On the two-way shape memory behavior in NiTi alloy—An experimental analysis

The present research aims to understand the mechanism of the two-way memory effect (TWME) in NiTi alloy through experimentally investigating shape recovery in the course of various thermomechanical processes. Particular attention was paid to the effect of martensitic strain, either due to pre-deformation or constrained forward transformation, on the TWME. Attempts were made to clarify the correlation of the stress-assisted two-way memory effect (SATWME) and the TWME to martensitic strain resulting from pre-straining and/or constrained cooling. When the martensitic strain resulting from constrained forward transformation exceeds the initial pre-strain, it directly influences the SATWME and TWME. Partial reverse transformation through thermal arrest during heating leads to accommodation of stress-assisted and detwinned martensite variants that result in formation of internal forward and back stresses. TWME is promoted through dominant internal forward stress formation, while the dominance of internal back stress reduces the TWME by reducing the martensitic strain.     

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.     

CuAlMn形状记忆合金的高阻尼特性

用悬臂梁弯曲共振法研究了新型CuAlMn系形状记忆合金在马氏体态和母相态的阻尼特性.研究结果表明:室温马氏体态Cu-10.5%Al-6%Mn(质量分数)和室温母相态Cu-11%Al-8%Mn形状记忆合金在表面应变振幅为4.05MPa时,都具有很高的阻尼性能,内耗Q-1接近0.1; 两种形态的CuAlMn形状记忆合金的阻尼性能都随应力振幅增加而下降,且母相态合金阻尼性能下降速度更快.     

Microstructure evolution of the undercooled Co-Ni-Ga magnetic shape memory alloy

Microstructure evolution of the martensite Co₅₀Ni₂₂Ga₂₈ magnetic shape memory alloy was investigated by the method of glass fluxing combined with superheating cycling. A maximum bulk undercooling of 220 K is achieved and a mass of sub-grains are introduced by the undercooling. A number of dislocations and large internal stress, due to the undercooling rapid solidification, give rise to the production of sub-grains during the recovery process. These sub-grains, as well as martensite variants, are remarkably refined with the increase of undercooling. The recrystallization and growth of sub-grains during the following annealing are also discussed.     

Stress generation by shape memory alloy wires embedded in polymer composites

Single shape memory alloy (SMA) wires are capable of generating high compressive stresses under constrained conditions when they are thermally activated. In this work, the ability of single SMA wires to act as internal stress generators in fibrous polymer composite systems has been investigated. The stress fields recorded both at the specimen level and internally in the reinforcing fibers by means of Raman microscopy measurements have confirmed that SMA wires can serve as effective stress-actuators. The efficiency of stress generation depends on the wire alloy composition and level of prestrain prior to incorporation into the composite medium. The internal stress distribution in the composite fibers decayed from a position just above the wire surface to the edges of the specimen reaching zero at a distance of approximately 3000 μm. The maximum value of stress recorded in the fibers at a distance of approximately one wire radius was 227 MPa. The significance of these results in the design and operation of adaptive composites systems similar to those examined here is discussed.     

Damping capacity of TiNi-based shape memory alloys

The damping capacity mainly comes from the motion of martensite boundaries, which is larger than that from different phase interfaces in TiNi-based shape memory alloys. The fine and disperse precipitation in the TiNi matrix will increase the damping capacity of the TiNiNbMo alloy in the martensitic state but little affect the damping capacity during the martensite transformation. The dispersed Nb-rich particles and the internal stress fields around them influence the change tendency of damping capacity with the increasing of strain amplitude.     

Transformational behaviour of constrained shape memory alloys

The transformational behaviour of shape memory alloy (SMA) wires embedded into a fibre reinforced epoxy composite was investigated and is discussed in this article. The effects on the transformational temperatures, and heats of the embedded SMA wires and the generation of recovery stresses within the composites on heating are shown to be related to the reversible martensitic transformation of the SMA wires. This article details the effects of the constraining matrix on the transformations of self-accommodating and preferentially oriented martensite. It was found that there is little change in the transformation temperatures of the constrained SMA wires with increasing pre-strain, but that the measurable transformation heats decrease significantly with increasing pre-strain. It is concluded that the transformation of self-accommodating martensite is nearly not affected by the constrained matrix, whereas the transformation of the preferentially oriented martensite is suppressed.     

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.     

Principle and realization of improving shape memory effect in Fe–Mn–Si–Cr–Ni alloy through aligned precipitations of second-phase particles

A principle that the shape memory effect of Fe–Mn–Si–Cr–Ni alloys can be remarkably improved through aligned precipitations of second-phase particles has been proposed and realized through ageing after pre-deformation at room temperature. A 79% shape recovery of an initial 5% strain in Fe–13.53Mn–4.86Si–8.16Cr–3.82Ni–0.16C alloy was attained by ageing at 1073 K after 10–20% tensile pre-deformation at room temperature. Transmission electron microscopy and in situ scanning electron microscopy observations showed that the aligned Cr₂₃C₆ particles were produced by directional interfaces resulting from stress-induced martensite. The aligned particles subdivided grains into even smaller domains, thus reducing and even suppressing the collisions between martensite bands belonging to different smaller domains. The aligned particles caused the stress-induced martensite to form in domains. It is expected that through ageing after moderate pre-deformation, other aligned carbides and nitrides can be attained and the shape memory effect improved further.     

电脉冲处理对不同预变形量Fe17Mn5Si8Cr5Ni0.5NbC合金NbC析出相与形状记忆效应的影响(英文)

为进一步提高铁基形状记忆合金的记忆效应并改善处理技术,研究了电脉冲处理对不同预变形量Fe17Mn5Si8Cr5Ni0.5NbC合金性能的影响。结果表明:当预变形量低于 10%时,电脉冲处理后合金的形状回复率随着预变形量的增大而增大,且最大值比800 °C热处理的合金高 8%左右。与热处理状态相比,当预变形量低于10%时,随着预变形量的增大,电脉冲处理诱导出NbC颗粒的数量更多、尺寸更小、析出速度更快,进而有效改善合金的记忆性能。     

弓状线形镍钛形状记忆合金的应力和相变数值模拟

利用有限元分析软件MSC．MARC,建立了弓状线形镍钛合金模型,完成了当合金受一定变形量和温度变化过程中各节点、各单元的应力变化和材料相变的模拟,得出了变形过程中马氏体的体积率和等效应力的分布。结果发现在最大变形处合金马氏体的体积率和等效应力也出现最大值。     

Ti-13at.-％Ni-37at.-％Pd记忆合金预形变与高温形状记忆效应SCIEI

本文着重地研究了Ti-13at.-％Ni-37at.-％Pd合金的高温形状记忆效应与预形变温度的关系.该合金A_s温度是620K,具有单程形状记忆效应和双程形状记忆效应。在620—640K温度预形变,可以得到最大的形状记忆应变,这与在此温区产生的最小可逆应变能有关。     

Martensite evolution in a NiTi shape memory alloy when thermal cycling under an applied load

The microstructural evolution of a near equiatomic NiTi shape memory alloy has been studied by in situ synchrotron diffraction, whilst being thermally cycled under an applied load. The martensite texture was found to be cyclically dynamic and an elongation of the sample was observed with each thermal cycle. The martensite formed when cooling under load exhibited progressive variant selection, in an attempt to accommodate the applied stress, accompanied by strain relaxation of the material. The characteristic transformation temperatures were found to decrease with cycling, possibly as a result of the generation and accumulation of dislocations. The magnitude of elongation, variant selection, lattice relaxation and damage accumulation were found to increase as a function of applied stress and their rate decrease with cycle number.     

Surface mechanical attrition treatment induced phase transformation behavior in NiTi shape memory alloy

The phase constituents and transformation behavior of the martensite B19′ NiTi shape memory alloy after undergoing surface mechanical attrition treatment (SMAT) are investigated. SMAT is found to induce the formation of a parent B2 phase from the martensite B19′ in the top surface layer. By removing the surface layer-by-layer, X-ray diffraction reveals that the amount of the B2 phase decreases with depth. Differential scanning calorimetry (DSC) further indicates that the deformed martensite in the sub-surface layer up to 300 μm deep exhibits the martensite stabilization effect. The graded phase structure and transformation behavior in the SMATed NiTi specimen can be attributed to the gradient change in strain with depth.     

HIGH TEMPERATURE SHAPE MEMORY EFFECT AND PRE-DEFORMATION FOR Ti_(50) Ni_(13) Pd_(37) ALLOY

The dependence of high temperature shape memory effect on the pre-deformation tempera-ture of the Ti_(50)Ni_3Pd_(37) alloy has been studied.This alloy,of which the reverse transformationstart temperature on heating is 620 K,has both one-way and two-way shape memory effects.The maximum shape memory strain of the ahoy may be obtained under pre-deformation inthe range of 620 to 640 K.This seems to be related to the minimum recoverable strain energyemerged from the above mentioned temperature range.     

退火温度对淬火态Fe-Mn-Si-Cr-Ni合金记忆效应的影响

研究了退火温度对淬火态预先存在热诱发ε马氏体的Fe-14Mn-5.5Si-8.0Cr-5.0Ni合金和无热诱发ε马氏体的Fe-19Mn-5.0Si-8.0Cr-6.0Ni合金记忆效应的影响。结果表明:预先存在热马氏体合金的形状回复率随退火温度的升高,先上升后下降,在500℃附近达到最大值。但无热马氏体存在合金的形状回复率随退火温度的变化却相反,在500℃附近达到最小值;两种合金的Ms温度都随退火温度的升高而下降,在500℃附近达到最低。预先存在热马氏体的合金由于退火后Ms温度的降低,减少了热诱发的马氏体量,因而形状记忆效应得到了提高;而无热马氏体存在的合金由于退火后Ms温度的进一步下降,使得应力诱发马氏体转变更不容易发生,因此形状记忆效应反而下降。     

一种实现低合金超高强钢超塑变形的新思路

将Mn-Si-Cr系低合金高强钢在过冷奥氏体状态下进行适当预变形,水冷后的组织主要由铁素体、球状碳化物和马氏体组成。研究表明：700℃时的低速率小变形可使上述组织转变为（铁素体＋球状碳化物）复相组织,对超塑性变形有利;预变形后水冷的试样在700℃应变速率为10-4 s-1～2×10-4 s-1范围时的m值可达0.48,流变抗力为40～60 MPa,激活能约158 kJ/mol,属于晶界滑动变形机制,具有超塑性变形的特征。     

Effect of high undercooling on Co-Ni-Ga ferromagnetic shape memory alloys

Co₄₅Ni₂₅Ga₃₀ ferromagnetic shape memory alloys were treated by glass fluxing combined with superheating cycling. The effect of high undercooling on solidified microstructure and transformation temperatures was investigated. The martensite lath is obviously refined and a mass of sub-grains are produced with the increase of undercooling. Undercooling rapid solidification introduces a number of dislocations and large internal stress, which give rise to the production of sub-grains in recovery. During the following annealing process, these sub-grains can gradually recrystallize and grow up with an amount of γ′ phase precipitated. Meanwhile, transformation temperatures of undercooled alloys are greatly elevated compared with those of as cast, which results from the large internal stress, and gradually reduce with the heating time.     

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。     

经形变热处理铜基形状记忆合金的晶粒生长、形状记忆和腐蚀行为

比较研究了Cu-11．8％Al-3．7％Ni-1％Mn和Cu-11％A1—5．6％Mn形状记忆合金（SMAS）的形状记忆、腐蚀性能。采用光学显微镜（0M）、扫描电子显微镜（SEM）、差示扫描量热法（DSC）、动电位极化、弯曲和拉伸试验,研究了晶粒细化对这些性能的影响。在800。C退火时,在首先的15S内静态再结晶和动态晶粒长达显示出一个快速的再结晶过程,随后才是晶粒生长。退火15S后得到的Cu—A1—Ni-Mn和Cu-Al—Mn合金的最小晶粒尺寸分别为90gm和260pm。拉伸试验表明2种合金呈现典型的三阶段曲线,由此可以看出,晶粒细化后合金具有高的断裂应力和应变。显微组织表明,Cu—Al-Ni—Mn合金中存在锯齿状的所马氏体形态,通过差示扫描量热法也证实了所和rf共存于Cu—A1-Mn合金中。评估了形变热处理前、后及800℃退火15min,随后进行水淬的合金的形状记忆性能。另外,采用动电位极化法分析了晶粒细化后合金的腐蚀行为。结果表明,铜溶解过程中主要为阳极反应,Cu—Al—Ni—Mn合金比Cu—A1-Mn合金具有更好的耐腐蚀性。