Effects of Predeformation on the Reverse Martensitic Transformation of TiNi Shape Memory Alloy

DSC was used to study the effects of predeformation on the reverse martensitic transformation of near-equiatomic TiNi alloy. Both the start temperature As and the finish temperature Af of the reverse transformation increased with increasing degree of predeformation, but the algebraic difference between As and Af decreased with increasing predeformation until it reached a minimum value, then remained unchanged with further deformation. Transformation heat also increased with increasing predeformation until it reached a maximum value, then decreased with further predeformation. All the phenomena above were considered to be closely related with the release of elastic strain energy during predeformation.

Effects of aging treatment on the microstructure and superelasticity of columnar-grained Cu71Al18Mn11 shape memory alloy

The effect of aging treatment on the superelasticity and martensitic transformation critical stress in columnar-grained Cu_（71）Al_（18）Mn_（11） shape memory alloy（SMA） at the temperature ranging from 250°C to 400°C was investigated. The microstructure evolution during the aging treatment was characterized by optical microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The results show that the plate-like bainite precipitates distribute homogeneously within austenitic grains and at grain boundaries. The volume fraction of bainite increases with the increase in aging temperature and aging time, which substantially improves the martensitic transformation critical stress of the alloy, whereas the bainite only slightly affects the superelasticity. This behavior is attributed to a coherent relationship between the bainite and the austenite, as well as to the bainite and the martensite exhibiting the same crystal structure. The variations of the martensitic transformation critical stress and the superelasticity of columnar-grained Cu_（71）Al_（18）Mn_（11） SMA with aging-temperature and aging time are described by the Austin-Rickett equation, where the activation energy of bainite precipitation is 77.2 kJ ·mol1. Finally, a columnar-grained Cu_（71）Al_（18）Mn_（11） SMA with both excellent superelasticity（5%-9%） and high martensitic transformation critical stress（443-677 MPa） is obtained through the application of the appropriate aging treatments.

New finite strain elastoplastic equations for accurately and explicitly simulating pseudoelastic-to-plastic transition effects of shape memory alloys

A new finite strain elatoplastic J2-flow model with coupling effects of both isotropic and anisotropic hardening is proposed with the co-rotational logarithmic rate.In terms of certain single-variable shape functions representing uniaxial loading and unloading curves,explicit multi-axial expressions for the three hardening quantities incorporated in the new model proposed are derived in unified forms for the purpose of automatically and accurately simulating complex pseudoelastic-to-plastic transition effects of shape memory alloys(SMAs)under multiple loading-unloading cycles.Numerical examples show that with only a single parameter of direct physical meaning for each cycle,accurate and explicit simulations may be achieved for extensive data from multiple cycle tests.

Effect of Ageing on Martensitic Transformation and Shape Memory Effect of Fe-Mn-Si-Co-Mo Alloy

This paper discusses the effect of ageing on the thermally induced martensitic transformation and its reverse transformation and shape memory effect of Fe-24Mn-5Si-8Co-4Mo shape memory alloy:the precipitation of Fe 2Mo particles increases the hardness and strength of the alloy as ageing goes on;ageing increases the transformation temperatures;ageing improves,the SME of the alloy so remarkably that a maximum shape recovery ratio is obtained while ageing at 600℃.

FCC/HCP martensitic transformation and shape memory effect in Co-Al binary system

It is known that pure Co undergoes martensitic transformation from γ phase (fcc) to ε phase (hcp) by the movement of a/6<112> Shockley partial dislocations at around 400 ℃, however, there have been few systematic works on the SM effect in Co and Co-based alloys. In this study, the fcc/hcp rnartensitic transformation and the SM effect were investigated in Co-A1 binary alloys(mole fraction of Al=0-16%). The γ/ε rnartensitic transformation temperatures were found from the DSC measurements to decrease with increasing Al content, while the transformation temperature hystereses were observed to increase from 60℃ at x(Al)=0 to 150℃ at x(Al) = 16%. The SM effect evaluated by a conventional bending test was enhanced by the addition of Al over 4% (mole fraction) and Co-Al alloys containing over 10%(mole fraction) exhibit a good SM effect associated with the hcpfee → reverse transformation above 200℃. The SM effect was significantly improved by precipitation of β (I32) phase and the max[real shape recovery strain of 2.2 % was obtained, which can be explained by precipitation hardening. The crystallographic orientations between the β, εand γ phases were also determined. Finally, the magnetic properties were investigated and it was found that the Curie temperature and saturation magnetization of Co-14% Al(mole fraction) are 690℃ and 120 emu/g, respectively. It is concluded that the Co-A1 alloys hold promise as new high-temperature and ferromagnetic SM alloys.

Martensitic transformation and shape memory effect in ausaged Fe-Ni-Si-based alloys

It is well known that the morphologies of the α’ martensite formed from the γ phase in ferrous alloys are classified into five types of lath, butterfly, （225）A type plate,lenticular and thin-plate. Among those α’ martensites, only the thin-plate martensite,which is characterized by containing a high density of transformation twins, has a potential of exhibiting a perfect shape memory （SM） effect.Recently the present authors found in Fe-Ni-Si alloys that the thin-plate martensite is formed by the introduction of fine and coherent γ’-（Ni,Fe）3Si particles with a L12 ordered structure in the austenite matrix due to ausaging. In the present study, the SM properties of the ausaged Fe-Ni-Si alloys with the thin-plate martensite are investigated by a conventional bending-test. The effects of the addition of Co to the Fe-Ni-Si alloys on the martensitic transformation and the SM properties are also investigated. It is shown that while the ausaged Fe-Ni-Si ternary alloys exhibit an imperfect SM effect due to reverse transformation from stress-induced thin-plate martensite to austenite, the SM properties are improved by the addition of Co. An almost perfect SM effect is confirmed in the Fe-Ni-Si-Co alloys by heating to 1 100 ℃ after deformation at -196 ℃.

Influence of recovery heating rate on shape memory effect in up-quenched Cu-Al-Mn alloy

The effect of recovery heating rate on shape memory effect of the up-quenched Cu-8.88Al-10.27Mn（mass fraction, %） alloy was investigated by optical microscopy, electron transmission microscopy（TEM） and electrical resistivity measurement. It is found that the shape recovery rate decreases as the heating rate decreases. It can reach 75% when the heating rate is 20 ℃/min, while it is only 8% when the heating rate is 1 ℃/min. In situ microstructure observation indicates that the dependence of shape memory effect on recovery heating rate is caused by the stabilization of twinned martensite induced by deformation. The analysis of electrical resistivity shows that the stabilization of twinned martensite may be ascribed to formation of compound defects of vacancies and dislocations at the boundaries of twinned martensite during the slow heating. The compound defects prevent the reverse transformation of twinned martensite.

Role of thermal martensite in shape memory effect of CoAl and CoNi alloys

To address the role of the HCP martensite in CoAl and CoNi shape memory alloys, the relationship between the shape memory effect （SME） and the content of the thermal and stress-induced HCP martensite was investigated in the solution-treated CoAl and CoNi alloys. In-situ optical observations were employed to investigate the contents of thermal HCP martensite before and after deep cooling and its influence on the stress-induced HCP martensite transformation and SME. The results show that the SME in both the CoAl and the CoNi alloys results from the stress-induced HCP martensite. The role of the thermal HCP martensite in both of them is the strengthening of the matrix. The much higher yield strength in the solution-treated CoAl alloy due to solution strengthening of Al is responsible for its better SME compared with the CoNi alloy.

STABILIZATION OF MARTENSITE IN Cu-Zn-Al ALLOY AND ITS REVERSE SHAPE MEMORY EFFECT

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Martensitic transformation and magnetic properties of aged Ni-Fe-Ga-Ti shape memory alloy

This article reports the effect of ageing on the microstructure, martensitic transformation, magnetic properties, and mechanical properties of Ni51FelsGa27Ti4 shape memory alloy. There are five specimens of this alloy aged at 573 up to 973 K for 3 h per each. This range of ageing temperature greatly affects the microstructure of the alloy. As the ageing temperature increased from 573 up to 973 K, the microstructure of Ni51FelsGa27Ti4 alloy gradually changed from the entirely martensitic matrix at 573 K to the fully austenitic microstructure at 973 K. The volume fraction of precipi- tated Ni3Ti particles increased with the ageing temperature increasing from 573 to 773 K. Further increasing the ageing temperature to 973 K decreased the content of Ni3Ti in the microstructure. The martensitic transformation tempera- ture was decreased steadily by increasing the ageing temperature. The magnetization saturation, remnant magnetization, and coercivity increased with the ageing temperature increasing up to 773 K. A further increase in ageing temperature decreased these raagnetic properties. Moreover, the hardness values were gradually increased at first by increasing the ageing temperature to 773 K, and then dramatically decreased to the lowest value at 973 K.

Effects of aging on the microstructure of a Cu-Al-Ni-Mn shape memory alloy

The influence of aging on the microstructure and mechanical properties of Cu-11.6wt%Al-3.9wt%Ni-2.5wt%Mn shape memory alloy（SMA） was studied by means of scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,X-ray diffractometer,and differential scanning calorimeter（DSC）.Experimental results show that bainite,γ2,and α phase precipitates occur with the aging effect in the alloy.After aging at 300°C,the bainitic precipitates appear at the early stages of aging,while the precipitates of γ2 phase are observed for a longer aging time.When the aging temperature increases,the bainite gradually evolves into γ2 phase and equilibrium α phase（bcc） precipitates from the remaining parent phase.Thus,the bainite,γ2,and α phases appear,while the martensite phase disappears progressively in the alloy.The bainitic precipitates decrease the reverse transformation temperature while the γ2 phase precipitates increase these temperatures with a decrease of solute content in the retained parent phase.On the other hand,these precipitations cause an increasing in hardness of the alloy.

Characteristics of shape memory and superelasticity for TiNi thin films

TiNi shape memory alloy thin films were deposited by using a RF magnetron sputtering apparatus. The transformation and shape memory characteristics of the thin films have been investigated by using DSC and tensile tests. After aging, perfect shape memory effect and superelasticity were achieved in TiNi thin films.

Martensite Aging Effect and Thermal Cyclic Characteristics in Ti-Pd and Ti-Pd-Ni High Temperature Shape Memory Alloys

The effect of thermal cycling and aging in martensitic state in Ti-Pd-Ni alloys were investigated by DSC and TEM observations. It is shown that the thermal cycling causes the decreases in M, and Af temperatures in Ti50Pd50-xNix (x=10, 20, 30) alloys, but no obvious thermal cycling effect was observed in Ti50Pd50Pd40Ni10 alloys and the aging effect shows a curious feature, i.e., the Af temperature does not saturate even after relatively long time aging, which is considered to be due to the occurrence of recovery recrystallization during aging.

Microstructure and high-temperature shape-memory effect in Ni_(54)Mn_(25)Ga_(21) alloy

Ni54Mn25Ga21 alloy was prepared to investigate the microstructure, martensitic transformation and high-temperature shape-memory effect. Ni54Mn25Ga21 alloy exhibits single phase of non-modulated martensite with tetragonal structure at room temperature. Its martensitic start temperature Ms, martensitic finish temperature Mf on cooling, and austenitic start temperature As, austenitic finish temperature Af on heating are 260.2, 237.8, 262.5 and 287.8 ℃, respectively. The compressive strength and strain of Ni54Mn25Ga21 single crystal were measured to be 845 MPa and 20.5%, respectively, with compressive axis along the growth direction of the rods. An excellent shape-memory strain of 6.1%, which is the best performance among high-temperature shape-memory alloys up to the present, is obtained when prestrained to 8%.

Effect of Thermal Cycling under Load on Martensite Transformation and Two-way Shape Memory Effect in a TiNi Alloy

The effect of thermal cycling under loading on martensitic transformation and two-way shape memory effect was investigated for Ti-49.8 at, pet Ni alloy. It is shown that M(s), and M(f) temperature increase with increasing the number of cycles, while A(s) and A(f) temperature decrease during thermal cycling. The total strain at and permanent strain epsilon (p) increase with increasing applied stress and number of cycles. The two-way shape memory effect can be improved by proper thermal cycling training under loading, while excessively high applied stress results in the deterioration of TWSME. The reason for the changes in martensitic transformation characteristics and two-way shape memory effect during thermal cycling under loading is discussed based on the analysis of microstructure by TEM observations.

Experimental investigation of the cyclic degradation of the one-way shape memory effect of NiTi alloys

Based on stress-and strain-controlled cyclic tension-unloading-heat-cooling tests,cyclic degradation of the one-way shape memory effect(OWSME)of NiTi shape memory alloys(SMAs)was investigated.It was seen,in thermo-mechanical coupled cyclic tests,that residual strain after each cycle accumulated,but the martensite reorientation stress and dissipation energy-per-cycle decreased as the number of cycles increased.Meanwhile,the cyclic degradation of OWSME was aggravated by increasing the stress/strain amplitude.In addition,the stress-strain response of NiTi SMAs was further investigated by performing simultaneous thermo-mechanical coupled cyclic tests with various phase-angle differences between the mechanical and thermal cyclic loadings.It can be concluded that such cyclic response depends significantly on prescribed phase-angle differences.Obtained experimental results are helpful for both the development of constitutive models and engineering applications of NiTi SMAs.

Study on Stress-softening Effect in CuZnAl Shape Memory Alloys

The effect of mechanical cyclings on the stress-strain(S-S) curves was studied systematically using tensile tests over a wide temperature range. Two-stage yielding was observed in the temperature range between Ms and the ultimate temperature to induce martensite, denoted by Md. The first stage was confirmed to be associated with stress-induced martensite(SIM) and the second stage was associated with the yielding of SIM. When the samples were mechanically cycled between Ms and Md, a stress-softening phenomenon was observed. The cyclings made the martensite amount increase, critical stress σc of SIM decrease according to exponential law, and the entropy(also enthalpy) of the parent to martensite transformation also decrease. The variations of SIM relative amount were determined by recording the changes of electrical resistances in the course of tension.

Microstructure and Shape Memory Effect of Cu-26.1Zn-4.8Al Alloy

The influence of processing parameters on the microstructure and shape memory effect of Cu-26.1Zn-4.8Al alloy was investigated. The treated specimens were characterized by metallography, X-ray diffraction （XRD） and transmission electron microscopy （TEM） to explain the mechanism of shape memory effect in Cu-26.1Zn-4.8Al alloy. The results reveal that the shape memory effect is markedly increased by appropriate quenching and ageing process. XRD shows that γ phase precipitates from martensite when aged at higher temperature and γ precipitates impair the shape memory effect. TEM analysis indicate that the substructure of plate-like martensite consists of twins and stacking faults.

Elastocaloric effect and mechanical behavior for NiTi shape memory alloys

The NiTi shape memory alloy exhibits excellent superelastic property and elastocaloric effect. The large temperature change（DT） value of 30 K upon loading and-19 K upon unloading are obtained at room temperature, which are higher than those of the other NiTi-based materials and among the highest values reported in the elastocaloric materials. The asymmetry of the measured DT values between the loading and unloading process is ascribed to the friction dissipation.The large temperature change originates from the large entropy change during the stress-induced martensite transformation（MT） and the reverse MT. A large coefficient-of-performance of the material is obtained to be 11.7 at ε= 1%, which decreases with increasing the applied strain. These results are very attractive in the present solid-state cooling, which potentially could replace the vapor compression refrigeration technologies.

Large elastocaloric effect in Ti–Ni shape memory alloy below austenite finish temperature

Solid refrigeration technology based on the elastocaloric effect has a great potential alternative to the conventional vapor compression cooling. Here we report the large elastocaloric effect in Ti-Ni （50 at%） shape memory alloy below its austenite finish temperature Af under different strain. Both Maxwell＇s and Clausius-Clapeyron equations are used to estimate the entropy change. The strain-induced entropy change increases with raising the strain and gets a maximum value at a few kelvins below Af. The maximum entropy changes ASrnax are -20.44 and -53.70 J/kg.K, respectively for 1% and 2% strain changes. Large entropy change may be obtained down to 20 K below Af. The temperature of the maximum entropy change remains unchanged before the plastic deformation appears but moves towards low temperature when the plastic deformation happens.