Effect of heat treatment on the transformation behavior and temperature memory effect in TiNiCu wires

The effect of heat treatment on the phase transformation behavior of TiNiCu shape memory alloy wires and the temperature memory effect in this alloy were investigated by the resistance method. These results showed that with increasing annealing temperature and annealing time, the phase transformation temperatures of TiNiCu wires were shifted to higher temperatures in the heating and cooling process. It was also found that incomplete thermal cycles, upon heating the TiNiCu wires, which were arrested at a temperature between the start and finish tem-peratures of the reverse martensite transformation, could induce a kinetic stop in the next complete thermal cycle. The kinetic stop tempera-ture was closely related to the previous arrested temperature. This phenomenon was defined as the temperature memory effect. The result of this study was consistent with the previous report on the phenomenon obtained using the differential scanning calorimetry method, indicating that temperature memory effect was a common phenomenon in shape memory alloys.     

热机械训练影响TiNiCu记忆合金DSC曲线特征的研究

利用示差扫描量热法(DSC)研究了加热伸长、冷却收缩和加热收缩、冷却伸长2种TiNiCu双向记忆效应弹簧训练过程中相变特征。研究结果表明，训练前升温时，DSC曲线表现出吸热峰由多个小峰组成，随着训练次数的增加，峰的个数减少，具有稳定双向记忆效应的试样中，升温时只出现1个锐利的峰。这是因为在训练过程中引入特定取向的位错，产生一定的应力场，使得马氏体变体的取向比较一致，相变点基本相同所导致。并由于位错的引入，引起相变点As和Af下降。     

Temperature memory effect of Ti50Ni30Cu20 (at.%) alloy

After the reverse thermal induced martensitic transformation process of shape memory alloy is arrested at a temperature between the reverse transformation start and finish temperatures (A_s and A_f), and then cooled to a temperature below M_f, a kinetic stop will occur in the next heat flow curve during the heating process. The kinetic stop is closely related to the arrested temperature. This phenomenon is called temperature memory effect (TME). TME of Ti₅₀Ni₃₀Cu₂₀ (at.%) shape memory alloy with phase transformation between B2 austenite and B19 martensite has been investigated by differential scanning calorimeter in this paper. The results indicate that TME of Ti₅₀Ni₃₀Cu₂₀ alloy only exists in the heating process.     

Effect of annealing and heating/cooling rate on the transformation temperatures of NiFeGa alloy

In this work, Ni₅₅Fe₁₈Ga₂₇ ferromagnetic shape memory alloy was prepared through a suck-casting method. The effects of annealing and heating/cooling rate on the martensitic transformation temperatures were investigated by differential scanning calorimetry (DSC). The results showed that the phase transformation temperatures increase with increasing the annealing temperature, upon the heating and cooling process. However, the start and the finish temperatures (M_s and M_f) of martensitic phase transformation increased firstly and then decreased upon the cooling process with the increase the annealing time at 300 °C. The start and the finish temperatures (A_s and A_f) of inverse phase transformation increased slightly upon the heating process with the increase of the annealing time. The results can be explained by the evolution of the microstructure after heat treatment. It was also found that the phase temperatures show great dependence on the heating/cooling rate of the DSC test, A_s and A_f increased and M_s and M_f decreased with the increase of the heating/cooling rate.     

Ti_(50+x)Pd_(30)Ni_(20-x)高温形状记忆合金的弹性行为

制备了三种 Ｔｉ５０＋ｘＰｄ３０Ｎｉ２０－ｘ高温形状记忆合金,使用热分析测试了合金的相变温度发现当 Ｔｉ的原子分数偏离５０％时,合金的相变温度随 Ｔｉ成分的升高而降低,并对这种合金的力学性能进行了测试．合金在奥氏体转变结束温度以上变形仍具有形状记忆效应,应力诱发的马氏体在应力去除后不能恢复到奥氏体研究了这种合金在不同的重复加载条件下的弹性行为,实验表明,弹性回线的形状特征与加载历程有关在奥氏体转变结束温度附近加载可以获得完全的线性伪弹性;而对于处于稳定单相的试样,在重复加载的条件下也可以获得弹性滞后回线     

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.     

TiNi合金激光焊接接头形状恢复温度的研究

利用热模拟试样模拟TiNi合金激光焊接接头焊缝热影响区,对焊接接头、焊缝金属、模拟热影响区和母材的相变温度进行差热分析,研究了焊接接头与其它三者逆相变温度的关系,并利用OM,SEM和XRD对母材、热影响区和焊缝金属的组织、析出相的分布以及晶体结构进行了研究.结果表明,TiNi合金激光焊接接头的形状恢复率与母材无明显差别,但形状恢复温度区间与母材差异较大,其形状恢复开始温度比母材低40℃;焊缝金属和热影响区逆相变开始温度(As)和结束温度(Af)均不同于母材,主要原因是焊缝金属经历了熔化-凝固过程,失去了母材中原有的晶体择优取向,且析出相尺寸小、分布不均;而热影响区As和Af降低,可能是细小析出相重新固溶于基体所致.整个接头的逆相变温度区间与焊缝金属近似,调控焊缝金属的相变温度是控制TiNi合金激光焊接接头形状记忆功能的关键.     

Investigation of Bending Trainings, Transformation Temperatures, and Stability of Two-Way Shape Memory Effect in NiTi-Based Ribbons

This paper discusses the application of rapid solidification by the melt-spinning method for the preparation of thin NiTi-based ribbons. Generally, the application of rapid solidification via melt-spinning can change the microstructure, improving the ductility and shape memory characteristics and lead to small-dimensioned samples. Several thousand thermal cycles were performed on the trained ribbons using bending deformation procedure, continuously observing the changes in the shape memory and transformation behaviors. These changes are due to the appearance of an intermediate phase which was stabilized probably by the accumulation of defects introduced by thermomechanical training. The influence of training and thermal cycling on characteristics of ribbons was studied by x-ray diffraction and transmission electron microscopy and differential scanning calorimetry. The results displayed that bending training methods were useful in developing a two-way shape memory effect (TWSME). All samples show a shape memory effect immediately after processing without further heat treatment. The addition of copper in NiTi alloys was effective to narrow the transformation hysteresis. The W addition has improved the stability of the TWSME and mechanical properties. The TWSME of ribbons and its stability are well suited for important applications such as microsensors and microactuators.     

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。     

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.     

Study of Thermal Scanning Rates on Transformations of Ti-19Nb-9Zr (at.%) by Means of Differential Scanning Calorimetry Analysis

Differential scanning calorimetry (DSC) thermal analysis is a well-accepted technique used to measure the transformation temperatures of shape memory alloy and its thermoelastic transformation energies. In this study, both forward and reverse transformation temperatures of a nickel-free Ti-19Nb-9Zr (at.%) SMA were investigated using DSC technique with different cooling and heating scanning rates in a range of 10 to 100 °C/min. The results showed that the transformation temperature intervals vary substantially with respect to the thermal scanning rates. It is found that the martensitic start (M _s) temperature decreases with decreasing the cooling rates. The optimal scanning rate was found to be 40 °C/min for obtaining the maximum thermoelastic transformation energies stored between the forward and the reverse martensitic transformations. It is believed that the thermoelastic transformation energy increases with the increase in the volume fraction of martensite. Based on these measurements, these thermoelastic transformation energies between the forward and the reverse martensitic transformations were estimated to be ~21 and ~27 J/g, respectively. The appropriate selection of scanning rate for SMA analysis will be discussed.     

Wide Range Temperature Memory Effect of an In Situ NiTi Alloys Composite

Incomplete martensitic transformation characteristics of NiTi shape memory alloys after cold deformation were studied. By designing the shape of the original surface curve, macroscopic domains with different dislocation density were introduced into the NiTi alloys and materials possessing the characteristics of composites were obtained. Due to the interactions between the dislocation texture and martensite variants, the reverse transformation temperatures were found to expand over a large temperature window, which gives temperature memory effect a higher potential for application.     

Mechanical and functional behavior of a Ni-rich Ni50.3Ti29.7Hf20 high temperature shape memory alloy

The mechanical and functional behaviors of a Ni-rich Ni_50.3Ti_29.7Hf₂₀ high temperature shape memory alloy were investigated through combined ex situ macroscopic experiments and in situ synchrotron X-ray diffraction. Isothermal tension and compression tests were conducted between room temperature and 260 °C, while isobaric thermomechanical cycling experiments were conducted at selected stresses up to 700 MPa. Isothermal testing of the martensite phase revealed no plastic strain up to the test limit of 1 GPa and near-perfect superelastic behavior up to 3% applied strain at temperatures above the austenite finish. Excellent dimensional stability with greater than 2.5% actuation strain without accumulation of noticeable residual strains (at stresses less than or equal to −400 MPa) were observed during isobaric thermal cycling experiments. The absence of residual strain accumulation during thermomechanical cycling was confirmed by the lattice strains, determined from X-ray spectra. Even in the untrained condition, the material exhibited little or no history or path dependence in behavior, consistent with measurements of the bulk texture after thermomechanical cycling using synchrotron X-ray diffraction. Post deformation cycling revealed the limited conditions under which a slight two-way shape memory effect (TWSME) was obtained, with a maximum of 0.34% two-way shape memory strain after thermomechanical cycling under −700 MPa.     

Gamma-phase influence on shape memory properties in Ni-Mn-Co-Ga-Gd high-temperature shape memory alloys

The effect of γ-phase on two-way shape memory effect(TWSME) of polycrystalline Ni_(56)Mn_(25-x)Co_xGa_(18.9)Gd_(0.1) alloys was investigated. The results show that an appropriate amount of ductile γ-phase significantly enhances the TWSME. The largest TWSME of 1.4% without training is observed in Ni_(56)Mn_(21)Co_4Ga_(18.9)Gd_(0.1) alloy, and this value is increased to 2.0% after thermomechanical training. The as-trained TWSME decays over the first five thermal cycles and then reaches a stable value as the number of cycles further increasing. Only the degradation of 0.2% is observed after 100 thermal cycles. The better TWSME and thermal stability are ascribed to the stable extra stress field formed by the plastically deformed γ-phase.     

A new method for fabricating SMA/CFRP smart hybrid composites

In order to suppress the microscopic mechanical damages in composite system, and thus to increase the reliability of carbon fiber reinforced plastics (CFRP), a smart composite SMA/CFRP is attracting much attention. In this application, the shape memory alloy (SMA) is expected to exert compressive stress to CFRP by reverse transformation upon heating after fabrication. However, one big problem in the fabrication process is that the curing temperature of the composite (130 °C) far exceeds the reverse transformation temperatures of Ti–Ni alloys (∼70 °C). This paper presents a new method to overcome this difficulty. The method consists of using heavily cold-worked wires to increase the reverse transformation temperatures, and of using flash electrical heating of the wires after fabrication in order to avoid damaging of the matrix around wires. By choosing the reduction of cold drawing and composition of TiNi alloys properly, it will be shown that the method is actually applicable without using special fixture jigs, which are necessary otherwise, to the fabrication of such smart composite as SMA/CFRP.     

Modeling and simulation of irradiation effects on martensitic transformations in shape memory alloys

A model is proposed for the calculation of the irradiation induced changes of the martensitic transformation temperatures of shape memory alloys. It considers the transition temperatures being determined by a chemical and a non-chemical term in the Gibbs free energy that can be affected by the irradiation induced changes of the vacancy density and the chemical ordering of the crystal structures. Numerical simulations for TiNi SMAs have shown that upon irradiation the austenite finish temperature A_f can be raised slightly at the beginning stage. After that, both A_f and the martensite start temperature M_s will decrease strongly and reach some stable values after extensive irradiations. Both the dose rate and the temperature of irradiations will affect the changes of M_s and A_f. According to the simulation, the increase of A_f at the beginning of irradiations is resulted from the irradiation induced point defect production that increases mainly the non-chemical term in the Gibbs free energy change of the martensitic transformation. The irradiation induced chemical disordering reduces both the chemical and non-chemical terms in the Gibbs free energy change and leads to the strong decreases of M_s and A_f.     

Ti-Ni-V形状记忆合金及其弹簧的相变和形变特性

用X射线衍射仪、示差扫描量热仪和拉伸实验研究了退火温度Tan和形变温度Td对Ti-50.8Ni-0.5V形状记忆合金丝及其弹簧的相变和形变特性的影响。结果表明:400～600℃退火态合金的室温相组成为B2和R相。冷却/加热时,350～400℃退火态合金发生A→R/R→A(A—母相,R—R相)一阶段可逆相变;450～500℃退火态合金发生A→R→M/M→R→A(M—马氏体相)两阶段可逆相变;550℃退火态合金发生A→R→M/M→A型相变;600℃退火态合金发生A→M/M→A一阶段可逆相变。随Tan升高,R相变温度先升高后降低,M相变温度先升高后趋于稳定,M相变热滞快速降低,而R相变热滞则几乎不受退火温度影响。随Td和Tan升高,退火态Ti-50.8Ni-0.5V合金弹簧的应力诱发M临界切应力升高。     

冷却速度对Ti-50.9%Ni形状记忆合金相变行为和组织的影响(英文)

为了探索冷却速度对镍钛形状记忆合金相变行为和组织的影响,通过差热扫描、X射线衍射、扫描电镜、X射线荧光分析等方法研究Ti-50.9%Ni(摩尔分数)合金的热处理。结果表明:当冷却速度非常低时(如炉冷)可诱发三级相变的产生;冷却速度对相变温度也有很大的影响,Ms和Mf都随冷却速度的降低而下降,降低冷却速度有利于提高M→A奥氏体相变温度,相变滞后宽度(Af–Mf)随着冷却速度的降低而增大;热处理不能消除镍钛合金热加工时所形成的织构,但能减弱其强度;冷却速度对晶粒大小影响不大。     

添加Sn及热处理对Co38Ni34Al28-xSnx磁性形状记忆合金显微组织和显微硬度的影响（英文）

在Co38Ni34Al28合金体系中添加 Sn,研究Sn含量及不同的热处理温度（1373 K,1473 K,1573 K）下,保温2h对Co38Ni34Al28-xSnx（x=1,2,3）合金显微组织和硬度的影响。结果表明,添加适量的Sn使合金中γ相组织减少;在1573K保温2h后,在室温下获得部分马氏体组织;当Sn 替代 2%Al 时,其显微组织中马氏体组织的比例较高。随着Sn含量的增多和热处理温度的升高,合金的硬度也随着增大。另外,合金马氏体的逆相变温度在Sn含量为1%和2%时升高,在Sn含量为3%时反而降低。     

Smart Control Systems for Smart Materials

Shape memory alloys (SMAs) are thermally activated smart materials. Due to their ability to change into a previously imprinted shape by the means of thermal activation, they are suitable as actuators for microsystems and, within certain limitations for macroscopic systems. Most commonly used SMAs for actuators are binary nickel-titanium alloys (NiTi). The shape memory effect relies on the martensitic phase transformation. On heating the material from the low temperature phase (martensite) the material starts to transform into the high temperature phase (austenite) at the austenite start temperature (A _s). The reverse transformation starts at the martensite start temperature after passing a hysteresis cycle. To apply these materials to a wide range of industrial applications, a simple method for controlling the actuator effect is required. Today??s control concepts for shape memory actuators, in applications as well as in test stands, are time-based. This often leads to overheating after transformation into the high temperature phase which results in early fatigue. Besides, the dynamic behavior of such systems is influenced by unnecessary heating, resulting in a poor time performance. To minimize these effects, a controller system with resistance feedback is required to hold the energy input on specific keypoints. These two key points are directly before transformation (A _s) and shortly before retransformation (M _s). This allows triggering of fast and energy-efficient transformation cycles. Both experimental results and a mechatronical demonstrator system, exhibit the advantages of systems concerning efficiency, dynamics, and reliability.