-packed hexagonal and body-centered cubic structure, and the crystal structure of β-type Ti alloy is body-centered cubic. Structure. The α-type Ti alloy is disadvantageous in that its cold workability is poor due to its crystal structure.

  α + β type Ti alloy is a two-phase alloy in which α and β phases coexist at room temperature by adding α stabilizing element and β stabilizing element in combination. The hot workability increases as the β phase increases. By performing rough forging of the ingot in the β region and finishing forging in the α + β region, the volume ratio of the α and β phases can be changed over a wide range, and the mechanical properties change correspondingly. Depending on the difference in hot working history, an equiaxed fine grain structure, a coarse equiaxed α phase structure or a coarse acicular α phase structure appears. When the equiaxed fine grain structure is used, the ductility is improved, but the fracture toughness is lowered. When a coarse equiaxed α phase structure or a coarse acicular α phase structure is used, the ductility is deteriorated but the fracture toughness is increased. That is, in the α + β type Ti alloy, it is difficult to simultaneously improve the ductility and fracture toughness by controlling the structure.

  Thus, the workability of the α-type Ti alloy and the α + β-type Ti alloy is not good. Compared to these alloys, it can be said that the workability of β-type Ti alloys is relatively good, but still, the cold working rate of the plate material made of Ti—Ni alloy, which is being put into practical use (cold working) When carried out, it refers to the limit degree of processing that does not cause defects such as ear cracks (cracks at the edges in the width direction). For example, if a 10 mm thick material finally becomes 9 mm thick by cold rolling, The processing rate is 10%), and the limit is about 10 to 15%. For this reason, the conventional Ti-Ni shape memory alloy plate manufacturing method is to hot-roll the ingot at 700 to 850 ° C., then to cold rolling followed by intermediate annealing and further cold rolling. A cold rolling-intermediate annealing process in which intermediate annealing is performed later is repeated a number of times, and finally the sheet is rolled to a desired thickness by cold rolling. As described above, since the annealing process and the cold rolling process are repeated many times to obtain the final product, the product cost is greatly increased.

  Therefore, in order to provide a titanium alloy having good cold workability, Patent Document 1 discloses that 5 to 40 atomic% of Nb, 10 atomic% or less of Mo, Al, Ge, Ga, and In. And a biosuperelastic titanium alloy containing Ti. Patent Document 2 discloses that 2 to 12 atomic% Mo, 14 atomic% or less Ga, or 8 atomic% or less Ge, and Ti. The biosuperelastic titanium alloy contained therein is disclosed, and the materials made of the alloys disclosed in Patent Documents 1 and 2 are both shown to have a total cold work rate of 50% or more.

Japanese Patent No. 40105080 Japanese Patent No. 3884316

  However, as disclosed in Patent Documents 1 and 2, if an element having a high melting point such as Nb having a melting point of 2470 ° C. or Mo having a melting point of 2620 ° C. is used, it is difficult to dissolve and alloying is performed. In this case, it is necessary to use a very high temperature, and a special manufacturing facility that can withstand the high temperature is required. Since a large amount of melting energy is required, the manufacturing cost increases. Moreover, evaporation of low melting point metals cannot be ignored at ultra high temperatures. Furthermore, when electron beam melting is used as the melting method, it is difficult to uniformly melt at a high temperature, and the alloy composition may change at the beginning and end of melting. It tends to occur and the alloy composition may vary depending on the location, and it is difficult to obtain a homogeneous alloy composition.

  The present invention has been made in view of such problems of the prior art, and the object thereof is a shape memory alloy excellent in cold workability containing an element having a low melting point (2000 ° C. or less). Is to provide.

  This inventor examined the preferable alloy composition based on the expression mechanism of a shape memory effect.

The shape memory effect occurs when a material deformed at a temperature below the A _S point is heated to a temperature above the A _f point, but the alloy sample in the parent phase state is converted to the M _f point (Martensite transformation end temperature). When cooled to the following temperature, it transforms from the parent phase to martensite. Martensite contains many lattice defects and is generally twin. When an external force is applied to such a sample, deformation of the sample proceeds by twin deformation. When this sample is heated to a temperature higher than the _Af point, it returns to its original state elastically as the reverse transformation progresses from the martensite phase to the parent phase. In this thermoelastic martensitic transformation, the reverse transformation occurs reversibly crystallographically, so that each twin crystal returns to the parent phase of the original orientation, and the whole sample also returns to its original shape. This is the shape memory effect. Therefore, in order to develop the shape memory effect, a martensitic transformation occurs at the deformation temperature or a stress-induced martensitic transformation is generated by applying stress during the deformation, and if it is unloaded, it is completely reversed. It is necessary that the martensitic transformation remains without transformation. This is because the strain corresponding to the twin deformation of martensite is recovered when reverse transformation occurs by heating to a temperature above the A _f point.

Therefore, in order to obtain an alloy that can easily develop the shape recovery phenomenon, it contains an element that lowers the martensite transformation start temperature and an element that effectively contributes to making the martensite thermoelastic. It is. In addition, the present inventor adopted Ti as the base metal because it is lightweight, excellent in strength, and has corrosion resistance.
(1) β-stabilizing element (decrease in martensitic transformation start temperature)
As described above, Ti alloys depending cold tissue, alpha-type Ti alloys, alpha + beta type Ti alloys, can be roughly divided into three types of beta-type Ti alloy, M _S point quenching stable beta at high temperatures ( When rapidly cooled to a temperature below the martensitic transformation start temperature), the martensitic transformation occurs. The M _S point depends on the amount of β-stabilizing element added to Ti, and the M _S point decreases as the amount of β-stabilizing element increases.

Therefore, inclusion of a β-stabilizing element is effective for lowering the M _S point.

However, if the amount of β-stabilizing element added becomes too large, the M _S point will drop to below room temperature, and even if quenched from a high temperature, the β phase will remain in a metastable state at room temperature without martensitic transformation. Become. When this residual β phase is heated, the α phase is precipitated and an ω phase may be generated as an intermediate stage. When this ω phase is generated, the alloy becomes hard, but at the same time it becomes brittle, so it is called ω brittleness. If the β-stabilizing element is too much, the ω phase is likely to be generated.

Therefore, in order to lower the M _S point, it is preferable to contain 1 mol% or more of the β stabilizing element, but in order to avoid ω brittleness, the β stabilizing element is preferably 9 mol% or less.

  As the β stabilizing element, Fe and / or Cr are particularly preferable.

  In particular, when the content of Fe and / or Cr exceeds 9 mol%, the β phase is excessively stabilized and martensitic transformation does not occur, or even if martensitic transformation occurs, it is easily reverse transformed.

Therefore, it is preferable to contain 1 to 9 mol% of any one of the first elements selected from Fe and Cr, or 1 to 9 mol% of the total of the two elements.
(2) Suppression of ω brittleness and thermoelasticization of martensite In order to suppress the appearance of ω phase during the aging treatment of β alloy, it may contain an element considered to contribute to α stabilization such as Sn. preferable. In addition, although Sn is such an element, it can make the martensite thermoelastic by destabilizing the β phase without substantially increasing the martensite transformation temperature. In addition, Au suppresses the appearance of the ω phase and contributes to the thermal elasticity of martensite.

  However, when these elements are excessive, the shape memory effect is lowered.

  Therefore, it is preferable to contain 1 mol% or more of any one or two of the second elements (Sn, Au) contributing to suppression of ω brittleness and thermoelasticity of martensite. In order to make the effect good, the total content of any one or two of the second elements is preferably 9 mol% or less.

In particular, Sn has the effects of suppressing the generation of the ω phase, strengthening the material, and promoting the generation of martensite. In addition to the same effects as Sn, Au has the effect of suppressing atomic diffusion and suppressing the generation of the ω phase. However, if Au exceeds 9 mol%, a large amount of embrittled phase (Ti ₃ Au) is generated, and mechanical properties are significantly deteriorated.
(3) Cold workability The present inventor has 1 to 9 mol% of the first element (Fe and / or Cr) contributing to stabilization of β-phase of Ti as an alloy that can easily develop a shape recovery phenomenon. An alloy containing 1 to 9 mol% of a second element (Sn and / or Au) contributing to suppression of ω brittleness and thermoelasticity of martensite, with the balance being Ti and inevitable impurities (total of 100 mol%) However, the melting points of Ti, Fe, Cr, Sn, and Au are 1660 ° C., 1540 ° C., 1860 ° C., 232 ° C., and 1064 ° C., respectively, and do not exceed 2000 ° C. In addition, since a β phase having many slip systems and good cold workability is generated, and generation of an ω phase that causes embrittlement of the material is suppressed, good cold workability can be exhibited.

  ADVANTAGE OF THE INVENTION According to this invention, the shape memory alloy excellent in the cold work property containing the low melting point element can be provided.

It is a figure explaining the bending method of the board | plate material for evaluating the shape recovery characteristic of a board | plate material. It is a figure explaining the measurement position of a curvature radius.

  Examples of the present invention will be described below. However, the present invention is not limited to the following examples, and can be appropriately changed or modified without departing from the technical scope of the present invention.

(1) Production of plate material A Ti-based alloy having the composition (mol%) shown in Table 1 below was melted using a non-consumable tungsten electrode type argon arc melting furnace. The obtained ingot was homogenized at 1000 ° C. for 2 hours. The ingot after the homogenization treatment was subjected to evaluation of cold workability described later.

Further, the ingot after the homogenization treatment was subjected to cold rolling subsequent to hot rolling, and further subjected to heat treatment at 900 ° C. for 5 minutes in a vacuum atmosphere (pressure of 10 ⁻⁵ Pa). The thickness of the plate material subjected to cold rolling was 6 mm, and finally a plate material having a thickness of 0.3 mm and a width of 10 mm was obtained. Although this processing rate is a high processing rate of 95%, the majority of the samples of the present invention (those whose processing rate is 95% or more in Table 1 and Tables 2 to 5 described later) This high working rate could be achieved by rolling. However, for some samples (with a processing rate of 40 to 50% in Table 1 and Tables 2 to 5 described later), achieving this high processing rate by one cold rolling is accompanied by rolling. Difficult for work hardening. Therefore, for some of the samples, after the combination treatment of cold rolling and heat treatment at 900 ° C. for 5 minutes is performed 1 to 4 times, the final thickness and width of the sample are finally reduced by cold rolling. A board was obtained.

  In addition, if it is Ti50mol% -Ni50mol% alloy of the conventional composition, if the combination process of the said cold rolling and heat processing is not repeated at least 10 times or more, it will achieve a processing rate of 95%. Although it seems to be difficult, the Ti-based alloy having the composition of the present invention is excellent in cold workability, so that the repetition of cold rolling and heat treatment could be suppressed to 4 times or less.

The shape memory Ti-based alloy plate material (thickness 0.3 mm, width 10 mm, length 400 mm) thus obtained is sandwiched between quartz plates in a vacuum atmosphere (pressure of 10 ⁻⁵ Pa), and is heated at 1000 ° C. for 30 minutes. After solution treatment, it was quenched in water.
(2) Evaluation of cold workability The above ingot is ground to form a plate having a thickness of 1 cm and a width of 2 cm, and cold rolling is performed at a processing rate of 2 to 3%. Cold rolling was performed until cracking occurred. Table 1 shows the processing rate (((t ₁ ) obtained from the thickness t ₂ at the time of cold rolling immediately before cracking in the plate material and the thickness t ₁ (1 cm) of the plate material subjected to cold rolling. −t ₂ ) / t ₁ ) × 100 (%)).

In Table 1, the sample number 1 of Ti92 mol% -Fe5 mol% -Sn3 mol% and the sample number 4 of Ti91 mol% -Cr5 mol% -Au4 mol% have a processing rate of 50% or more and are excellent. It can be seen that it has cold workability.
(3) Evaluation of shape recovery characteristics of plate material In order to evaluate the shape recovery characteristics of the plate material, as shown in FIG. 1, the plate material (thickness 0.3 mm, width 10 mm) 1 after quenching was set at room temperature (20 to 25 ° C.). ) Was held for 30 seconds in a state bent at 180 ° C. along a stainless steel round bar 2 having a diameter of 7 mm. Then, the board | plate material 1 was removed from the stainless steel round bar 2, and the curvature radius R1 of the board | plate material 1 was measured as shown to Fig.2 (a).

  Thereafter, the plate material 1 is inserted into an electric furnace, heated to 400 ° C. or higher while heating at a heating rate of 10 ° C./min or higher, and then air-cooled to room temperature, as shown in FIG. A radius of curvature R2 of 1 was measured.

  Then, curvature ρ1 (1 / R1) and ρ2 (1 / R2) are obtained from the curvature radii R1 and R2, and [(ρ1−ρ2) / ρ1] × 100 (%) is the shape recovery rate (maximum surface recovery strain). As shown in Table 1.

  In Table 1, all samples, namely sample number 1 of Ti 92 mol% -Fe 5 mol% -Sn 3 mol%, sample number 2 of Ti 93 mol% -Fe 3 mol% -Au 4 mol%, and Ti 90 mol% -Cr 7 mol Sample No. 3 of% -Sn 3 mol% and Sample No. 4 of Ti 91 mol% -Cr 5 mol% -Au 4 mol% all have a shape recovery rate of 50% or more and have good shape recovery characteristics at high temperatures. It was confirmed that there was.

  A Ti-based alloy having the composition (mol%) shown in Tables 2 to 5 below is obtained by the same method as in Example 1 to obtain an ingot and a plate material having a thickness of 0.3 mm and a width of 10 mm. (Thickness 0.3 mm, width 10 mm, length 400 mm) was quenched in water by the same method as in Example 1.

  The ingot was evaluated for cold workability by the same method as in Example 1, and the plate material after quenching was evaluated for shape recovery characteristics by the same method as in Example 1.

  Tables 2 to 5 show processing rates for evaluating cold workability and shape recovery rates for evaluating shape recovery characteristics. The meanings of the processing rate and the shape recovery rate are the same as those in the first embodiment.

  In Table 2 and Table 3, “impossible to experiment” means that the cracking during cold rolling was remarkable and the experiment could not be continued.

  Table 2 shows the processing rate and the shape recovery rate of the Ti—Cr—Sn alloy. In Table 2, it can be seen that the working examples of Sample Nos. 5 to 13 have a working rate of 46% or more and have excellent cold workability. A higher processing rate is preferable in that practical applications are expanded. On the other hand, with respect to the shape recovery rate, even if the shape recovery rate is not necessarily high, such as Sample No. 13, Sample No. 28 in Table 3 to be described later, and Sample No. 35 in Table 4 to be described later, the shape recovery rate is not limited. There are matching practical uses.

  However, all of the Ti—Cr—Sn alloys of the comparative examples of sample numbers 14 to 21 do not have shape recovery characteristics, and the Ti—Cr—Sn alloys of sample numbers 16, 17 and 19 are substantially cold. Processability is so bad that it cannot be processed.

  Table 3 shows the processing rate and shape recovery rate of the Ti—Cr—Au alloy. In Table 3, it can be seen that the working examples of Sample Nos. 22 to 28 have a working rate of 50% or more and have excellent cold workability.

  However, the Ti—Cr—Au alloys of Comparative Examples Nos. 29 to 32 do not have any shape recovery characteristics, and the Ti—Cr—Au alloys of Nos. 29, 30 and 32 are substantially cold. Processability is so bad that it cannot be processed.

  Table 4 shows the processing rate and shape recovery rate of the Ti—Fe—Sn alloy. In Table 4, it can be seen that the examples of the present invention with sample numbers 33 to 35 have a processing rate of 95% or more and have extremely excellent cold workability.

  However, the Ti—Fe—Sn alloy of the comparative examples of sample numbers 37 to 40 has a working rate of 40% or less and poor cold workability, and the Ti—Fe—Sn of the comparative examples of sample numbers 41 and 42. The system alloy is so poor in workability that it cannot substantially be cold worked. Further, the Ti—Fe—Sn alloys of the comparative examples of sample numbers 36, 38, 40, 41 and 42 do not have shape recovery characteristics.

  Table 5 shows the processing rate and shape recovery rate of the Ti—Fe—Au alloy. In Table 5, it can be seen that the examples of the present invention with sample numbers 43 to 46 have a processing rate of 95% or more and have extremely excellent cold workability and extremely excellent shape recovery characteristics.

  However, the Ti—Fe—Au alloy of the comparative example of sample number 47 does not have shape recovery characteristics.

The experimental results of Example 2 are summarized below.
(1) As shown in Table 2, 5.0 to 8.0 mol% of the first element (Cr) contributing to Ti β-stabilization, ω brittleness suppression, and martensite thermoelasticity The total amount of the contributing second element (Sn) is 8.0 to 11.0 mol%, the balance is made of Ti and inevitable impurities, and 33% ≦ (content of second element / first The Ti-based alloy having a composition ratio of (element content) ≦ 73% has excellent cold workability and good shape recovery characteristics.
(2) As shown in Table 3, 3.0 to 8.0 mol% of the first element (Cr) contributing to stabilization of Ti β phase, suppression of ω brittleness, and thermal elasticity of martensite The total amount of the contributing second element (Au) is 5.0 to 14.0 mol%, the balance is made of Ti and inevitable impurities, and 25% ≦ (content of second element / first The Ti-based alloy having a composition ratio of ≦ 80% has excellent cold workability and good shape recovery characteristics.
(3) As shown in Table 4, 4.0 to 6.0 mol% of the first element (Fe) contributing to Ti β-phase stabilization, ω brittleness suppression, and martensite thermoelasticity The total amount of the second element (Sn) that contributes is 7.0 to 12.0 mol%, the balance is made of Ti and inevitable impurities, and 60% ≦ (content of second element / first The Ti-based alloy having a composition ratio of ≦ 100% has extremely good cold workability and good shape recovery characteristics.
(4) As shown in Table 5, 3.0 to 7.0 mol% of the first element (Fe) contributing to stabilization of Ti β phase, suppression of ω brittleness, and thermal elasticity of martensite The total amount of the contributing second element (Au) is 7.0 to 11.0 mol%, the balance is made of Ti and inevitable impurities, and 57% ≦ (content of second element / first The Ti-based alloy having a composition ratio of (element content) ≦ 133% has extremely excellent cold workability and extremely excellent shape recovery characteristics.

  INDUSTRIAL APPLICABILITY The present invention can be widely used for orthodontic wires, microwave oven dampers, air conditioner wind direction control members, rice cooker steam pressure regulating valves, architectural vents, mobile phone antennas, bra wires, glasses frames, and the like.

1 Sheet material 2 Stainless steel round bar

Claims (5)

Containing 3.0 to 8.0 mol% of Cr or Fe as the first element contributing to stabilization of the β phase of Ti,
It contains 2.0 to 6.0 mol% of Sn or Au as the second element contributing to the suppression of ω brittleness and the thermal elasticity of martensite,
A total of 5.0 mol% to 14.0 mol% of the first element and the second element,
A shape memory alloy which is a thermoelastic martensitic Ti alloy obtained by cold working and quenching a Ti alloy consisting of Ti and inevitable impurities. 5.0 to 8.0 mole percent Cr;
2.0 to 4.0 mol% Sn,
A total of 8.0 to 11.0 mol% of Cr and Sn,
2. The shape memory alloy according to claim 1, wherein the balance is made of Ti and inevitable impurities and has a composition ratio of 33% ≦ (Sn content / Cr content) × 100 (%) ≦ 73%. . 3.0 to 8.0 mol% Cr,
2.0 to 6.0 mol% Au,
Containing 5.0 to 14.0 mol% of Cr and Au in total,
2. The shape memory alloy according to claim 1, wherein the balance is made of Ti and inevitable impurities and has a composition ratio of 25% ≦ (Au content / Cr content) × 100 (%) ≦ 80%. . 4.0 to 6.0 mol% Fe;
3.0 to 6.0 mol% Sn,
Containing 7.0 to 12.0 mol% of Fe and Sn in total,
The shape memory alloy according to claim 1, wherein the balance is made of Ti and inevitable impurities and has a composition ratio of 60% ≦ (Sn content / Fe content) × 100 (%) ≦ 100%. . 3.0 to 7.0 mol% Fe;
4.0 mol% Au,
Containing 7.0 to 11.0 mol% of Fe and Au in total,
2. The shape memory alloy according to claim 1, wherein the balance is made of Ti and inevitable impurities and has a composition ratio of 57% ≦ (Au content / Fe content) × 100 (%) ≦ 133%. .

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