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.     

Transformation hysteresis and shape memory effect of an ultrafine-grained TiNiNb shape memory alloy

In present work, transformation hysteresis and shape memory effect of an ultrafine-grained Ti₄₄Ni₄₇Nb₉ alloy processed by ECAP were studied. After deformation, the ECAPed sample showed a much wider transformation hysteresis than the initial sample due to the enlarged strength mismatch between matrix and β-Nb phase. The shape memory effect and its cycling stability of the ECAPed sample were obviously improved.     

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.     

Microstructure and martensitic transformation of an ultrafine-grained TiNiNb shape memory alloy processed by equal channel angular pressing

Microstructure, martensitic transformation and mechanical properties of an ultrafine-grained Ti₄₄Ni₄₇Nb₉ shape memory alloy processed by equal channel angular pressing were investigated. The as-ECAP processed sample is characterized by an inhomogeneous and refined microstructure. In β-Nb phase-rich region, the grains of matrix are elongated with high density dislocations. In β-Nb phase-free region, the microstructure is partial recovery and characterized by near-equiaxed grains. The heterogeneous microstructure is attributed to presence of β-Nb phase. Martensitic transformation behavior of the as-ECAP processed sample is characterized by a single-stage transformation. The thermal cycling stability of transformation and the mechanical properties are considerably improved due to a strengthening effect resulting from refined grain size and high dislocation density.     

DEFORMATION BEHAVIOUR OF Cu－Zn－Al ALLOYS AND ITS EFFECT ON TRANSFORMATION HYSTERESIS

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Ni—Ti—Nb宽滞后形状记忆合金的形变诱发马氏体相变及其可逆性

用透射电镜、高分辨电镜、不同温度下的拉伸试验以及电阻率－温度曲线测试研究了Ｎｉ－Ｔｉ－Ｎｂ合金形变诱发马氏体相变及其可逆性，分析了形变诱发马氏体的稳定性和可逆性与其变体界面结构之间的关系。结果表明，Ｎｉ－Ｔｉ－Ｎｂ合金在Ｍｓ－Ｍｓ＾σ温度区间加应力时发生应力诱发马氏体相变，而在Ｍｓ＾σ以上温度加应力时，发生应变诱发马氏体相变。形变对Ｎｉ－Ｔｉ－Ｎｂ合金的应力诱发马氏体界面结构有明显影响，随着拉伸变     

Microstructural evolution in NiTi alloy subjected to surface mechanical attrition treatment and mechanism

Both nanocrystalline and amorphous phases are observed from the near surface of nickel titanium shape memory alloy (NiTi SMA) with the B2 austenite phase after surface mechanical attrition treatment (SMAT). The microstructure and phase changes are systematically studied by cross-sectional and plane-view transmission electron microscopy. The strain induces grain refinement and it is accompanied by increased strain in the surface layer triggering the onset of highly dense dislocations and dislocation tangles (DTs), formation of the martensite plate via stress-induced martensite (SIM) transformation (B2 to B19′), and dislocation lines (DLs) as well as dense dislocation walls (DDWs) inside the martensite plate leading to the subdivision of the martensite plate. In addition, reverse martensite transformation (B19′ to B2) and amorphization take place concurrently in the surface region, and successive subdivision and amorphization finally result in the formation of well separated nanocrystalline and amorphous phases in the near surface. The average grain size of the nanocrystallites is about 20 nm. Owing to the almost complete reverse martensite transformation as well as thermal stability, the strain-induced nanocrystalline structure has the B2 austenite phase in the surface layer and no transformation occurs.     

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.     

低应力／应变幅控制的对称拉压疲劳对Ti—49.6Ni合金相变行为的影响

采用电阻法研究了TiNi形状记忆合金在对称拉压应力或应变疲劳条件下的相变行为,结果表明,经过低应力/应变疲劳,材料发生R相变,试样的相变温度变得稳定,马氏体相变温度Ms和逆相变温度As低于未疲劳试样,随着循环次数的增加,应变疲劳后材料的Ms点、As点与循环次数无明显关系,材料具有良好的相变稳定性。     

Ti-Ni-Nb宽滞后形状记忆合金的应力诱发马氏体相变行为

用在（Ms＋30℃）温度下的拉伸实验和差示扫描量热仪（DSC）较系统地研究了Ti44Ni47Nb9宽滞后形状记忆合金应力诱发马氏体的相变行为。研究结果表明：当形变量达到14％左右时，应力诱发马氏体相变过程基本完成。应力诱发马氏体的逆相变温度间隔要比热诱发马氏体约小一个数量级。形变对该合金应力诱发马氏体的逆转变开始温度、逆转变温度间隔以及相变潜热均有明显影响，随着拉伸变形量的增加而增加。而在随后的冷却循环中，相变潜热和马氏体相变开始温度均随着形变的增加缓慢降低。     

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 借助于X射线衍射，研究了C、Mn、Cr和Ni 含量对304奥氏体不锈钢拉伸力学性能和应变诱发 马氏体相变倾向的影响。结果表明：C、Mn、Cr和Ni在允许的成分范围内变化，应变诱发α′马氏体相变倾向差异很大，这导致屈服强度和抗拉强度复杂的变化，尽管应变诱发α′马氏体相变使加工硬化速率提高，相变可以诱发塑性，但相变速率较快，相变倾向较大的钢塑性反而下降，此外，由于室温变形还增大热诱发马氏体相变倾向，从而限制了C、Mn、Cr和Ni下限钢在高精度和低温环境下构件的应用。     

Strain-induced martensitic transformation in stainless steels: A three-dimensional phase-field study

A three-dimensional elastoplastic phase-field model is developed to study the microstructure evolution during strain-induced martensitic transformation in stainless steels under different stress states. The model also incorporates linear isotropic strain hardening. The input simulation data is acquired from different sources, such as CALPHAD, ab initio calculations and experimental measurements. The results indicate that certain stress states, namely uniaxial tensile, biaxial compressive and shear strain loadings, lead to single variant formation in the entire grain, whereas others, such as uniaxial compressive, biaxial tensile and triaxial strain loadings, lead to multivariant microstructure formation. The effects of stress states, strain rate as well as temperature on the mechanical behavior of steels are also studied. The material exhibits different yield stresses and hardening behavior under different stress states. The equivalent stress is higher at low strain rate, whereas a higher elongation is obtained at high strain rate. The deformation temperature mainly affects the hardening behavior of the material as well as the transformation, i.e. martensite volume fraction decreases with increasing temperature. Some of the typical characteristics of strain-induced martensite, such as the formation of thin elongated martensite laths, shear band formation and nucleation of martensite in highly plasticized areas, as well as at shear band intersections, are also observed.     

Effects of nanoprecipitation on the shape memory and material properties of an Ni-rich NiTiHf high temperature shape memory alloy

Shape memory properties of a Ni_50.3Ti_29.7Hf₂₀ (at.%) polycrystalline alloy were characterized after selected heat treatments. The effects of heat treatment temperature and time on the transformation temperatures (TTs) and temperature hysteresis were determined by differential scanning calorimetry. Thermal cycling under constant compressive stress was carried out to reveal the changes in transformation strain, temperature hysteresis, and TT as a function of stress. Isothermal stress cycling experiments were conducted to reveal the critical stresses, transformation strain, and stress hysteresis as a function of temperature. The crystal structure and lattice parameters of the transforming phases were determined by X-ray diffraction at selected temperatures. Precipitate characteristics and martensite morphology were revealed by transmission electron microscopy. Precipitation was found to alter the martensite morphology and significantly improve the shape memory properties of the Ni-rich NiTiHf alloy. For the peak aged condition shape memory strains of up to 3.6%, the lowest hysteresis, and a fully reversible superelastic response were observed at temperatures up to 240 °C. In general, the nickel-rich NiTiHf polycrystalline alloy exhibited a higher work output (≈16.5 J cm^?3) than other NiTi-based high temperature alloys.     

Energetic evaluation for inducing the thermoelastic martensitic transformation by mechanical stress in Cu–Zn–Al single crystals

The ease with which the thermoelastic martensitic transformation is induced by mechanical stress in four Cu–Zn–Al single crystals with pseudoelasticity effect has been studied. Mechanical tests have been carried out at different temperatures, and show an increase in the slope of the stress–strain curve corresponding to the β → stress-induced martensite (SIM) transformation with an increase the test temperature. The relationship between the slope and the temperature follows a power law equation, depending on the chemical composition, transformation temperature and defect density as the most important factors. The stored energy has been calculated from the stress–strain curves as the area under the curve when SIM transformation is finished, and the friction energy as the area of the mechanical hysteresis, for four single crystals of Cu–Zn–Al alloys. The results indicate that when the M_s of the alloy increases, a decrease in stored energy can be observed. Also, the increase of the test temperature produces an increase in the stored energy.     

In-situ synchrotron X-ray diffraction investigation on deformation behavior of Nb/NiTi composite during pre-straining process

The mechanisms responsible for deformation behavior in Nb/NiTi composite during pre-straining were investigated systematically using in-situ synchrotron X-ray diffraction, transmission electron microscopy and tensile test. It is shown that upon loading, the composite experiences elastic elongation and slight plastic deformation of B19′, B2 and β-Nb phases, together with the forward stress-induced martensitic (SIM) transformation from B2 to B19′. Upon unloading, the deformation mechanisms of the composite mainly involve elastic recovery of B19′, B2 and β-Nb phases, compression deformation of β-Nb phase and incomplete B19′→B2 reverse SIM transformation. In the tensile loading?unloading procedure, besides the inherent elastic deformation and SIM transformation, the (001) compound twins in B19′ martensite can also be conducive to the elastic deformation occurring in B19′-phase of the composite. Therefore, this composite can exhibit a large recoverable strain after unloading owing to the elastic deformation, and the partially reversible and consecutive SIM transformation together with the (001) compound twins.     

Texture development and strain hysteresis in a NiTi shape-memory alloy during thermal cycling under load

Thermal cycling experiments were conducted on a NiTi shape-memory alloy at different constant applied stresses below the yield strength of the martensite. The mechanical strain response manifested as strain hysteresis loops, whose range was proportional to the applied stress. In situ neutron diffraction experiments show that the strain hysteresis occurs as a result of the establishment of a stress-dependent crystallographic texture of the martensite during the first cool-down from austenite, and thereafter repeated during thermal cycling under the same load. This texture is found to depend on the stress during the thermal cycling experiments. A strain-pole map is derived and shown to explain the observed texture during thermal cycling. The strain-pole methodology is shown to work with similar martensitic transformations in other material systems.     

Nb合金化对电弧增材制造NiTi基形状记忆合金的影响

NiTi合金是一种应用广泛的形状记忆合金,其中Ti₄₇Ni₄₄Nb₉成分的合金是一种可靠的航空管接头材料. 采用双丝电弧增材制造(WAAM)的方法制备了Ni₅₂Ti₄₈合金,并以Nb元素进行了原位合金化得到了Ti₄₇Ni₄₄Nb₉合金,研究了其典型组织、压缩性能、相变温度与形状记忆效应,分析了Nb元素的添加对WAAM镍钛合金组织及性能的影响. 结果表明,加入Nb元素后,合金的组织除B2相晶粒外,还在晶界处有细小的β-Nb相析出,使得合金的压缩强度在横向与纵向上分别增加了7.9%与3.1%,形状记忆回复率则下降了4.0%,相变温度滞后从?6.4 ℃提升至40.9 ℃,使得该材料作为记忆合金管接头时更加利于储存与装配.     

Full-field strain evolution during intermartensitic transformations in single-crystal NiFeGa

Using in situ digital image correlation to obtain full-field measurements, we study the intermartensitic transformations in single-crystal NiFeGa. Full-field strain measurements identify the coexistence of modulated martensite phases during the first plateau of the multistage stress–strain curve at room temperature. At a higher temperature, the measurements indicate the bypassing of one of the modulated phases. Strain as high as 13% is measured as a result of the transformation to the intermediate monoclinic modulated and final tetragonal phase. Based on the full-field strain measurements, the phase fractions during the nucleation and the progression of the transformation are obtained. The evolution of the local strain and the phase fractions prove critical in explaining strain softening, hysteresis and other phenomena observed in the stress–strain curves.     

Strain glass in defect-containing ferroelastic Ti_(44)Ni_(51)Nb_5 alloy

An abnormal phenomenon was investigated in Ti_(44)Ni_(51)Nb_5 alloy which exhibits an absence of martensite transformation and a negative temperature dependence of electrical resistivity with the temperature decreasing.X-ray diffraction(XRD) analysis shows the matrix phase keeps a B2 structure during cooling without a martensite transformation,and dynamic mechanical analysis displays a frequency-dependent AC modulus/loss anomaly at T_g according to Vogel-Fulcher relation.Simultaneously,transmission electronic microscope(TEM) analysis manifests the superlattice spots at near 1/3 commensurate position,implying an existence of strain nanodomains with an R-like structure.And above experiment results provide evidence for the "strain glass" transition in defect-containing ferroelastic Ti_(44)Ni_(51)Nb_5 alloy.     

One-dimensional model of martensitic transformations

A modified Landau–Ginzburg theory for martensitic transformations is suggested. An accommodation strain energy in parent phase is added into the free energy of the system besides the Landau free energy and gradient energy which have been generally considered. Both the equilibrium state and the dynamic process of martensitic transformations are investigated by solving the equation of motion numerically. Two important features of martensitic transformations, i.e., transformational hysteresis and athermal transformation, can be displayed by the modified model. The results of numerical study of the model agree with experiments well, including surface martensite, thermoelastic and nonthermoelastic martensite formations, autocatalysis and burst transformation.