Size effects under martensitic deformation of shape-memory alloys

The effect of the grain size and film thickness on the parameters of martensitic transitions and deformation behavior of shape-memory alloys is studied in terms of the theory of smeared martensitic transitions. Comparison of theoretical results with experimental data suggests that the transformation kinetics related to the growth mechanism and shrinkage of martensite lamellas when transformation steps (dislocations) move along their boundaries is most sensitive to the size factor.     

Martensitic transformation and related magnetic effects in Ni-Mn-based ferromagnetic shape memory alloys

Ferromagnetic shape memory alloys, which undergo the martensitic transformation, are famous multifunctional materials. They exhibit many interesting magnetic properties around the martensitic transformation temperature due to the strong coupling between magnetism and structure. Tuning magnetic phase transition and optimizing the magnetic effects in these alloys are of great importance. In this paper, the regulation of martensitic transformation and the investigation of some related magnetic effects in Ni-Mn-based alloys are reviewed based on our recent research results.     

Martensitic transformation in Ni-Mn-Ga alloys

Single crystals of different compositions in shape memory alloys Ni-Mn-Ga have been studied by electron and low temperature X-ray diffraction as well as by differential scanning calorimetry. It is shown that the cooling-induced martensitic phases are long-periodic ones modulated along the (110) directions by a transverse wave of atomic shifts with 5 and 7 atomic layers periodicity for the alloys studied exhibiting a martensitic transformation at 180 K and 446 K, respectively. The transformation heats appeared to be about 10 times different for both alloys.     

Shape memory effect and superelasticity in a strain glass alloy

The shape memory effect and superelasticity are usually found in alloys exhibiting spontaneous martensitic transformation. Thus it is hard to imagine that such interesting effects can appear in a system without a martensitic transformation. In this Letter we show shape memory and the superelasticity effect in a nonmartensitic Ti48.5Ni51.5 alloy, which has no martensitic transformation but undergoes a "strain glass" transition. In situ x-ray diffraction experiment showed that the shape memory and superelasticity in strain glass stem from a stress-induced strain glass to martensite transformation and its reverse transformation. The new shape memory and superelasticity in strain glass extends the regime of the shape memory effect and superelasticity and may lead to novel applications.     

Behavior of the nickel-titanium alloys with the shape memory effect under conditions of shock wave loading

The behavior of the Ti_51.1Ni_48.9 and Ti_49.4Ni_50.6 alloys with shape memory effects has been studied under submicrosecond shock wave loading in the temperature range from −80 to 160°C, which includes both the regions of the stable state of the specimens in the austenite and martensite phases and the regions of thermoelastic martensitic transformations. The grain size of the studied alloys varies from initial values 15–30 to 0.05–0.30 μm. The dependences of the dynamic elastic limit on the temperature and on the elemental composition are similar to the dependences of the yield stress of these alloys under low strain rate loading. The rarefaction shock wave formation as a consequence of the pseudoelastic behavior of the alloy during a reversible martensitic transformation has been revealed. A decrease in the grain size leads to an increase in the dynamic elastic limit and decreases the temperatures of martensitic transformations.     

Modelling of hysteresis loops taken during the stress- and temperature-induced martensitic transformations

In this article, it has been shown that the process of reconfiguration of the crystal defects system noticeably contributes to the width of the stress–strain and strain–temperature hysteresis loops taken during the stress- and temperature-induced martensitic transformations of the shape memory alloys. It has been demonstrated that the contribution of the defects system to the hysteresis width strongly depends on the alloy temperature and the transformation cycle duration. It has been shown that the hysteresis effect can be observed not only in the course of the first-order phase transition of martensitic type, but also in the course of the gradual deformation of crystal lattice. The obtained results are applicable to the ferroelastic phase transitions in the different crystalline solids.     

Effect of cobalt doping on thermoelastic martensitic transformations and physical properties of magnetic shape memory alloys Ni50 − x Co x Mn29Ga21

The magnetic and thermoelastic martensitic transformations and physical properties (magnetization, electrical resistivity, thermoelectric power, relative elongation, and thermal expansion coefficient) of multicomponent magnetic shape memory alloys Ni_{50 ? x} Co _x Mn₂₉Ga₂₁ (x = 0, 1, 2, 3, 10 at %) have been investigated. The critical temperatures of thermoelastic martensitic transformation and magnetic transitions have been determined. It has been found that the alloy with 10 at % Co undergoes a martensitic transformation in the temperature range of 6–10 K.     

Ni-Fe-Ga磁致形状记忆合金中间马氏体相变

采用差示扫描量热和x射线衍射技术研究Ni-Fe-Ga磁致形状记忆合金的马氏体相变行为.结 果发现,在多晶Ni565_56.5Fe190_19.0Ga245_{2 4.5}和Ni563_56.3Fe170_17.0 Ga267_26.7合金中除马氏体相变外,还观察到一次完整的、正相变和逆相 变对应出现、单 纯由温度诱发的中间马氏体相变.该中间马氏体相变与马氏体相变均为热弹性相变. 关键词： Ni-Fe-Ga 中间马氏体相变 磁致形状记忆合金     

The influence of Al substitution on the phase transitions and magnetocaloric effect in Ni43Mn46Sn11−xAlx alloys

The effects of Al substitution on the phase transitions and magnetocaloric effect of Ni₄₃Mn₄₆Sn_11−xAl_x (x=0-2) ferromagnetic shape memory alloys were investigated by X-ray diffraction and magnetization measurements. With the increase of Al content, the cell volume decreases due to the smaller radius of Al, and the martensitic transformation temperature increases rapidly, while the Curie temperature of austenitic phase shows a small increase. A large positive and a negative magnetic entropy change were observed near the first-order martensitic transition and the second-order magnetic transition, respectively. The magnetic entropy changes, hysteresis behavior, and refrigerant capacity near the two transitions are compared.     

Effect of the Conditions of Crystallization of Rapidly Quenched TiNiCu Amorphous Alloys with High Copper Contents on the Characteristics of Martensite Transformations and Shape Memory Effect

Amorphous TiNiCu alloys with copper contents of 28 to 38 at % are fabricated via melt spinning. The isothermal crystallization of alloys is conducted at 500°C with variable durations of the heat treatment (100 to 300 s). It is shown that shortening the duration of crystallization prevents the formation of brittle phases of the Ti–Cu system and contributes to the martensitic B2 ? B19 transformation, with the temperature intervals of transformation shifting to higher values and a marked increase in the enthalpy of the martensitic transitions and the magnitude of the shape memory effect.     

CoNiZ系列合金的结构和马氏体相变性质

利用X射线衍射研究了CoNiZ(Z＝Si，Sb，Sn，Ga 等)合金在不同热处理条件下的相组成.当Z元素为Sn，Sb时，材料是完全的B2结构；但Z为Si时，材料变成面心立方的γ相.形成B2还是γ相由电子浓度和原子尺寸效应两种因素共同决定.而CoNiGa的研究结果表明，在合金中除了形成B2结构的同时还容易形成γ相，常表现出两相共存的特性.对材料进行不同方式的热处理可以使合金中两相的含量有所消长,γ相含量的多少对CoNiGa合金的马氏体相变有很大的影响.分析指出，两相共存及其所带来的物性变化是CoNiGa铁磁性形状记忆合金非常有利用价值的物理性质. 关键词： Heusler合金 马氏体相变 γ相     

Martensitic transformations in some ferrous and non-ferrous alloys under magnetic field and hydrostatic pressure

Martensitic transformations are extensively influenced by external fields, such as temperature and uniaxial stress, in transformation temperatures, crystallography and amount and morphology of the product martensites. Therefore, to clarify the effect of external fields on martensitic transformations it is very important to understand the essential problems of the transformation, such as thermodynamics, kinetics and the origin of the transformation, whose information is naturally useful in technological applications using the transformation. Magnetic field and hydrostatic pressure are important in such external fields because there exist some significant differences in magnetic moment and atomic volume between the parent and martensitic states. In the present paper, therefore, we summarizz the effects of magnetic field and hydrostatic pressure on martensitic transfonnations in some ferrous and non-ferrous alloys by referring to past and recent works made by our group and many other researchers. The transformation start temperatures of all the ferrous alloys examined increase with increasing magnetic field, but those of non-ferrous alloys, such as Ti-Ni and Cu-Al-Ni shape memory alloys, are not affected. On the other hand, the transformation start temperature decreases with increasing hydrostatic pressure in some ferrous alloys, but increases in Cu-Al-Ni alloys. The magnetic field and hydrostatic pressure dependencies of the martensitic start temperature are in good agreement with those calculated by our proposed equations.

During investigations of ferrous Fe-Ni-Co-Ti shape memory alloy, we found that a magnetoelastic martensitic transformation appears and, in addition, several martensite plates grow nearly parallel to the direction of the applied magnetic field in a specimen of Fe-Ni alloy single crystal.

We further found that the isothermal process in Fe-Ni-Mn alloy changes to athermal under a magnetic field and the athermal process changes to isothermal under hydrostatic pressure. Based on these facts, a phenomenological theory has been constructed, which unifies the two transformation processes.     

Shape memory characteristics in the L1 0 -fcc order-disorder transformation of FePd

Shape recovery through L1 0 -fcc order-disorder transformation of FePd is examined. Under a uniaxial compressive stress, a reversible shape change associated with the order-disorder transformation is observed. Shape memory characteristics (transformation strain, time required for the transformation and temperature hysteresis) for single-crystal and polycrystal specimens are determined by a compression test under a constant stress. The transformation strain (4% for a single crystal) and time required for disordering (about 10 s for a single crystal) are comparable with those of conventional martensitic shape memory alloys. The alloys can be used as shape memory materials with relatively high transformation temperature.     

Martensite-induced stress relaxation and deformation effects in membranes made of shape memory materials

The theory of diffuse martensitic transitions is used to analyze martensitic transformation and stress relaxation in a planar round membrane made of a shape memory material under a constant pressure. The plastic flexure of the membrane is found as a function of the temperature and applied pressure.     

Combined experimental and theoretical study on the effect of Nb content on martensitic transformation of NbRu shape memory alloys

The effect of Nb content on the martensitic transformation of NbRu high-temperature shape memory alloys is investigated by experiments and first-principles calculations. We calculate the lattice parameters, density of states, charge density, and heats of formation of Nb_50+xRu_50-x β phase. The results show that an increase in Nb content increases the stability of Nb_50+xRu_50-x β phase, leading to a significant decrease of the β to β' martensitic transformation temperature. In addition, the mechanism of the effects of Nb content on phase stability and martensitic transformation temperature is studied on the basis of electronic structure.     

Annealing effect on phase transformation in nano structured Ni–Mn–Ga ferromagnetic shape memory alloy

Nano structured Ni_52.6Mn_23.7Ga_24.3 alloy was prepared using the ball milling technique. High martensitic transition temperatures are observed in the range between 336 and 367 K. The X-ray diffraction profile revealed that annealed Ni–Mn–Ga powder at 1073 K displays mixture phases of austenite and martensite. Annealing at 1173 K induces phase transformation from mixture phase to Heusler L2₁ structure, which confirms the high-temperature shape memory effect. On the contrary, the milled sample shows no evidence of shape memory effect. Furthermore, annealing at higher temperature (1273 K) shows the accumulation of oxidation, which leads to the loss of shape memory effect. The grain size increases with increasing annealing temperature and causes deterioration in the soft magnetic properties.     

Strain glass behaviour of Ni–Co–Mn–Sn ferromagnetic shape memory alloys

We report the direct experimental observations of the glassy behaviour in Ni–Co–Mn–Sn ferromagnetic shape memory alloys by doping sufficient substitutional point defect Co into the Ni sites (9 at%). The results showed that high level of Co doping had caused the complete suppression of the martensitic transformation and introduction of a strain glass transition in Ni–Co–Mn–Sn alloys. The strain glass transition was definitively characterized by the dynamic mechanical anomalies following the Vogel–Fulcher relationship and the signature nonergodicity of the frozen glass using a zero‐field‐cooled/field‐cooled heating measurement of static strain. The findings clarified the cause of vanishing of the martensitic transformation in Ni–Co–Mn–Sn alloy with high Co doping levels and the generality of glassy state in Ni–Mn based ferromagnetic shape memory alloys with high level of foreign elements doping. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Electronic structures and magnetisms of the Co_2TiSb_(1-x)Sn_x(x= 0, 0.25, 0.5) Heusler alloys: A theoretical study of the shape-memory behavior

The total energy, electronic structures, and magnetisms of the Al Cu2Mn-type Co2TiSb1-xSnx(x = 0, 0.25, 0.5) with the different lattice parameter ratios of c/a are studied by using the first-principles calculations. It is found that the phase transformation from the cubic to the tetragonal structure lowers the total energy, indicating that the martensitic phase is more stable and that a phase transition from austenite to martensite may happen at a lower temperature. Thus, a ferromagnetic shape memory effect can be expected to occur in these alloys. The Al Cu2Mn-type Co2TiSb1-xSnx(x = 0, 0.25, 0.5) alloys are weak ferrimagnets in the austenitic phase and martensitic phase.     

Shape memory alloys — characterization techniques

Shape memory alloys are the generic class of alloys that show both thermal and mechanical memory. The basic physics involved in the shape memory effect is the reversible thermoelastic martensitic transformation. In general, there exists two phases in shape memory alloys, viz., a high-temperature phase or austenitic phase (A) and a low-temperature phase or martensitic phase (M). In addition, an intermediate R phase exists in some special cases. The M↔A transformation is associated with a recoverable strain of about 6.5–8% and the R↔A transformation is associated with a recoverable strain of about 1%. The former transformation has been widely used in the applications like antenna deployment of satellite, aerospace couplings, orthodontic arch wires, medical guide wires for diagnostic and therapeutic catheters and other industrial applications. Our group has been giving emphasis to the characterization techniques for R phase, using differential scanning calorimetry (DSC), electrical resistivity probe (ER) and thermomechanical analyzer (TMA). R phase is found to have attractive features like stability against thermal cycling, a small thermal hysteresis and a negligible strain recovery fatigue. DSC has been used successfully to characterize the recoverable strain parameters, apart from the determination of transformation temperatures. ER is used, for the systematic study of the dependence of various phases on heat-treatment temperatures. TMA has been effectively employed for the study of the mixed phases. A space-rotating platform is designed and fabricated, using an actuator of shape memory spring, for obtaining controlled rotations. The efficiency and the reliability of this actuator has been tested, over a million thermal cycles.     

Effects of pre-deformation on the martensitic transformation and magnetocaloric property in Ni-Mn-Co-Sn ribbons

This paper investigates the martensitic transformation and magnetocaloric effect in pre-deformed Ni-Mn-Co-Sn ribbons.The experimental results show that the reverse martensitic transformation temperature T M increases with the increasing pre-pressure,suggesting that pre-deformation is another effective way to adjust T M in ferromagnetic shape memory alloys.Large magnetic entropy changes and refrigerant capacities are obtained in these ribbons as well.It also discusses the origin of the enhanced martensitic transformation temperature and magnetocaloric property in pre-deformed Ni-Mn-Co-Sn ribbons.