CN100561175C - The method of ball-shape pressure head for determining shape memory alloy phase change property - Google Patents

Abstract

The invention discloses the method for a kind of ball-shape pressure head for determining shape memory alloy (SMAs) phase-change characteristic.It is to utilize spherical indenter to be pressed into the shape memory alloy material surface, makes it that stress-induced phase transformation take place, and detects load and the displacement signal that adds in the uninstall process simultaneously by sensor, obtains load F-displacement h _tCurve is converted into corresponding nominal stress σ according to analyzing load-displacement curve that test is obtained then _m-apparent strain ε _mCurve, and then obtain performances such as the transformation stress of SMAs and elastic modulus.This method of testing is simple, can realize nondestructive measurement substantially to material, not only be widely used in the measurement of various super bullets and shape memory SMAs phase-change characteristic, and be specially adapted to thickness and be low to moderate the test of several microns SMAs film or typical structure size at the SMAs of micron dimension micro element phase-change characteristic, measured value is accurate, the precision height can provide reliable phase-change characteristic test foundation in the application of MEMS (micro electro mechanical system) for SMAs.

Description

The method of ball-shape pressure head for determining shape memory alloy phase change property

Technical field

The invention belongs to shape memory alloy material measuring mechanical property technical field.

Background technology

Marmem (Shape Memory Alloys writes a Chinese character in simplified form SMAs) can show unique shape memory characteristic or the super characteristic that plays under different temperatures.In conjunction with outstanding advantages such as its high power density (output power of unit volume), big power output and output displacements, SMAs has become the ideal material of development mems driver and sensor.In various SMAs, Nitinol (NiTi) is the highest marmem of degree of being practical, and its merit density is up to 2.5 * 10 ⁷J/m ³, than high two orders of magnitude of the little driving material of other type.The driver that the high power density material development goes out, output power is constant but physical dimension is littler, and perhaps physical dimension is constant but output power is bigger.

To surplus the research of Nitinol (NiTi) microdrive existing 20 year historical, developed various devices such as little valve, micro switch, micro-machine arm and Micropump.Because above-mentioned NiTi microdrive element mainly utilizes the martensite phase transformation to finish the displacement or the stress drive function of little driving, characteristics such as the elastic modulus of NiTi alloy and transformation stress just become the main parameters of decision microdrive element function.Therefore, when using the NiTi alloy as the microdrive element, performances such as elastic modulus that at first must clear and definite element and transformation stress, thus provide reference frame for NiTi alloy microdrive circuit elements design and use.Yet, in these microdrives, the Nitinol element mostly be thickness only several microns nano-multicrystal film or typical structure size at little member of micron dimension, can't test crucial phase-change characteristic such as its elastic modulus and transformation stress with traditional stretching test method, thereby need badly and develop a kind of method of testing of testing Nitinol and other various SMAs films and SMAs micro element phase-change characteristic.

Shown in drawing stress σ-strain stress curve synoptic diagram when carrying out the unilateral stretching experiment: be different from traditional material as the marmem of accompanying drawing 1, marmems such as Nitinol can successively experience the stress plateau of martensite phase transformation and two distortion of martensite surrender, corresponding martensite transformation stress σ respectively in unilateral stretching or compression process _tWith martensite yield stress σ _nTherefore, although be widely used in measuring the consistency and elasticity modulus of elasticity or elastic-plastic material based on the nano impress method of Berkovich pressure head, yet, because the coupling of phase transformation distortion and plastic yield in the SMAs deformation process, can't measure the phase-change characteristic of SMAs with the Berkovich indentation of routine: as shown in Figure 2, since the Berkovich pressure head be shaped as the very little triangular taper of tip curvature radius, under very little load-up condition, the martensite yield deformation will be brought out in the indented area central area, and edge region is brought out martensite phase transformation distortion simultaneously.For this reason, measured hardness will be martensite transformation stress σ _tWith martensite yield stress σ _nConcentrated expression owing to can't know the size of martensite yield range in the contact region, also just can't further analyze the phase-change characteristics such as martensite transformation stress that obtain SMAs.

Summary of the invention

The method that the purpose of this invention is to provide a kind of ball-shape pressure head for determining shape memory alloy phase change property, this kind method can be measured the phase-change characteristic of different shape memorial alloy, is particularly suitable for the test of shape memory alloy film and marmem micro element phase-change characteristic.

The technical scheme that the present invention is adopted for its goal of the invention of solution is: 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.

Compared with prior art, the invention has the beneficial effects as follows:

One, conventional Berkovich point vertebra shape pressure head can't be isolated the coupling of phase transformation distortion and plastic yield because the tip curvature radius is little in process of press in, also promptly can't detect the respective loads F-displacement h that the phase transformation distortion is only arranged _tCurve, thus the phase-change characteristic of marmem can not be tested.Be different from conventional Berkovich point vertebra shape pressure head, spherical indenter of the present invention is because pressure head tip curvature radius is big, can apply and the less load of energy measurement under, it can only bring out the martensite phase transformation distortion of marmem in the contact region, and under big load, the martensitic plastic deformation of just further bringing out marmem.Therefore, it by sensor to add in the load and displacement data in the uninstall process, load and corresponding displacement that martensite phase transformation deformation process only takes place are arranged, thus can be according to analyzing the load F-displacement h that test is obtained _tCurve is converted into corresponding nominal stress σ _m-apparent strain ε _mCurve, and then obtain the transformation stress of marmem.Therefore, measuring method of the present invention can be widely used in the measurement of different shape memorial alloy phase-change characteristic.Particularly for the shape memory alloy film (thickness is low to moderate several microns) or the marmem micro element (the typical structure size is in micron dimension) that adopt drawing process to test, especially be fit to adopt method of the present invention, can reach existing method of testing inaccessiable test effect of institute and precision, the favorable reproducibility of data can provide reliable test basis in the application of MEMS (micro electro mechanical system) for SMAs.

Two, simple, convenient, can on many indentation equipments, realize.Only need spherical indenter to be installed, SMAs to be measured is radially exerted pressure, detect corresponding load and displacement signal in loading and the uninstall process simultaneously, then data are carried out analytical calculation and get final product at indentation equipment.

Three, pilot region is little, and is little to the infringement of detected materials.Only in test block to be measured, form the big bulb-shaped recess of radius-of-curvature, and whole material production homogeneous deformation, Berkovich pressure head are produced the sharp vertebra shape depression of concentrating than stretching experiment, the destruction that material is caused can be avoided testing, nondestructive measurement can be realized substantially material.

After above-mentioned d goes on foot and draws the transformation stress of marmem to be measured, can 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.Like this, can provide more phase-change characteristic performance test foundation in the application of MEMS (micro electro mechanical system) for SMAs.

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

Description of drawings

Fig. 1 is that marmem carries out the unilateral stretching drawing stress σ-strain stress curve synoptic diagram in when experiment.

Fig. 2 is the contact region synoptic diagram of Berkovich pressure head when being pressed into marmem.

When Fig. 3 was the inventive method test, spherical indenter was pressed into the synoptic diagram of shape memory alloy material.

Fig. 4 is that with the inventive method length and width to be 10mm, thickness be that two shape memory Nitinol polycrystalline thin slice samples of 0.5mm are tested, one of them sample is in super bullet state SE, another sample is in shape memory state SME, thereby obtains the load F-displacement h of the marmem sample SME of the marmem sample SE of super bullet state of same shape and shape memory state respectively _tCurve.

Fig. 5 is that the inventive method is two marmem samples to Fig. 4, carries out test analysis and curve that the contact stiffness S of the sample SME of curve that the shape memory sample SE contact stiffness S of the super bullet state that obtains respectively changes with load F and marmem state changes with load F.

Solid line among Fig. 6 its nominal stress σ that to be the inventive method obtain the shape memory sample SE test analysis of the super bullet state of Fig. 4 _m-apparent strain ε _mCurve; Dotted line is the unilateral stretching stress σ-strain stress curve of the same sample that adopts existing drawing process and measure.

Solid line among Fig. 7 is the marmem sample SME of the inventive method to the shape memory state of Fig. 4, its nominal stress σ that test analysis obtains _m-apparent strain ε _mCurve; Dotted line is unilateral stretching stress σ-strain stress curve of the same sample SME that adopts existing drawing process and measure.

To be the inventive method carry out test analysis to two samples of Fig. 4 to Fig. 8, and the contact stiffness S of the sample SE of the super bullet state that obtains and the sample SME of shape memory state is with contact radius a _cThe curve that changes, two slope of a curves are 2E _Eff(E _EffThe complex elastic-modulus that contacts with SMAs for spherical indenter).

Embodiment

Embodiment

With length and width be 10mm, thickness be two Nitinol polycrystalline thin slices of 0.5mm as specimen, one of them is the sample SE of super bullet state, another is the sample SME of shape memory state.Test with these two samples is an example, and the concrete operations step of the inventive method is described.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 simultaneously continuous detecting load and uninstall process in load F and displacement h _tSignal obtains the load F-displacement h of marmem _tCurve.When Fig. 3 was test, spherical indenter was pressed into the synoptic diagram of shape memory alloy material, and wherein R is the radius-of-curvature of spherical indenter, can be characterized the radius of curvature R of pressure head in this example=20 μ m by scanning electron microscope; a _cBe contact radius, h _tBe the displacement that produces behind the sample pressurized, h _cBe the vertical range between pressure head sample engagement edge and sample depression bosom.

Fig. 4 is that 100mN tests respectively two samples for adopting peak load, the load F-displacement h of the sample SE of the super bullet state that obtains and the sample SME of shape memory state _tCurve.As seen, when the pressure head radius-of-curvature is 20 μ m, when the test peak load is 100mN, surpass the load F-displacement h that plays sample SE _tCurve recovers after unloading substantially fully, and shows as one than the large time delay ring before recovery, shows that the super alloy that plays sample SE before plastic yield takes place stress-induced reversible transition has taken place.And the alloy of the sample SME of shape memory state has produced bigger residual deformation in process of the test, and its phase transformation distortion basic not recovery after unloading is described, has the characteristic of shape memory.

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.The curve that the contact stiffness S of this routine sample that Fig. 5 is test, simulate changes with load F.Utilize this matched curve can obtain contact stiffness numerical value under the different loads easily.

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.The load F-displacement h of the marmem sample SE of this example super bullet state when in actual measurement, selecting peak load for use and the sample SME of shape memory state for 100mN _tCurve, as shown in Figure 4.

Calculate under the selected 100mN peak load, mean pressure and the representative strain of impression contact region in loading and uninstall process is defined as nominal stress σ respectively _mWith apparent strain ε _mIts 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.

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

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.

Solid line among Fig. 6 is the nominal stress σ of the super bullet state sample SE that surveys out _m-apparent strain ε _mCurve; For the ease of the comparison of the inventive method and stretching experiment method, adopt the test of stretching experiment method to obtain its unilateral stretching stress σ-strain stress curve, i.e. dotted line among Fig. 6 to this sample SE simultaneously.Solid line among Fig. 7 is the nominal stress σ of the sample SME of the shape memory state of surveying out _m-apparent strain ε _mCurve; Be similarly and be convenient to comparison, also provided unilateral stretching stress σ-strain stress curve that this sample adopts the test of stretching experiment method to obtain, i.e. dotted line among Fig. 7.

From Fig. 6, Fig. 7 as can be seen, the nominal stress σ that the inventive method obtains _m-apparent strain ε _mCurve is very consistent with stress σ-strain stress curve shape that tension test obtains.In loading procedure, because stress-induced martensite phase transformation, stress plateau has all appearred in NiTi marmem two kinds of stress-strain curves in Fig. 6, Fig. 7 respectively.In the uninstall process: owing to the elastic performance of super bullet (SE) NiTi alloy, its phase transformation has all obtained good recovery among Fig. 6, and shape memory (SME) NiTi alloy all has only less recovery of elasticity among Fig. 7 under two kinds of method of testings.

With drawing stress σ-strain stress class of a curve seemingly, nominal stress σ among Fig. 6 and Fig. 7 _mWith apparent strain ε _mThe platform of curve has reflected that also the phase transformation of Nitinol in impression process of the present invention is mobile: according to the theory of Tabor, and the nominal stress σ when phase transformation is flowed _mBe about three times of unidirectional when compression transformation stress.In addition, because the anisotropy of Nitinol when stretching and compress, the transformation stress during its compression is about 1.5 times when stretching.The nominal stress σ that phase transformation was flowed when therefore, Nitinol compressed _mBe about 4.5 times of its unilateral stretching transformation stress σ.

The nominal stress σ of the sample SE of super bullet state among Fig. 6 _m-apparent strain ε _mThe stress σ that curve loaded segment transformation stress platform begins to locate _fBe 1780MPa, the forward transformation stress when phase transformation was flowed when the sample SE of corresponding super bullet state compressed, calculating corresponding stretching transformation stress σ is 395MPa.It is very approaching that one directional tensile test is directly measured the transformation stress σ value 370MPa of (sign) among this value and Fig. 6.Nominal stress σ among Fig. 6 _m-apparent strain ε _mThe stress σ of the recovery stress platform end of curve unloading segment _r=666MPa, corresponding super bullet state sample SE resilience the time reverse transformation stress, calculating corresponding stretching transformation stress σ is 148MPa, also with Fig. 6 in the direct measured value 137MPa of tension test identical preferably.

The nominal stress σ of the sample SE of shape memory state among Fig. 7 _m-apparent strain ε _mThe stress σ that curve loaded segment transformation stress platform begins to locate _fBe 750MPa, the transformation stress when phase transformation was flowed when the sample SE of correspondingly-shaped remember condition compressed, calculating corresponding stretching transformation stress σ is 167MPa.It is very approaching that one directional tensile test is directly measured the transformation stress σ value 162MPa of (sign) among this value and Fig. 7.

Need to prove: the marmem load F-displacement h that obtains in b pacing amount _tIn the curve, when peak load produces the minimum load of martensite phase transformation less than it, marmem will only produce elastic deformation, and not produce the martensite phase transformation, its load F-displacement h _tThe loaded segment of curve and unloading segment are then overlapping fully.Obviously, two steps of c, d select not produce phase transformation these test resulting load F-displacement h _tCurve carries out analytical calculation, can't draw its transformation stress.

And the nonoverlapping any load F-displacement h of loaded segment and unloading segment _tThe martensite phase transformation all greater than its minimum load that produces the martensite phase transformation, has all taken place in curve, the peak load of its loading in the test process, for the marmem of super bullet state, the d step is analyzed the nominal stress σ that draws _m-apparent strain ε _mCurve just must have the transformation stress platform of loaded segment and the recovery stress platform of unloading segment, final corresponding forward transformation stress is the stress of loaded segment transformation stress platform initial place, and final corresponding reverse transformation stress is the stress of unloading segment recovery stress platform end.And for the marmem of shape memory state, the d step is analyzed the nominal stress σ that draws _m-apparent strain ε _mCurve just must have the transformation stress platform of loaded segment, and final corresponding transformation stress is the stress of loaded segment transformation stress platform initial place.Therefore, select any loaded segment and the nonoverlapping load F-displacement of unloading segment h _tCurve all can obtain corresponding transformation stress by the step in two steps of c, d, and with which bar load F-displacement h of concrete selection _tCurve is irrelevant.At loaded segment and the nonoverlapping load F-displacement of unloading segment h _tSelect different curves in the curve, just the nominal stress σ in d step _m-apparent strain ε _mIn the curve, stress plateau length has difference (the nominal stress σ that peak load is big _m-apparent strain ε _mCurve, stress plateau is longer; On the contrary, stress plateau is shorter).

After this routine 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, can be assumed to 0.3 usually; v _iAnd E _iBe the Poisson ratio and the elastic modulus of spherical indenter, its value is v _t=0.07 and E _i=1141GPa.

Because the complex elastic-modulus E that spherical indenter contacts with SMAs _EffWith contact stiffness S, contact radius a _cBetween have following relation: E _Eff=0.5S/a _c, wherein: $1/E_{\mathrm{eff}}=(1-v_m^2)/E_m+(1-v_i^2)/E_i,$ So have: $E_m=\frac{E_i(1-v_m^2)S}{2a_c{E}_i-(1-v_i^2)S}.$

According to contact radius-stiffness curve of above formula and Fig. 8, the elastic modulus that actual measurement obtains super bullet of this example (SE) and shape memory (SME) NiTi alloy is respectively 42GPa and 48GPa, the 41GPa and the 42GPa basically identical that obtain with tension test.

In sum, utilize the spherical indenter plunging can record the load F-displacement h of shape-memory material easily _tCurve, and according to analyzing the nominal stress σ that can be translated into correspondence _mWith-apparent strain ε _mCurve, and then obtain performances such as the transformation stress of shape-memory material and elastic modulus.This method of testing is simple, can realize nondestructive measurement substantially to material, be applicable to different shape memorial alloy (SMAs) material under super bullet state or the phase-change characteristic under shape memory state test, and be specially adapted to the test and the research of SMAs film or SMAs micro element phase-change characteristic.

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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