CN100561175C - Method for Measuring Phase Transformation Properties of Shape Memory Alloys with Spherical Indenter - Google Patents

Abstract

The invention discloses a method for measuring the phase transition characteristics of shape memory alloys (SMAs) with a spherical indenter. It uses a spherical indenter to press into the surface of the shape memory alloy material to cause a stress-induced phase transition, and simultaneously detects the load and displacement signals during the loading and unloading process through the sensor to obtain the load F-displacement _ht curve, and then according to the analysis The load-displacement curve obtained from the test is converted into the corresponding nominal stress σ _m - nominal strain ε _m curve, and then the phase transformation stress and elastic modulus of SMAs are obtained. The test method is simple and easy, and can basically realize the non-destructive measurement of materials. It is not only widely applicable to the measurement of the phase transition characteristics of various hyperelastic and shape memory SMAs, but also especially suitable for SMAs films with a thickness as low as several microns or typical structural dimensions. The test of the phase change characteristics of SMAs micro-devices at the micron level has accurate measurement values and high precision, which can provide a reliable test basis for the phase change characteristics of SMAs in the application of micro-electromechanical systems.

Description

Method for Measuring Phase Transformation Properties of Shape Memory Alloys with Spherical Indenter

technical field

The invention belongs to the technical field of mechanical performance testing of shape memory alloy materials.

Background technique

Shape memory alloys (Shape Memory Alloys, SMAs for short) can exhibit unique shape memory properties or superelastic properties at different temperatures. Combined with its outstanding advantages such as high power density (output power per unit volume), large output force and output displacement, SMAs have become ideal materials for the development of MEMS drivers and sensors. Among various SMAs, nickel-titanium alloy (NiTi) is the most practical shape memory alloy, and its work density is as high as 2.5×10 ⁷ J/m ³ , which is two orders of magnitude higher than other types of micro-actuation materials. The driver developed with high power density materials has the same output power but smaller structural size, or the same structural size but higher output power.

The research on nickel-titanium alloy (NiTi) micro-actuator has a history of more than 20 years, and various devices such as micro-valve, micro-switch, micro-machine arm and micro-pump have been developed. Since the above-mentioned NiTi micro-actuator components mainly use martensitic phase transformation to complete the displacement or stress driving function of the micro-drive, the elastic modulus and phase transformation stress of the NiTi alloy become the main parameters that determine the performance of the micro-drive components. Therefore, when using NiTi alloy as a micro-actuator element, the elastic modulus and phase transformation stress of the element must first be clarified, so as to provide a reference for the design and use of NiTi alloy micro-actuator elements. However, in these micro-actuators, the nickel-titanium alloy components are mostly nano-polycrystalline films with a thickness of only a few microns or micro-components with a typical structural size on the order of microns, and the traditional tensile test method cannot be used to test their elastic modulus. Therefore, there is an urgent need to develop a test method that can test the phase transition properties of nickel-titanium alloys and other various SMAs thin films and SMAs micro-devices.

As shown in the schematic diagram of the tensile stress σ-strain ε curve of the shape memory alloy in the uniaxial tensile test in Figure 1: different from traditional materials, shape memory alloys such as nickel-titanium alloys undergo uniaxial tension or compression It will successively experience two stress platforms of martensitic transformation and martensitic yield, corresponding to martensitic transformation stress σ _t and martensitic yield stress σ _n respectively. Therefore, although the nanoindentation method based on the Berkovich indenter has been widely used to measure the hardness and elastic modulus of elastic or elastoplastic materials, however, due to the coupling of phase change deformation and plastic deformation during the deformation of SMAs, it cannot be used with conventional Berkovich indentation technology to measure the phase transition characteristics of SMAs: as shown in Figure 2, since the shape of the Berkovich indenter is a triangular pyramid with a small radius of curvature at the tip, under a small load condition, the central area of the indentation area will be Induce martensitic yield deformation, and at the same time induce martensitic transformation deformation in the edge region. Therefore, the measured hardness will be a comprehensive reflection of the martensitic transformation stress σ _t and the martensitic yield stress σ _n . Since the size of the martensitic yield area in the contact zone cannot be determined, further analysis cannot be done. The transformation characteristics such as the martensitic transformation stress of SMAs are obtained.

Contents of the invention

The purpose of the present invention is to provide a method for measuring the phase transition characteristics of shape memory alloys with a spherical indenter, which can measure the phase transition characteristics of various shape memory alloys, and is especially suitable for shape memory alloy thin films and shape memory alloy microstructures. Testing of phase transition characteristics of devices.

The technical scheme that the present invention adopts for solving the object of the invention is: a kind of spherical indenter measures the method for the phase change characteristic of shape memory alloy, and its steps are:

a. Using indentation equipment, a spherical indenter is used to press radially into the surface of the shape memory alloy material to cause a stress-induced phase transition, and the sensor simultaneously detects the load F and displacement h _t signals in the loading and unloading process, and obtains Load F-displacement h _t curve of shape memory alloy;

b. Using different peak loads, repeat step a to obtain the load F-displacement _ht curves of the shape memory alloy under different peak loads. The initial unloading slope of these load F-displacement _ht curves is the contact pair under the corresponding load The contact stiffness S of , so as to fit the curve of the contact stiffness S changing with the load F;

c. From the load F-displacement _ht curve of the shape memory alloy measured in step b, select any load F-displacement _ht curve that does not overlap between the loading section and the unloading section, and calculate the loading and unloading corresponding to the selected curve The average pressure and representative strain in the indentation contact area during the process are defined as the nominal stress σ _m and the nominal strain ε _m , respectively. The calculation process is: first calculate the contact radius a _c , $a_c=\sqrt{2(h_t-0.75f/S)R-{(h_t-0.75f/S)}^2};$ Then calculate the nominal stress σ _m , σ _m =F/πa _c ² and the nominal strain ε _m , ε _m =0.75a _c /πR; in the formula: R is the radius of curvature of the spherical indenter, and S is the fitting curve of step b The contact stiffness corresponding to the load F in

d. According to the nominal stress _σm and nominal strain _εm calculated in step c, the nominal stress _σm -nominal strain _εm curve of the shape memory alloy to be tested is obtained; for the shape memory alloy to be tested in the hyperelastic state, the nominal The stress σ f at the beginning of the phase transition stress plateau in the loading section of the stress σ _m - nominal strain ε _m curve corresponds to the forward phase transition stress of the shape memory alloy to be tested, and the stress σ _r at the end of the recovery stress plateau _in the unloading section corresponds to the Measure the reverse phase transition stress of the shape memory alloy; for the shape memory alloy to be tested in the shape memory state, the stress σ _f at the beginning of the phase transition stress plateau in the loading section of the nominal stress σ _m - nominal strain ε _m curve corresponds to the shape to be measured Phase transformation stress of memory alloy.

Compared with prior art, the beneficial effect of the present invention is:

1. Due to the small curvature radius of the tip of the conventional Berkovich cone-shaped indenter, it is impossible to separate the coupling of phase transformation deformation and plastic deformation during the indentation process, that is, it is impossible to detect the corresponding load F-displacement h of only phase transformation deformation _t curve, so the phase transformation characteristics of shape memory alloys cannot be tested. Different from the conventional Berkovich cone-shaped indenter, the spherical indenter of the present invention has a large radius of curvature at the tip of the indenter, and it can induce the deformation of the shape memory alloy only in the contact area under a small load that can be applied and measured. The martensitic transformation deformation of the shape memory alloy is further induced under a larger load. Therefore, among the load and displacement data in the loading and unloading process detected by the sensor, there are loads and corresponding displacements that only occur in the martensitic transformation deformation process, so that the load F-displacement h _t obtained from the test can be analyzed according to the analysis The curve is converted into the corresponding nominal stress σ _m - nominal strain ε _m curve, and then the phase transformation stress of the shape memory alloy is obtained. Therefore, the measurement method of the invention can be widely applied to the measurement of the phase transition characteristics of various shape memory alloys. Especially for shape memory alloy films (thickness as low as several microns) or shape memory alloy microdevices (typical structural size in micron order) that cannot be tested by stretching method, it is especially suitable to adopt the method of the present invention, which can reach the existing The test effect and accuracy that the test method cannot achieve, and the reproducibility of the data are good, which can provide a reliable test basis for the application of SMAs in micro-electromechanical systems.

2. The operation is simple and convenient, and it can be realized on multiple indentation equipment. It is only necessary to install a spherical indenter on the indentation equipment, apply radial pressure on the SMAs to be tested, and simultaneously detect the corresponding load and displacement signals during the loading and unloading process, and then analyze and calculate the data.

3. The test area is small, and the damage to the material to be tested is small. Only a spherical depression with a large radius of curvature is formed in the test area to be tested. Compared with the uniform deformation of the entire material in the tensile test and the concentrated pointed cone-shaped depression produced by the Berkovich indenter, it can avoid the damage caused by the test to the material, and can basically Realize non-destructive measurement of materials.

After the phase transition stress of the shape memory alloy to be tested is obtained in the above step d, the elastic modulus E _m of the shape memory alloy can be further calculated, ${\mathrm{E.}}_m=\frac{{\mathrm{E.}}_i(1-v_m^2)S}{{2a}_c{\mathrm{E.}}_i-(1-v_i^2)S};$ In the formula: S is the contact stiffness corresponding to the load F in the fitting curve of step b, v _m is the Poisson's ratio of the shape memory alloy to be tested, and v _i and E _i are the Poisson's ratio and elastic modulus of the spherical indenter. In this way, it can provide more test basis for phase change characteristic performance for the application of SMAs in MEMS.

The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

Description of drawings

Fig. 1 is a schematic diagram of a tensile stress σ-strain ε curve when a shape memory alloy is subjected to a uniaxial tensile test.

Fig. 2 is a schematic diagram of the contact area when the Berkovich indenter presses into the shape memory alloy.

Fig. 3 is a schematic diagram of a spherical indenter pressing into a shape memory alloy material when the method of the present invention is tested.

Fig. 4 is to use the method of the present invention to test two shape-memory nickel-titanium alloy polycrystalline sheet samples whose length and width are 10mm and thickness 0.5mm, wherein one sample is in the superelastic state SE, and the other sample is in the shape-memory state SME , so that the load F-displacement h _t curves of the shape memory alloy sample SE in the superelastic state and the shape memory alloy sample SME in the shape memory state of the same shape are respectively obtained.

Fig. 5 is the curve of the contact stiffness S of the shape memory sample SE in the superelastic state obtained by testing and analyzing the two shape memory alloy samples in Fig. 4 and the sample SME in the state of the shape memory alloy according to the load F. The contact stiffness S varies with the load F curve.

The solid line in Fig. 6 is its nominal stress σ _m -nominal strain ε _m curve obtained by the present invention method to the shape memory sample SE test analysis of hyperelastic state of Fig. 4; Dotted line is that adopts existing stretching method to measure Uniaxial tensile stress σ-strain ε curve of the same sample.

The solid line in Fig. 7 is the shape memory alloy sample SME of the shape memory state of Fig. 4 according to the present invention, and its nominal stress _σm -nominal strain _εm curve obtained by test and analysis; the dotted line is that adopts existing stretching method to measure The uniaxial tensile stress σ-strain ε curve of the same sample SME is obtained.

Fig. 8 is the curve that the contact stiffness S of the sample SE of the superelastic state obtained and the sample SME of the shape memory state are changed with the contact radius a _c by testing and analyzing the two samples of Fig. 4 by the method of the present invention, the slopes of the two curves is 2E _eff (E _eff is the complex modulus of elasticity of the contact between the spherical indenter and the SMAs).

Detailed ways

Example

Two nickel-titanium alloy polycrystalline sheets with a length and width of 10 mm and a thickness of 0.5 mm were used as test samples, one of which was a sample SE in a superelastic state, and the other was a sample SME in a shape memory state. Taking the test of these two samples as an example, the specific operation steps of the method of the present invention are illustrated. The steps are:

a. Using indentation equipment, a spherical indenter is used to radially press the surface of the shape memory alloy material to cause a stress-induced phase transition, and the load F and displacement h _t signals during the loading and unloading process are continuously detected by the sensor at the same time, The load F-displacement h _t curve of the shape memory alloy is obtained. Figure 3 is a schematic diagram of the spherical indenter pressing into the shape memory alloy material during the test, where R is the radius of curvature of the spherical indenter, which can be characterized by a scanning electron microscope. In this example, the radius of curvature of the indenter R=20 μm; a _c is the contact Radius, h _t is the displacement of the sample after being pressed, h _c is the vertical distance between the contact edge of the indenter sample and the deepest part of the sample depression.

Figure 4 shows the load F-displacement h _t curves of the sample SE in the hyperelastic state and the sample SME in the shape memory state obtained by testing two samples with a peak load of 100 mN. It can be seen that when the radius of curvature of the indenter is 20 μm and the test peak load is 100 mN, the load F-displacement h _t curve of the hyperelastic sample SE is basically completely restored after unloading, and shows a large hysteresis loop before recovery, indicating that the hyperelastic sample SE The alloy of the elastic sample SE has undergone a stress-induced reversible phase transformation before undergoing plastic deformation. The SME alloy in the shape memory state produced a large residual deformation during the test, indicating that its phase transformation deformation basically did not recover after unloading, and it has the characteristics of shape memory.

b. Using different peak loads, repeat step a to obtain the load F-displacement _ht curves of the shape memory alloy under different peak loads. The initial unloading slope of these load F-displacement _ht curves is the contact pair under the corresponding load The contact stiffness S, so as to fit the curve of the contact stiffness S changing with the load F. Figure 5 is the test and fitting curve of the contact stiffness S of the sample in this example changing with the load F. The contact stiffness values under different loads can be easily obtained by using this fitting curve.

c. From the load F-displacement h _t curves _of the shape memory alloy measured in step b, select any load F-displacement h t curve in which the loading section and the unloading section do not overlap. In this example, the load F-displacement _ht curves of the shape memory alloy sample SE in the superelastic state and the sample SME in the shape memory state when the peak load is 100mN are selected in the actual measurement, as shown in Figure 4.

Under the selected peak load of 100mN, the average pressure and representative strain of the indentation contact area during loading and unloading are calculated, which are defined as nominal stress σ _m and nominal strain ε _m , respectively. The calculation process is: first calculate the contact radius a _c , $a_c=\sqrt{2(h_t-0.75f/S)R-{(h_t-0.75f/S)}^2};$ Then calculate the nominal stress σ _m , σ _m =F/πa _c ² and the nominal strain ε _m , ε _m =0.75a _c /πR; in the formula: R is the radius of curvature of the spherical indenter, and S is the fitting curve of step b The contact stiffness corresponding to the load F in .

The derivation process of the contact radius a _c formula is: According to the Oliver-Pharr theory, $a_c=\sqrt{{2h}_{c}R-h_{\mathrm{<figure-callout\; id="2"\; label="c"\; filenames="C2007100500400011H1.png"\; state="\{\{state\}\}">c</figure-callout>}}^2},$ For a spherical indenter, h _c =h _t -0.75F/S, so $a_c=\sqrt{2(h_t-0.75f/S)R-{(h_t-0.75f/S)}^2}.$

d. According to the nominal stress _σm and nominal strain _εm calculated in step c, the nominal stress _σm -nominal strain _εm curve of the shape memory alloy to be tested is obtained; for the shape memory alloy to be tested in the hyperelastic state, the nominal The stress σ f at the beginning of the phase transition stress plateau in the loading section of the stress σ _m - nominal strain ε _m curve corresponds to the forward phase transition stress of the shape memory alloy to be tested, and the stress σ _r at the end of the recovery stress plateau _in the unloading section corresponds to the Measure the reverse phase transition stress of the shape memory alloy; for the shape memory alloy to be tested in the shape memory state, the stress σ _f at the beginning of the phase transition stress plateau in the loading section of the nominal stress σ _m - nominal strain ε _m curve corresponds to the shape to be measured Phase transformation stress of memory alloy.

The solid line in Fig. 6 is the nominal stress σ _m -nominal strain ε _m curve of the measured hyperelastic state sample SE; in order to facilitate the comparison between the method of the present invention and the tensile test method, the tensile test is adopted for the sample SE at the same time The method test obtained its uniaxial tensile stress σ-strain ε curve, which is the dotted line in Figure 6. The solid line in Figure 7 is the measured nominal stress σ _m - nominal strain ε _m curve of the sample SME in the shape memory state; also for the convenience of comparison, the unidirectional Tensile stress σ-strain ε curve, namely the dotted line in Fig. 7.

It can be seen from Fig. 6 and Fig. 7 that the nominal stress σ _m -nominal strain ε _m curve obtained by the method of the present invention is very consistent with the shape of the stress σ-strain ε curve obtained by the tensile test. During the loading process, due to the stress-induced martensitic transformation, stress plateaus appeared in the two stress-strain curves of NiTi shape memory alloy in Fig. 6 and Fig. 7 respectively. During the unloading process: due to the superelastic properties of the superelastic (SE) NiTi alloy in Figure 6, its phase transformation has been well recovered, while the shape memory (SME) NiTi alloy in Figure 7 has both Only minor elastic recovery.

Similar to the tensile stress σ-strain ε curve, the platforms of the nominal stress σ _m and nominal strain ε _m curves in Fig. 6 and Fig. 7 also reflect the phase change flow of nickel-titanium alloy in the indentation process of the present invention: according to Tabor According to the theory of phase change flow, the nominal stress σ _m is about three times of the phase change stress in unidirectional compression. In addition, due to the anisotropy of nickel-titanium alloy in tension and compression, the phase transformation stress in compression is about 1.5 times that in tension. Therefore, the nominal stress σ _m of phase transition flow of NiTi alloy in compression is about 4.5 times of its uniaxial tensile phase transition stress σ.

In Fig. 6, the nominal stress σ _m - nominal strain ε _m curve of the sample SE in the hyperelastic state, the stress σ _f at the beginning of the phase transition stress plateau in the loading section is 1780 MPa, which corresponds to the positive direction of the phase change flow when the sample SE in the hyperelastic state is compressed Phase transition stress, the calculated corresponding tensile phase transition stress σ is 395MPa. This value is very close to the phase transformation stress σ value of 370MPa directly measured (characterized) by the uniaxial tensile test in Fig. 6 . The stress at the end of the recovery stress plateau in the unloading section of the nominal stress σ _m -nominal strain ε _m curve in Figure 6 is σ _r = 666MPa, which corresponds to the reverse phase transition stress of the sample SE in the hyperelastic state when it rebounds, and the corresponding The tensile phase transition stress σ is 148MPa, which is also in good agreement with the direct measurement value of 137MPa in the tensile test in Figure 6.

The nominal stress σ _m -nominal strain ε _m curve of the sample SE in the shape memory state in Fig. 7, the stress σ _f at the beginning of the phase transition stress plateau in the loading section is 750 MPa, which corresponds to the phase change flow when the sample SE in the shape memory state is compressed The corresponding tensile phase transition stress σ is calculated to be 167MPa. This value is very close to the phase transformation stress σ value of 162 MPa directly measured (characterized) by the uniaxial tensile test in Fig. 7 .

It should be noted that: in the shape memory alloy load F-displacement h _t curve measured in step b, when the peak load is less than the minimum load for martensitic transformation, the shape memory alloy will only produce elastic deformation without For martensitic transformation, the loading section and unloading section of the load F-displacement h _t curve overlap completely. Obviously, the load F-displacement h _t curves obtained from these tests without phase transition are selected in steps c and d for analysis and calculation, and the phase transition stress cannot be obtained.

However, for any load F-displacement h _t curve that does not overlap between the loading section and the unloading section, the peak load of the load is greater than the minimum load for martensitic transformation, and martensitic transformation occurs during the test. For a shape memory alloy in a hyperelastic state, the nominal stress σ _m -nominal strain ε _m curve obtained from the d-step analysis must have a phase transition stress platform in the loading section and a recovery stress platform in the unloading section, and the final corresponding positive phase transition stress is the stress at the beginning of the phase change stress platform in the loading section, and the final corresponding reverse phase transition stress is the stress at the end of the recovery stress platform in the unloading section. For shape memory alloys in the shape memory state, the nominal stress σ _m -nominal strain ε _m curve obtained from the d-step analysis must have a phase transition stress plateau in the loading section, and the final corresponding phase transition stress is the phase transition stress plateau in the loading section initial stress. Therefore, choosing any load F-displacement h _t curve that does not overlap between the loading section and the unloading section, the corresponding phase change stress can be obtained through steps c and d, and which load F-displacement h _{t curve} is selected specifically Curves are irrelevant. Choose different curves in the load F-displacement h _t curves that do not overlap between the loading section and the unloading section, but the length of the stress plateau is different in the nominal stress σ _m -nominal strain ε _m curve of step d (nominal with a large peak load Stress σ _m - nominal strain ε _m curve, the stress plateau is longer; conversely, the stress plateau is shorter).

After obtaining the phase transition stress of the shape memory alloy to be tested in step d of this example, the elastic modulus E _m of the shape memory alloy is further calculated, ${\mathrm{E.}}_m=\frac{{\mathrm{E.}}_i(1-v_m^2)S}{2a_c{\mathrm{E.}}_i-(1-v_i^2)S},$ In the formula: S is the contact stiffness corresponding to the load F in the fitting curve of step b, v _m is the Poisson's ratio of the shape memory alloy to be tested, which can usually be assumed to be 0.3; v _i and E _i are the Poisson of the spherical indenter Ratio and modulus of elasticity, the values of which are v _t = 0.07 and E _i = 1141 GPa.

Since the complex elastic modulus E _eff of the contact between the spherical indenter and the SMAs has the following relationship with the contact stiffness S and the contact radius a _c : E _eff =0.5S/a _c , where: $1/{\mathrm{E.}}_{\mathrm{eff}}=(1-v_m^2)/{\mathrm{E.}}_m+(1-v_i^2)/{\mathrm{E.}}_i,$ So there are: ${\mathrm{E.}}_m=\frac{{\mathrm{E.}}_i(1-v_m^2)S}{2a_c{\mathrm{E.}}_i-(1-v_i^2)S}.$

According to the above formula and the contact radius-stiffness curve in Figure 8, the elastic modulus of the superelastic (SE) and shape memory (SME) NiTi alloys in this example are measured to be 42GPa and 48GPa, respectively, which are basically the same as the 41GPa and 42GPa obtained by the tensile test. unanimous.

In summary, the load F-displacement h _t curve of shape memory materials can be easily measured by using the spherical indenter indentation method, and can be converted into corresponding nominal stress σ _m and -nominal strain ε _m curves according to the analysis , and then the phase transition stress and elastic modulus of the shape memory material are obtained. The test method is simple and easy, and can basically realize the non-destructive measurement of the material. It is suitable for the phase change characteristics test of various shape memory alloy (SMAs) materials in the superelastic state or shape memory state, and is especially suitable for SMAs thin films or Testing and research on phase transition characteristics of SMAs micro-devices.

Claims (2)

1, a kind of method of ball-shape pressure head for determining shape memory alloy phase change property the steps include:

A, utilize indentation equipment, adopt spherical indenter radially to be pressed into the shape memory alloy material surface, make it that stress-induced phase transformation take place, and by sensor detect simultaneously load and uninstall process in load F and displacement h _tSignal obtains the load F-displacement h of marmem _tCurve;

B, the different peak load of employing repeat a step in step, obtain the load F-displacement h of marmem under the different peak loads _tCurve, these load F-displacement h _tThe initial unloading slope of curve is the contact stiffness S of Contact Pair under the respective loads, thereby simulates the curve that contact stiffness S changes with load F;

C, the marmem load F-displacement h that obtains in b pacing amount _tIn the curve, selected any loaded segment and the nonoverlapping load F-displacement of unloading segment h _tCurve calculates mean pressure and representative strain in the impression contact region in the loading of this selected curve correspondence and the uninstall process, is defined as nominal stress σ respectively _mWith apparent strain ε _m, its computation process is: calculate contact radius a earlier _c, $a_c=\sqrt{2(h_t-0.75F/S)R-{(h_t-0.75F/S)}^2};$ Calculate nominal stress σ again _m, σ _m=F/ π a _c ²With apparent strain ε _m, ε _m=0.75a _c/ π R; In the formula: R is the radius-of-curvature of spherical indenter, and S is the contact stiffness of corresponding load F in the b matched curve in step;

D, the nominal stress σ that calculates according to c step _mWith apparent strain ε _m, obtain the nominal stress σ of marmem to be measured _m-apparent strain ε _mCurve; For the marmem to be measured of super bullet state, this nominal stress σ _m-apparent strain ε _mThe stress σ that curve loaded segment transformation stress platform begins to locate _fBe the forward transformation stress of corresponding marmem to be measured, the stress σ of the recovery stress platform end of unloading segment _rIt is the reverse transformation stress of corresponding marmem to be measured; For the marmem to be measured of shape memory state, this nominal stress σ _m-apparent strain ε _mThe stress σ that curve loaded segment transformation stress platform begins to locate _fIt is the transformation stress of corresponding marmem to be measured.

2, the method for a kind of ball-shape pressure head for determining shape memory alloy phase change property according to claim 1 is characterized in that: after described d goes on foot and draws the transformation stress of marmem to be measured, also further calculate the elastic modulus E of marmem _m, $E_m=\frac{E_i(1-v_m^2)S}{2a_c{E}_i-(1-v_i^2)S};$ In the formula: S is the contact stiffness of corresponding load F in the b matched curve in step, v _mBe the Poisson ratio of marmem to be measured, v _iAnd E _iPoisson ratio and elastic modulus for spherical indenter.

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