Stress-induced martensitic transformation in （Ni47Ti44）100-xNbx shape memory alloys with wide hysteresis

The effect of deformation via stress-induced martensitic transformation on the reverse transformation behavior of the （Ni47Ti44）100-xNbx （x=3, 9, 15, 20, 30, mole fraction, %） shape memory alloys was investigated in detail by differential scanning calorimetry （DSC） after performing cryogenic tensile tests at a temperature of Ms＋30 ℃. The results show that Nb-content has obvious effect on the process of stress-induced martensitic transformation. It is also observed that the stress-induced martensite is stabilized relative to the thermally-induced martensite （TIM） formed on cooling, and Nb-content in Ni-Ti-Nb alloy has great influence on the reverse transformation start temperature and transformation temperature hysteresis of stress-induced martensite（SIM）. The mechanism of wide transformation temperature hysteresis was fully explained based on the microscopic structure and the distribution of the elastic strain energy of （Ni47Ti44）100-xNbx alloys.     

Changes in martensitic transformation temperatures during thermal cycling in Ti−Ni−Zr shape memory alloys

Changes in the transformation behavior and transformation temperatures of a 40Ti-50Ni-10Zr (at%) alloy during thermal cycling have been investigated by means of differential scanning calorimetry, X-ray diffraction and thermal cycling tests under a constant load. A 40Ti−50Ni−10Zr alloy showed two stage transformation behavior, i.e., from the B2 to the B19 and then from the B19 to the B19, although they are not clearly separated. With the increasing of the number of thermal cycling, the decreasing rate of the transformation temperature corresponding to the B2–B19 was higher than that corresponding to the B19–B19, and consequently the former overlapped with the latter. Transformation temperatures of 40Ti−50Ni−10Zr greatly decreased (95K) during thermal cycling without applied stress, since only the thermal cycling effect which suppresses martensitic transformation had an influence on transformation temperatures. Decreases in transformation temperatures during thermal cycling with applied stress was smaller than that during thermal cycling without applied stress because both the structural refinement effect which assists martensitic transformation and the thermal cycling effect had an influence on transformation temperatures.     

Effect of deformation on the stress-induced martensitic transformation in (Ni47Ti44)100−xNbx shape memory alloys with wide hysteresis

Martensite in TiNi-based alloys is reported to be thermally stabilized after a moderate deformation. Hence, this paper investigates the effect of deformation via stress-induced martensitic transformation on the reverse transformation behavior of (Ni₄₇Ti₄₄)_100−xNb_x (x=3, 9, 15, 20, 30 at.%) alloys. The stress-induced martensite appears to be stabilized in relation to the thermal-induced martensite that forms on cooling. This observation is confirmed by an increase in the reverse transformation start temperature, during which time the transformation temperature hysteresis reaches about 200°C. Moreover, the Nb content in Ni−Ti−Nb alloy has a great influence on the transformation temperature hysteresis of stress-induced martensite as well as on the process of stress-induced martensitic transformation. The mechanism of wide transformation temperature hysteresis is explained in terms of the microscopic structure of (Ni₄₇Ti₄₄)_100−xNb_x alloys. Furthermore, the temperature interval of the reverse transformation of stress-induced martensite was found to increase slightly as the strain of the high Nb-content alloy increased, though the value was much smaller than that of the thermally induced martensite. Finally, the paper explains the relation between this unique phenomenon and the elastic strain energy.     

Multistage martensitic transformation in high temperature aged Ti48Ni52 shape memory alloy

Aging effects on the transformation behavior of Ti₄₈Ni₅₂ SMA aged at 600 °C and 550 °C were investigated. The transformation evolution of specimens aged at 600 °C and at 550 °C can be classified into three and two periods, respectively. During aging at 600 °C, the specimens undergo B2↔M₁ in period I, B2↔M₁ + B2↔M₂ in period II, and B2↔M₂ in period III (M represents the B19′ martensite). During aging at 550 °C, the specimens undergo B2→R→M₁/M₁→B2 in periods I and II, and an additional B2↔M₂ in period II. In period I, only Ti₃Ni₄ precipitates (ppts) form in the B2 matrix. In period II, Ti₃Ni₄ and Ti₂Ni₃ ppts coexist, and Ti₃Ni₄ depletion zones occur around Ti₂Ni₃ plates, inducing B2↔M₂. In period III, only Ti₂Ni₃ plates exist. The occurrence of B2→R→M₁ or B2→M₁ in period I is related to whether the Ti₃Ni₄ ppts have a coherent interface or not. Schematic diagrams for periods I ∼ III are proposed to interpret the observed transformation behavior. The difference in Ni-content in the matrix near the Ti₃Ni₄ and Ti₂Ni₃ ppts affects the transformation temperatures of B2↔B19′.     

Transition from many domain to single domain martensite morphology in small-scale shape memory alloys

The morphology of the martensitic transformation during a superelastic cycle is studied by in situ scanning electron microscopy deformation experiments in microwires of Cu–Zn–Al. The diameters of the wires studied (21–136 μm) span the range in which significant size effects upon transformation hysteresis have been observed. In larger wires the transformation is accommodated by the continual nucleation of many new martensite plates that grow and eventually coalesce with their neighbors. In small wires a single martensite plate nucleates at the start of transformation and then proceeds to grow in a monolithic fashion; the wire transforms by smooth axial propagation of a single interface. The transition from many domain to single domain transformation is gradual with wire diameter, and is based upon scaling of the domain density with sample size. We attribute it to a crossover from bulk to surface obstacle control of transformation front propagation. This observation also sheds light on reported size effects in energy dissipation in shape memory alloys.     

Effects of aging on stress-induced martensitic transformation in ductile Cu–Al–Mn-based shape memory alloys

Effects of aging at 473–573 K on stress-induced martensitic transformation for textured Cu_71.9Al_16.6Mn_9.3Ni₂B_0.2 and random-textured Cu_72.1Al_16.9Mn_10.5Co_0.5 shape memory alloy (SMA) wires with a large relative grain size d/D = 6 were investigated by cyclic tensile testing at room temperature, where d and D indicate mean grain size and wire diameter, respectively. The random-textured Cu_72.1Al_16.9Mn_10.5Co_0.5 wire cannot be uniformly deformed and the ductility is drastically reduced by aging treatment. On the other hand, in the textured Cu_71.9Al_16.6Mn_9.3Ni₂B_0.2 SMA wire, the critical stress for martensitic transformation σ_t and the tensile strength σ_f are increased by aging without the associated loss of superelasticity (SE). Even in textured wire with a high σ_t of over 750 MPa, an excellent SE strain of about 6% can be obtained due to the formation of a fine bainite phase. Moreover, it was confirmed by in situ observation that stress-induced martensite plates grow, accompanying distortion of the bainite plates.     

The effect of electronic and magnetic valences on the martensitic transformation of CoNiGa shape memory alloys

Martensite start temperatures (M_s) and magnetic properties of several Co–Ni–Ga shape memory alloys (SMAs) with B2 austenite structure were investigated as a function of composition. It was found that, in addition to the well-known effect of the valence electron concentration (e/a ratio) in increasing M_s, the magnetic character of the alloy also plays a decisive role in determining M_s. These results were further corroborated through electronic structure calculations based on density functional theory. Experiments and calculations suggest that, for a given composition of Ga, the higher e/a ratios result in the higher M_s. Moreover, at a constant e/a ratio, the higher the magnetic valence number (Z_m) of the alloy is, the lower the M_s becomes. It was concluded that Z_m can be used as an indicator for the compositional trends in M_s in the present ferromagnetic SMAs in addition to the e/a ratio. Statistical analysis of the experimental results suggests that an increase in e/a of 0.1 yields an increase in M_s of about 190 K, while a change in Z_m of +0.1 results in a decrease in M_s of about 160 K. The calculations also confirmed that ferromagnetism may ultimately result in entropic and/or energetic stabilization of the austenite (cubic) phase, yielding lower M_s. These findings seem to be valid for other ferromagnetic SMA systems, where different M_s temperatures were reported for a given e/a ratio.     

Martensitic transition and shape memory effect of Ti-Zr-Mo series alloys

Development of shape memory alloys is always one of the most important directions for functional Ti alloys. The Ti-Zr-Mo series alloys with various Mo contents were prepared. The main aim of the current work is to investigate the effects of Mo on martensitic transition and shape memory effect of Ti-Zr alloy. The X-ray diffraction and transmission electron microscope results indicate that the phase constitution of the examined alloys is greatly dependent on Mo content. The Ti-Zr-Mo alloy with 2 wt% Mo is composed mainly of α′ martensite and a few β phase. As the Mo content increased to 4 wt%, the Ti-50Zr-4Mo alloy consists of α″ martensite and β phase. As the Mo content further increased to 8 wt%, the alloy consists mainly β phase and a barely detectable amount of α″ martensite. Thermal analysis shows that the reverse martensitic transition temperature of the examined alloys decreases with the increasing of Mo. The reverse martensitic transition start, A_s, temperature is approximately 584 °C for Ti-50Zr-2Mo alloy and 519 °C for Ti-50Zr-4Mo, respectively. And the martensitic transition start, M_s, temperature is approximately 553 °C and 501 °C for that two alloys, respectively. But no obvious exothermic and/or endothermic peak can be observed in DSC curve of Ti-50Zr-8Mo alloy. Furthermore, the effect of Mo content on shape memory recovery ratio, η, of the examined alloys was also investigated. Results show that the η first increases and then decreases with the increasing of Mo. The alloy with 4 wt% Mo has the maximum η approximately 13.8%. The influencing mechanism of Mo content on shape memory effect of the examined alloys was also discussed. This findings not only supplied a series of shape memory TiZr-based alloys, but also enriched and deepened the theory of shape memory effect.     

Role of magnetism on the martensitic transformation in Ni-Mn-based magnetic shape memory alloys

The effect of magnetism on the martensitic structural transformation has been analyzed through the evolution of the transformation temperatures of several Ni-Mn-Ga and Ni-Mn-In alloys subjected to high-temperature quenching and post-quench annealing thermal treatments. It is found that the atomic order variations associated with the thermal treatments affect the structural transformation in different ways depending on the character of the magnetic ordering in the austenitic and the martensitic phases. In particular, regardless of composition, the variation in the atomic order affects the martensitic transformation temperature only in those alloys in which at least one of the structural phases show magnetic order at the transformation temperature, whereas those transformations taking place between paramagnetic phases remain unaffected. The observed behaviors are explained in terms of the effect of the magnetic exchange coupling variations on the free energy difference between austenite and martensite. The results confirm the key role of magnetism in the martensitic transformation.     

Influence of compositional ratio on microstructure and martensitic transformation of CuZr shape memory alloys

In the present work the effect of Cu/Zr atomic ratio on structural and calorimetric properties of this high temperature shape memory alloy (HTSMA) is studied. It has been discussed the changes induced by Cu/Zr ratio on the martensitic transformation temperatures and the corresponding transformation heats coupled with the phases microstructure. The modification of the Cu content in the range ±2% at, around the equiatomic composition, does not drastically change the thermal properties of the alloys. Moreover, the Cu/Zr ratio strongly influences the microstructure in terms of the presence and amount of the other characteristic phases, Cu₁₀Zr₇ and CuZr₂, in the place of the CuZr phase. The understanding of the basic properties of the binary system can be of great help for further investigations on CuZr based systems with other alloying elements.     

激冷Ti-46Ni合金薄带的组织、相变和形状记忆效应

用激冷甩带法制备了Ti-46Ni合金薄带,用扫描电子显微镜、X射线衍射仪、示差扫描热分析仪和弯曲试验研究了合金薄带的组织特征、相变行为和形状记忆效应。结果表明,铸态及400~600℃退火态Ti-46Ni合金薄带的显微组织形态呈树枝状,晶粒细小,由B19?马氏体和B2母相组成;在冷却、加热过程中,该合金薄带发生一阶段马氏体相变;铸态和退火态Ti-46Ni合金薄带均具有良好的形状记忆效应。     

Microstructure, phase transformation and mechanical properties of Ni-Mn-Ga-Y magnetic shape memory alloys

Influence of rare earth Y addition on the microstructure and phase transitions and mechanical properties of polycrystalline Ni₅₀Mn₂₉Ga₂₁ ferromagnetic-shape memory alloy (FSMA) are investigated. It is shown that microstructure of the Ni-Mn-Ga-Y alloys consists of the matrix and the Y-rich phase. The Y-rich phase firstly disperses homogeneously in the matrix with small amounts and then tends to segregate at the grain boundaries with increasing Y substitution for Ga. The Y-rich phase is indexed to Y(Ni,Mn)₄Ga phase with a hexagonal CaCu₅ type structure. The structural transition from 5 M to 7 M, and then to non-modulated T martensite appears with the increase of Y content. The martensitic transformation temperature increases remarkably with increasing Y content, whereas the Curie temperature almost keeps unchanged. It is revealed that the appropriate addition of Y significantly enhances the yield strength and improves the ductility of the alloys. The mechanism on the influence of Y content on the improved mechanical properties and martensitic transformation temperature is also discussed.     

双晶体NiTi形状记忆合金在无/有应力作用时效下的Ni_4Ti_3相沉淀行为模拟(英文)

运用相场方法研究双晶体NiTi形状记忆合金分别在无应力和有应力作用下时效过程中Ni4Ti3相的沉淀行为,模拟研究两种不同初始过饱和度(Ni含量分别为51.5%和52.5%,摩尔分数)的NiTi基体并考虑外加应力的影响。模拟结果表明,在无应力作用的体系中,当体系Ni原子初始浓度相对较低(51.5%Ni)时,Ni4Ti3相以非均匀的方式析出,其中晶界上存在大量的Ni4Ti3变体,晶界内大部分区域无变体;当体系Ni原子初始浓度较高(52.5%Ni)时,Ni4Ti3在整个双晶体系中均匀析出。在所研究的两种初始浓度下,一定大小的外加应力将直接导致Ni4Ti3变体在整个模拟体系中均匀析出,而两晶粒中变体的类型分布有所不同。     

Influence of Nb content on martensitic transformation and mechanical properties of TiNiCuNb shape memory alloys

In present work, microstructure, martensitic transformation behavior and mechanical properties of (Ti₅₀Ni₄₀Cu₁₀)_100−xNb_x (x = 0, 5, 10, 15 at.%) alloys were investigated as a function of Nb content. The addition of Nb to TiNiCu alloy leads to the presence of β-Nb phase. During cooling and heating, the alloys show one-step B2 ↔ B19 transformation. As the Nb content increases, the transformation temperatures almost linearly decrease and the transformation hysteresis monotonously increases due to the decrease of middle eigenvalue of the phase transformation matrix. The addition of Nb is effective in improving the elongation because of the introduction of β-Nb phase. With the increase of Nb content, both the yield strength and the critical stress to induce martensitic transformation increase, resulting in the improved superelastic strain.     

Effect of hydrostatic pressure on martensitic transformation in a ferromagnetic shape memory alloy Ni2MnGa

Ni₂MnGa transforms in the cooling process from the parent (P-) phase to the intermediate (I-) phase and then to the martensite (M-) phase. Under a uniaxial stress, a new phase (X-phase) appears, and the P → I transformation distinctively separates to the P → X, and the X → I transformations. In the present study, we have studied the effect of hydrostatic pressure on these transformation temperatures. As a result, we found that the distinct separation described above does not occur under a hydrostatic pressure of up to 0.9 GPa. The P → X and the I → M transformation temperatures increase with increasing hydrostatic pressure, and the ratio is 13.1 K/GPa and 16.2 K/GPa, respectively. The change in molar volume and/or thermal expansion coefficient associated with the transformation is estimated from these values.     

Martensitic transformation,shape memory effect and superelasticity of Ti-xZr-(30-x)Nb-4Ta alloys

Martensitic transformations,mechanical properties,shape memory effect and superelasticity of Ti-xZr-(30-x)Nb-4Ta(x=15,16,17 and 18;at%) alloys were investigated.X-ray diffraction(XRD),optical microscopy(OM) and transmission electron microscopy(TEM) results indicated that the Ti-16Zr-14Nb-4Ta,Ti-17Zr-13Nb-4Ta and Ti-18Zr-12Nb4Ta alloys were mainly composed of α″-martensite,while the Ti-15Zr-15Nb-4Ta alloy was characterized by predominant p phase.The reverse martensitic transformation temperatures increased when Nb was replaced by Zr,indicating stronger p-stabilizing effect for the former.The Ti-15Zr-15Nb-4Ta alloy displayed superelasticity during tensile deformation with a recovery strain of 3.51%.For the other three alloys with higher Zr content,the martensitic reorientation occurred during tensile deformation,resulting in shape memory recovery upon subsequent heating.The maximum shape memory effect was 3.46% in the Ti-18Zr-12Nb-4Ta alloy.