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)     

Shape Memory Effect and Superelasticity in Ti–Ni and Fe–Ni–Co–Ti Single Crystals

The dependence of shape memory effect and superelasticity on the crystal orientation, size, and particle volume fraction is studied for Ti–Ni and Fe–Ni–Co–Ti single crystals. The shape memory effect and superelasticity are shown to decrease with increase in the particle volume fraction and depend on the number of crystallographic variants of particles that can vary due to aging under loading. The influence of disperse particles undergoing no martensite transformations on the martensitic crystal growth and their fine structure is examined.     

Martensitic transformation and shape memory effect of a Ni–Fe–Ga polycrystalline alloy

The martensitic transformation and shape memory effect of Ni—23.9 wt%–Fe—28.7 wt%Ga polycrystalline alloy are studied in the present paper. It is shown that martensitic transformation temperatures and its reverse transformation temperatures are all raised by annealing treatment. A large reversible transformation strain, about 2100ppm, is obtained in Ni—23.9 wt%–Fe—28.7 wt%Ga polycrystalline alloy due to martensitic transformation and its reverse transformation. This transformation strain is also increased by the external magnetic field. It is believed that the effect of field on the preferential orientation of martensitic variants when transforming increases the transformation strain.     

Ni4Ti3沉淀相对NiTi形状记忆合金相变行为的影响

采用第二近邻修正型嵌入原子势的分子动力学方法,建立了共格沉淀相与半共格沉淀相块状/柱状模型,模拟了温度诱发相变和应力诱发相变,分析了Ni₄Ti₃沉淀相对Ni Ti形状记忆合金相变行为的影响．结果表明,Ni₄Ti₃沉淀相本征应变诱发的弹性应力场对相变中马氏体变体类型、形核位置、分布等有重要影响.在温度诱发相变时,共格沉淀相促进部分马氏体变体的形核生长,能显著提高Ni Ti超弹性形状记忆合金的马氏体相变开始温度;在应力诱发相变时,Ni₄Ti₃沉淀相使马氏体早于无沉淀相区域形核,导致了相变应力降低、抑制了马氏体解孪,减小了应力-应变曲线的滞回环.     

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.     

High temperature strain glass in Ti–Au and Ti–Pt based shape memory alloys

Strain glass is a frozen short-range strain ordered state found in shape memory alloys recently, which exhibits novel properties around the ideal glass transition temperature T₀. However, the T₀ of current strain glass systems is still very low, limiting their potential applications and experimental studies. In this paper, we reported two new strain glass systems with relatively high T₀. In Ti₅₀Au_50-xCr_x alloys, the strain glass appears at x = 25, and exhibits a T₀ of 251 K, while in Ti₅₀Pt_50-yFey alloys, the strain glass takes place at y = 30, and shows a T₀ of 272 K. Both of them are comparable with the highest T₀ value reported so far. Moreover, the phase diagrams of main strain glass systems in Ti-based alloys were summarized. It is found that the influence of the martensitic transformation temperature of the host alloy on the T₀ of the strain glass is limited. This work may help to design new strain glass systems with higher T₀ above ambient temperature.     

Ni54Fe19Ga27单晶的形状记忆和超弹性

本文研究了单晶Ni₅₄Fe₁₉Ga₂₇不同方向的形状记忆效应、超弹性和磁性. 研究发现,单晶样品具有良好的双向形状记忆效应.不同晶体学方向的相变应变随着热循环次数的变化而改变. 在外应力作用下,通过应力诱发马氏体相变,样品在[001],[110],[111]方向分别产生了3.3%, 2% 和3%的可回复应变平台.磁性测量结果表明马氏体的磁晶各向异性能约为4.8× 10⁵ erg/cm³,远远小于变体孪生所需机械应力能,因此磁场的作用是使磁矩发生转动而不是使孪晶界移动, 成功揭示了不能在NiFeGa中获得大磁感生应变的物理根源.     

Alternate stresses and temperature variation as factors of influence of ultrasonic vibration on mechanical and functional properties of shape memory alloys

It is known that the main factors in a variation in the shape memory alloy properties under insonation are heating of the material and alternate stresses action. In the present work the experimental study of the mechanical behaviour and functional properties of shape memory alloy under the action of alternate stresses and varying temperature was carried out. The data obtained had demonstrated that an increase in temperature of the sample resulted in a decrease or increase in deformation stress depending on the structural state of the TiNi sample. It was shown that in the case of the alloy in the martensitic state, a decrease in stress was observed, and on the other hand, in the austenitic state an increase in stress took place. It was found that action of alternate stresses led to appearance of strain jumps on the strain–temperature curves during cooling and heating the sample through the temperature range of martensitic transformation under the constant stress. The value of the strain jumps depended on the amplitude of alternate stresses and the completeness of martensitic transformation. It was shown that the heat action of ultrasonic vibration to the mechanical behaviour of shape memory alloys was due to the non-monotonic dependence of yield stress on the temperature. The force action of ultrasonic vibration to the functional properties was caused by formation of additional oriented martensite.     

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.     

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.     

Neutron-irradiation-induced shape memory effect in a TiNi alloy

The recovery of inelastic strains in Ti-Ni alloy samples irradiated in a nuclear reactor under isothermal conditions was studied. Before irradiation, the cylindrical samples were compressed to a residual strain of 3–6% in the martenstici state at room temperature. The samples were irradiated at a temperature of 45°C, which does not exceed the temperature of the onset of the reverse martensitic transformation A _S . Irradiation with a fastneutron fluence of 5 × 10²⁰ cm^?2 is established to result in the recovery of the residual strain. The value of the recoverable strain is comparable to that observed under the conditions of the shape memory effect on heating of the deformed alloy and even somewhat exceeds it. The obtained data show that neutron irradiation can induce the shape-memory effect in the TiNi alloy. This is due to a decrease in the temperatures of the martensitic transformations under irradiation.     

Si掺杂的铁磁形状记忆合金Co50Ni21Ga29Six的物性研究

成功生长了Co₅₀Ni₂₁Ga₂₉:Si(x=1,2)单晶样品,对其磁性,马氏体相变及其相关性质进行了细致的测量.发现掺Si成分的单晶具有非常迅速的马氏体相变行为、2.5％的大相变应变、大于100 ppm的磁感生应变和4.5％的相变电阻.进一步研究指出,在CoNiGa合金中掺入适量Si元素,能够降低材料的马氏体相变温度,减小相变热滞后,提高材料的居里温度,并使得磁性原子的磁矩有所降低.尤其重要的是Si元素的添加能够增大材料马氏体的磁晶各向异性能,改善马氏体变体的迁移特性,从而获得更大的磁感生应变. 关键词： 铁磁形状记忆合金 Heusler合金 50Ni₂₁Ga₂₉Si_x')" href="#">Co₅₀Ni₂₁Ga₂₉Si_x     

Effect of the temperature and way of preliminary torsion in the austenitic state on the shape memory effect in TiNi alloy

The effect of temperature and direction of preliminary torsion in the austenitic state on the degree of strain recovery upon heating of a TiNi alloy has been investigated. It is shown that an increase in the preliminary deformation temperature from 500 to 700 K leads to an increase in the degree of shape recovery upon heating of the material studied. In particular, a 20% strain at a temperature of 500 K decreases the recovery coefficient by 20%, whereas the same preliminary strain at 700 K deteriorates the shape recovery by only 4%. It is established that, applying preliminary torsion in the austenitic and martensitic states in opposite directions, one can obtain an increase in the shape memory strain with an increase in the preliminary plastic strain. Thus, at some plastic strains (λ _pl > 10%), the strain recovered upon heating may even exceed the strain set in the martensitic state.     

Transformation plasticity and shape memory effects in TiNi alloy after low-temperature treatment

The effect of low-temperature annealing on the structure, kinetics of martensitic transformations, and functional properties of an equiatomic TiNi shape memory alloy is studied. Low-temperature annealing of the TiNi alloy is shown to decrease the temperature of the end of the forward martensite transformation M _f and the temperature of the onset of the reverse transformation A _s , which increases the transformation temperature range. As a result, the shape memory effect is improved due to a decrease in the irreversible strain. These phenomena are assumed to be caused by the hardening of the TiNi alloy induced by low-temperature annealing.     

The effect of Si content on the martensitic transformation temperature of Ni55.5Fe18Ga26.5-xSix alloys

This paper investigates the effects of substitution of Si for Ga on the martensitic transformation behaviours in Ni-Fe-Ga alloys by using optical metallographic microscope and differential scanning calorimetry (DSC) methods. The structure type of Ni55.5Fe18Ga26.5-xSix alloys is determined by x-ray diffraction (XRD),and the XRD patterns show the microstructure of Ni-Fe-Ga-Si alloys transformed from body-centred tetragonal martensite (with Si content x = 0) to body-centred cubic austenite (with x = 2) at room temperature. The martensitic transformation temperatures of the Ni55.5Fe18Ga26.5-xSix alloys decrease almost linearly with increasing Si content in the Si content range of x ≤ 3. Thermal treatment also plays an important role on martensitic transformation temperatures in the Ni-Fe-Ga-Si alloy. The valence electronic concentrations,size factor,L21 degree of order and strength of parent phase influence the martensitic transformation temperatures of the Ni-Fe-Ga-Si alloys. An understanding of the relationship between martensitic transformation temperatures and Si content will be significant for designing an appropriate Ni-Fe-Ga-Si alloy for a specific application at a given temperature.     

Pressure effects on magnetic properties and martensitic transformation of Ni–Mn–Sn magnetic shape memory alloys

The mechanism for the effects of pressure on the magnetic properties and the martensitic transformation of Ni-Mn- Sn shape memory alloys is revealed by first-principles calculations. It is found that the total energy difference between paramagnetic and ferromagnetic austenite states plays an important role in the magnetic transition of Ni-Mn-Sn under pressure. The pressure increases the relative stability of the martensite with respect to the anstenite, leading to an increase of the martensitic transformation temperature. Moreover, the effects of pressure on the magnetic properties and the martensitic transformation are discussed based on the electronic structure.     

Nanoscopic size effects on martensitic transformations in shape memory alloys

The effect of the grain size and transverse film size in nano-and micrometer ranges on the parameters of martensitic transitions in shape memory alloys is theoretically considered in the framework of the theory of diffuse martensitic transitions. A quantitative analysis of the size effects is performed including not only the thermodynamic aspect of the martensitic transformation but also its kinetic aspect, which is particularly sensitive to structural and size factors. This complex approach makes it possible to explain the following three basic facts associated with the influence of a decreased grain size or transverse film size on the parameters of the martensitic transition in shape memory alloys: a decrease in the critical (characteristic) transition temperature, an increase in the transition temperature smearing, and the existence of a critical grain size or film thickness below which the martensitic transformation in alloys is blocked.     

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.     

Martensitic transformation of TiNiPd high-temperature shape memory alloys: A first-principles study

Heat of formation, elastic property and electronic structure of TiNiPd high-temperature shape memory alloys have been investigated by first-principles calculations using the pseudopotentials plane-wave method. The results show that the heat of formation difference between austenite and martensite plays an important role in the martensitic transformation. The effect of Pd content on the martensitic transformation temperature and transformation type is clarified based on the elastic constants of the B2 phases. High martensitic transformation temperature can be attributed to a low shear resistance C′. Furthermore, the mechanism of the effect of Pd addition on elastic constants is explained on the basis of the electronic structure.     

New aspects of martensite stabilization in Ni–Mn–Ga high-temperature shape memory alloy

We report on new aspects of martensite stabilization in high-temperature shape memory alloys. We show that, due to the difference in activation energies among various structural defects, an incomplete stabilization of martensite can be realized. In material aged at high temperatures, this gives rise to a variety of unusual features which are found to occur in the martensitic transformation. Specifically, it is shown that both forward and reverse martensitic transformations in a Ni–Mn–Ga high-temperature shape memory alloy can occur in two steps. The observed abnormal behaviour is evidence that, in certain circumstances, thermoelastic martensitic transformation can be induced by diffusion.