Methods for obtaining two way memory effect and stressed two way memory effect of CuAlNi single crystals

1　ＩＮＴＲＯＤＵＣＴＩＯＮＴｗｏｗａｙｍｅｍｏｒｙｅｆｆｅｃｔ (ＴＷＭＥ)ｒｅｆｅｒｓｔｏｔｈｅｒｅｖｅｒｓｉｂｌｅｓｐｏｎｔａｎｅｏｕｓｓｈａｐｅｃｈａｎｇｅｂｙｈｅａｔｉｎｇａｎｄｃｏｏｌｉｎｇｂｅｔｗｅｅｎａ“ｈｏｔ”ａｕｓｔｅｎｉｔｅｓｈａｐｅａｎｄａ“ｃｏｌｄ”ｍａｒｔｅｎｓｉｔｅｓｈａｐｅｗｉｔｈｏｕｔａｎｙｍａｃｒｏ ｓｔｒｅｓｓａｐｐｌｉｃａ ｔｉｏｎ[1] .Ｉｎｏｔｈｅｒｗｏｒｄｓ ,ｔｈｉｓｅｆｆｅｃｔｉｓｉｎｔｒｉｎｓｉｃｗｈｉｃｈｉｓｄｉｓｔｉｎｇｕｉｓｈｅｄｆｒｏｍｓｔｒｅｓ…     

Two-way shape memory effect and its stability in Ti-Ni-Hf high temperature shape memory alloy

The two-way shape memory effect(TWSME) in a Ti36 Ni49 Hf15 high temperature shape memory alloy (SMA) was systematically studied by bending tests. In the TiNiHf alloy, the martensite deformation is an effective method to get two-way shape memory effect even with a small deformation strain. The results indicate that the internal stress field formed by the bending deformation is in the direction of the preferentially oriented martensite variants formed during the bending deformation. Upon cooling the preferentially oriented martensite variants form under such an oriented stress field, which should be responsible for the generation of the two-way shape memory effect.Proper training process benefits the formation of the oriented stress field, resulting in the improvement of the twoway shape memory effect. A maximum TWSME of 0.88% is obtained in the present alloy.     

Generation,development and degradation of the intrinsic two-way shape memory effect in different alloy systems

A uniaxial two-way shape memory effect is induced in wire specimens by a thermomechancial treatment. In order to enable a systematic variation of the microstructure with respect to dislocation density, second phase particles and grain size, the investigations are carried out on three different alloy systems. 4000 thermal cycles are performed on the trained shape memory elements, continuously observing the changes in the deformation behavior. The influence of work hardening, grain size and high internal stress-fields on the development and the stability of the intrinsic two-way shape memory effect is discussed, supported by microstructural investigations.The degradation due to cyclic application is related to instabilities in the high- and low-temperature modification. The decrease of the effect size is found to consist of two characteristic stages, where the decay is distinguished by different origin and rate. Definite conclusions about the degradation mechanisms are drawn by analyzing the influence of the preliminary heat treatment on the fatigue behavior. It is shown that the degradation can be minimized and hence the stability of the two-way shape memory effect can be optimized by establishing an appropriate microstructure.     

Compressive training of the shape memory alloy washer

This paper evaluates the performance and engineering aspects of 44Ti-47Ni-9Nb alloy in a pretensioning washer application. Previously the behavior of this alloy has mainly been studied for tensile predeformation with wires and strips up to 1 mm thickness. The present work investigated the effect of compression deformation and different homogenization temperatures on ring-shaped washers with larger dimensions. The best results with the compression-trained washer were achieved after homogenization at 1073 K where the compression training deformation was -10.4%, the lowest applied. The deformation training was carried out at 213 K near the martensitic reaction start temperature of the material. The expansion method, applied to the commercial reference ring, seemed to be a fairly efficient training method for the studied application, even though the parallel course of the radial surfaces was poor.     

Factors affecting the generation of stress-assisted two-way memory effect in NiTi shape memory alloy

The combined effects of martensite pre-deformation and constrained stress on the generation of stress-assisted two-way memory effect (SATWME) were studied. Particular attention was paid to the generation of internal stress during thermal cycling under constant stress. The result highlights that the maximum SATWME is not determined solely by the maximum internal stress developed. The SATWME strain when subjected to 18% martensite pre-strain coincides, independently of constrained stress applied and internal stress developed. This study also provides experimental evidence that when under 400 MPa constrained stress the maximum SATWME shifts toward higher pre-deformation amplitude and this phenomenon is comparable to the increased cold-work. The results further suggest that the mechanism that associates with the change of M_s temperature plays critical role in determining the SATWME.     

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.     

热循环对Fe-Mn-Si-Cr-Ni合金形状记忆效应的影响

采用电阻法和OM等手段研究了不同温度区间的热循环对Fe-14Mn-5.5Si-8Cr-5Ni合金微观组织和形状记忆效应的影响。结果表明:298 K～353 K、520 K、773 K和77 K～773 K热循环1次后,合金形状回复率均大幅度提高;298～773 K循环5次后的形状回复率提高了26%,77～773 K热循环5次后的形状回复率提高了36%。热循环显著减少了因淬火热应力导致的热诱发ε马氏体量,并且热诱发ε马氏体的逆转变向基体引入了大量的堆垛层错。与固溶态相比,热循环后再变形不仅使α’马氏体的数量显著减少,而且应力诱发ε马氏体以区域化的方式形成,因而合金的形状记忆效应显著提高。77～773 K的热循环比298～773 K的热循环更能显著提高合金形状记忆效应的原因是由于其引入了更多的堆垛层错。     

Two-way shape memory effect and alternating current driving characteristics of a TiNi alloy spring

Two-way shape memory effect (TWSME) was induced into the TiNi shape memory alloys (SMAs) spring by thermomechanical training after annealing treatment, which has promising application in micro-actuating fields. The TWSME spring can contract upon heating and extend upon cooling. The results show that there is an increase of the recovery ratio up to a maximum TWSME of 45%. During the training procedure, transformation temperatures and hysteresis were measured by different scanning calorimetry (DSC). The results show that As (reverse transformation start temperature) and Af (reverse transformation fmish temperature) shift to lower temperature after training. The intervals of ArAs and Ms-Mf (Ms and Mf are the martensite start and finish temperatures, respectively) increase and the heat of transformation decreases after training. The electrothermal driving characteristics of the TWSME springs were also investigated with alternating current density of 3.2-14.7A/mm^2. It is found that the time response and the maximum contraction ratio greatly depend on the magnitude of the electrical current density.     

Effect of ageing on the phase transformation temperatures and shape memory effect of a NiAl-Fe alloy

Effect of ageing at temperatures of 200–600°C on the phase transformation temperatures and shape memory effect of a NiAl-Fe alloy were investigated. It was found that the martensitic transformation temperature and the reverse temperature of Ni-24.1 at% Al–18.2 at% Fe specimens decreased gradually as the ageing temperature increased from 200 to 400°C, and then increased as the ageing temperature increased higher than 400°C. The amount of one-way, without load, shape recovery of Ni-24.1 at%Al–18.2 at% Fe specimens, aged, increased as the ageing temperature increased from 200 to 400°C for 1 h, and decreased dramatically as the ageing temperature increased to 600°C.     

The effect of nanoprecipitates on the superelastic properties of FeNiCoAlTa shape memory alloy single crystals

FeNiCoAlTa shape memory alloys were recently discovered to possess a combination of high transformation strain and high resistance to plastic deformation. However, the transformation strain observed from single crystals is much smaller than theoretically predicted, which could be related to γ′ (L1₂) precipitates. Therefore, we examined the effect of nanosized γ′ precipitates on various superelastic properties of Fe–28%Ni–17%Co–11.5%Al–2.5%Ta single crystals, and found that they have profound influence on the superelastic stress hysteresis, transformation temperatures, stress–temperature phase diagram, and the characteristics of the stress–strain response along the 〈1 0 0〉 orientations. The size and volume fractions of precipitates were determined quantitatively with 3-D atom probe tomography, and the composition of these precipitates was determined for the first time. Aging at 600 °C for 7 h and above resulted in little or no change in the volume fraction of the precipitates, but coarsening of the precipitates accompanied by modest changes in their chemical compositions was observed with increasing aging time, which resulted in an increase in the transformation temperatures. Furthermore, the change in the precipitate size affected tensile and compressive superelastic behavior differently. An increasing size of the precipitates, and thus decreasing number density, caused tension–compression asymmetry in the superelastic characteristics, such as recoverability, stress hysteresis, and intensified the critical stress vs. temperature slope. We explain this observation based on the inherent differences in the morphology and variant structures of the martensite formed during tension and compression superelastic experiments.     

The suppression and recovery of martensitic transformation in a Ni-Co-Mn-In magnetic shape memory alloy

The intrinsic mechanism of the martensitic transformation (MT) suppression observed in Ni-Co-Mn-In alloys fabricated under non-equilibrium conditions still remains mysterious. Here, we used the undercooling technique to obtain a solidified microstructure in non-equilibrium state, subsequently leading to MT suppression even further cooling to 10 K. It was found that primary dendrite-like In-depleted precipitates occurred during solidification under a large undercooling. After a prolonged annealing, the MT interestingly appeared again due to the dissolution of the precipitates and the recovery of equilibrium chemical composition in the matrix.     

新型Cu-Zn-Cr合金的形变热处理及对性能的影响

设计了一种新型导电结构材料Cu-Zn-Cr合金。通过金相观察、硬度测量、电导率测量和透射电子显微分析(TEM)以及高分辨分析的方法,研究了形变热处理工艺对Cu-Zn-Cr合金性能的影响以及Cu-Zn-Cr合金的强化机理。结果表明,由均匀化、热轧、固溶、冷轧、时效组成的形变热处理工艺能显著提高合金性能;合金的最佳均匀化温度为900℃,最佳时效温度为400℃,最佳时效时间为1 h;960℃固溶处理2 h能提高时效强化效果。经过固溶处理后冷变形80%,再在400℃时效1 h后合金综合性能最佳,硬度为194 HV2,电导率为42%IACS。时效过程中Cr以纳米级的第二相粒子形式从过饱和固溶体中析出,产生沉淀强化效果,同时净化了基体,提高了电导率。     

Shape memory behavior and tension-compression asymmetry of a FeNiCoAlTa single-crystalline shape memory alloy

A 〈1 0 0〉 textured polycrystalline FeNiCoAlTa shape memory alloy was recently shown to possess large superelastic strain and stress levels. In this study, the shape memory behavior of a Fe-28Ni-17Co-11.5Al-2.5Ta (at.%) single-crystalline material oriented along the 〈1 0 0〉 direction was studied, for the first time, by thermal cycling under constant stress levels in both tension and compression. When γ′ precipitates with an average size of 5 nm are introduced by an aging heat treatment, the single crystals show fully recoverable transformation strains up to 3.75% in tension and 2% in compression. The change in transformation temperatures for a unit change in applied stress level was higher in compression than in tension, in accord with the lower transformation strains in compression obtained both from theoretical calculations and experimental observations. However, in all specimens, the observed transformation strain levels were lower than theoretically predicted, possibly owing to significant volume fraction of non-transforming precipitates, incomplete martensite reorientation due to martensite variant interactions, and a slightly higher-than-expected martensite c/a ratio in the samples used in this study. The ramifications of relevant structural parameters and microstructural features on reaching theoretical transformation strain and high strength levels are also discussed.     

High temperature deformation behavior and processing map for a nickel-titanium shape memory alloy

The hot deformation behavior of 49.2Ti-50.8Ni shape memory alloy was studied using hot compressive deformation testing in the temperature range of 1023-1323 K and at strain rates of 0.01-10 s^-1. The work-hardening rate was induced to analyze the stress-strain curves, and the critical stress σc and the dynamic recovery saturation stress σ_sat were measured which can be specified approximately by the expressions: σ_sat-1.12σ_p and σ_c-0.86σ_p. An Arrhenius model was calculated to describe the relationship between peak stress and the Z parameter. The relationship between deformation activation energy, the deformation conditions and the effect of Ni component in a binary TiNi alloy on the activation energy were discussed in this work. With the help of electron backscattering diffraction, a connected mode dynamic recrystallization microstructure was confirmed in peak efficiency regimes (850 °C & 0.01 s^-1 and 1050 °C & 10 s^-1) of the processing map.     

回复加热速度对上淬Cu-Al-Mn合金的形状记忆效应的影响

采用光学显微镜、透射电子显微镜和电阻率测量技术,研究回复加热速度对上淬后Cu-8.8Al-10.27Mn合金的形状记忆效应的影响。结果表明：材料形状回复率随回复加热速度的减小而减小。当回复加热速度为20°C/min时,形状回复率可达75%;而当回复加热速度为1°C/min时,其形状回复率仅为8%。原位金相观察表明：材料的形状记忆效应是由马氏体逆转变后的残余孪晶形变马氏体的稳定性引起的。电阻率分析结果表明：残余孪晶形变马氏体的稳定性与其界面处空位与位错在慢速加热时形成的复合缺陷相关。而复合缺陷的形成会阻碍孪晶形变马氏体的逆转变。     

Cu-Al-Mn合金的低温形状记忆效应与晶体结构的关系

利用自制的合金低温特性测量装置研究马氏体相变温度Ms为100 K左右的Cu-Al-Mn低温形状记忆合金的形状记忆性能,并采用X射线衍射仪、光学显微镜和扫描电镜等研究其微观结构。结果表明：Cu-Al-Mn合金具有良好的形状记忆性能,其XRD谱中除含β相的衍射峰外没有其他相的衍射峰,并且有序度越高,合金的形状记忆性能越好;其它相的析出尤其是α（Cu）相的大量析出使得合金的形状记忆性能变差;在Cu-Al-Mn合金中,随着Mn和Al含量的增加,其马氏体相变温度降低。