A study on a Ti52Ni47Al1 shape memory alloy

The Ti₅₂Ni₄₇Al₁ alloy has 16% volume fraction Ti₂Ni particles in the B2 matrix with Ti₂Ni particles having a higher Al content than the B2 matrix. Transformation temperatures M^* and A^* of this alloy are lower than those of the Ti₅₁Ni₄₉ alloy due to the solid solution of the Al atoms. M^* and A^* decrease with increasing aging time at 400°C because the Al atoms diffuse slightly from the Ti₂Ni to the B2 matrix. The hardness increment of this alloy is more than that of the Ti₅₁Ni₄₉ alloy under the same degree of cold rolling. At the same time, M^* and A^* of this alloy can be more depressed by thermal cycling than those of the Ti₅₁Ni₄₉ alloy, especially in the first ten cycles. All of these features result from the fact that this alloy has a higher inherent hardness due to the solid solution of the Al atoms. This also causes the R-phase transformation to be more easily promoted by both cold rolling and thermal cycling in this alloy. The strengthening effects of cold rolling and thermal cycling on the M^* (Ms) temperature of this alloy follows the expression Ms = T₀–K_y, in which K values are affected by different strengthening processes. It is found that the higher the inherent hardness of the TiNi and TiNiX alloys, the higher the K values they have.     

The study of constitutional phases in a Ni47Ti44Nb9 shape memory alloy

The constitutional phases and microstructure of Ni₄₇Ti₄₄Nb₉ alloy have been studied by means of optical microscopy, electron probe X-ray microanalyses (EPMA) and X-ray diffraction. It has been shown that the microstructure of the experimental alloy consists of three phases:TiNi matrix, niobium-rich phase and Ti₃(Ni,Nb)₂ compound. The Nb-rich phase is determined to be β -Nb with bcc structure containing a small amount of Ni and Ti. The β -Nb is a soft phase which forms a eutectic structure with TiNi phase during solidification. After hot working the soft β -Nb phase is dispersed in TiNi matrix and gives rise to a wide transformation hysteresis in the alloy. The Ti3(Ni,Nb)₂ is a harder and embrittlemental phase.     

预变形和热循环对Ni50Ti45Ta5形状记忆合金相变行为的影响

利用示差扫描量热仪研究了预变形和热循环对Ni54Ti45Ta5形状记忆合金相变行为的影响。研究结果表明：预变形后合金的马氏体发生了稳定化现象。形变后第一次热循环．逆相变温度增加。正相变温度降低；而第二次相变循环。马氏体的稳定化现象消失。热循环使相变温度降低。减少了相变热。     

Characteristics of Deformation and Recovery in Ti50Ni47Fe3 Shape Memory Alloy

The mechanical behavior and the effect of pre-strain on recovery behavior of Ti50Ni47Fe3 (at. pct) alloy were investigated systematically by tensile and recovered tests accompanied by electrical resistance measurement. Ti50Ni47Fe2 alloy has different deformation behaviors at different temperature ranges, the deformation curves in different temperature range can be classified into four kinds. The start temperature of recovery increases with the increase of pre-strain.There exists an optimal deformation condition, at which the specimen exhibits maximum free recovery strain. With increasing pre-strain the recovery stress increases and reaches the maximum at 8% pre-strain. R-phase to parent transition offered about 0.2% recovery strain. With pre-strain increasing the recovery stress increases and reaches to the maximum at 8% pre-strain. The recovery stress is corresponding with the critical stress of stress-induced martensitic transformation.     

Crystallization behavior of Ti50Ni25Cu25 amorphous alloy

Crystallization behavior of the Ti₅₀Ni₂₅Cu₂₅ alloy was studied by means of scanning and isothermal differential calorimetry, X-ray diffraction, conventional and high-resolution transmission electron microscopy. A single stage polymorphic-type transformation of the amorphous phase forming a Ti₂CuNi crystalline phase was observed. The activation energy for such a single stage crystallization of the amorphous phase was determined by Kissinger analysis. Kinetics of the crystallization was analyzed on the basis of Johnson-Mehl-Avrami equation and discussed regarding to the value of Avrami exponent obtained. Relatively high average value of Avrami exponent of 5.5 at the range from 702 to 709 K suggests nucleation with an increasing nucleation rate.     

NiTiHf高温形状记忆合金的相变行为和形状记忆效应

利用透射电镜、X射线衍射仪和示差热分析仪系统分析了Ni49Ti36Hf15合金的相变行为及其形状记忆效应。结果表明，Ni49Ti36Hf15合金的热马氏体变体间构成典型的自协作组态，主要呈矛头状、镶嵌块状和楔状3种形态，亚结构主要为（001）复合孪晶。随着热循环次数的增加，相变温度降低，经50次热循环后，相变温度随热循环次数增加变化趋势不明显。固溶处理Ni49Ti36Hf15合金在20－184℃范围内弯曲变形时，呈现良好的形状记忆效应，其最大可恢复应变可达3％。形状恢复率随着弯曲变形温度的增加而下降，当弯曲变形温度大于317℃时，形状恢复率下降为0。     

时效Ti49Ni51合金的显微组织与双程形状记忆效应

采用弯曲试验和透射电镜分析研究了时效Ｔi49Ni51合金的显微组织与双程形状记忆应用,结果表明,５００６℃,１h时效处理试样中,相呈透镜片状,周围有较强的应变场初度,而５００℃,３０h时效试样中,ＴiNi4相已聚集长大成粗片状,与基体失去共格,５００℃,１h时效处理试样在训练后可获得较好的双程形记忆效应,时效Ｔi49Ni51合金中ＴiNi4相粒子周围的共格应变场和主地位错交互作用可以产生有效内应力场,从而诱发双程形状记忆效应。     

Ni50.1Mn24.1Ga20.3Fe5.5形状记忆合金多晶纤维的双程形状记忆效应

通过熔体抽拉技术制备Ni_50.1Mn_24.1Ga_20.3Fe_5.5多晶纤维,采用步进式热处理释放因快速凝固引起的内应力和缺陷,利用场发射扫描电子显微镜、透射电子显微镜、XRD衍射仪对其微结构和相结构进行表征,采用动态机械拉伸仪测试其相变行为和双程形状记忆性能。结果表明:热处理后原子有序度显著提高,孪晶界平直,在恒应力作用下一个热循环中母相和马氏体相的形状得到完全恢复。双程形状记忆曲线显示了热弹性马氏体相变的两个基本特征:可逆性和热滞性。在热循环实验中,纤维被加载到198 MPa时,其马氏体态总应变达到1.32%。根据热机械拉伸测量,发现相变温度遵循Clausius-Clapeyron关系式。与诸如Ti-Ni和Cu-Al-Ni的其他合金相比,Fe掺杂的纤维显示出较小的应变-应力依赖性,在恒应变输出的驱动中是有益的。     

Corrosion behavior of a laser-welded NiTi shape memory alloy

Corrosion behaviors of the laser-welded Ni–49.4 at.% Ti shape memory alloy and base metal in 0.9% NaCl solution were investigated by means of electrochemical techniques (the open circuit potential measurement, linear and potentiodynamic polarizations). The results showed that corrosion resistance of the laser-welded NiTi alloy is better than that of the base metal. Compared to the base metal, the laser-welded NiTi alloy exhibits higher open circuit potential, higher polarization resistance, a wider passive region and higher breakdown potential. The improvement of corrosion resistance of the laser-welded NiTi alloy is ascribed to a smoother, defect free surface and an absence of carbides.     

Optimisation of Ni–Ti shape memory alloy response time by transient heat transfer analysis

Nickel–Titanium (Ni–Ti) shape memory alloys (SMAs) are commonly applied in commercial actuator design due to the high associated fatigue and tensile strengths, low cost and high activation temperature. Consequently, Ni–Ti SMAs provide an opportunity for the development of novel electromechanical actuators. However, the cooling response time is typically of significantly larger duration than the associated heating response time. The applicability of SMA actuators would be significantly greater if the cooling response time was reduced to allow a symmetric, high speed activation profile. This work provides insight into the opportunities associated with enhancing thermal heat transfer efficiency to achieve this objective. An explicit model of the temperature of Ni–Ti SMA wire is developed to estimate the temperature–time profile during resistive heating. A finite-difference equation is developed to predict the associated temperature during cooling. These models are used to confirm that for a typical scenario, the cooling stage dominates the total response time, and that lagging with a highly conductive media can be used to dramatically reduce the cooling response time. The finite-difference equation is validated against steady state data, and extended to provide insight into the effects of SMA lagging, including the effects of periodic excitation on cooling rate and the minimum observed SMA temperature during a heating cycle. The outcomes of this work are generally applicable to any axisymmetric transient heat transfer optimisation problem.     

Nanocrystalline Ti49.2Ni50.8 shape memory alloy as orthopaedic implant material with better performance

TiNi alloys, with their unique shape memory effects and super elastic properties, occupy an indispensable place in the family of metallic biomaterials. In the past years, surface treatment is the main technique to improve the bioinert nature of microcrystalline TiNi alloys and inhibit on the release of toxic nickel ions to obtain excellent osteogenesis and osseointegration function. In the present study, nanocrystalline Ti_49.2Ni_50.8 alloy has been fabricated via equal channel angular pressing (ECAP), and the in vitro and in vivo studies revealed that it had enhanced cell viability, adhesion, proliferation, ALP (Alkaline phosphatase) activity and mineralization, and increased periphery thickness of new bone, in comparison to the commercial coarse-grained counterpart. These findings indicate that the reduction of grain size is beneficial to increasing the biocompatibility of Ti_49.2Ni_50.8 shape memory alloy.     

Effect of Y addition on microstructure and martensitic transformation of a Ni-rich Ti–Ni shape memory alloy

In the present work, 1 at% Y was added to Ti-50.7at%Ni alloy to prepare Ti-50.2Ni-1Y alloy and the effects of rare earth Y addition on the microstructure and martensitic transformation behavior of Ti-50.7at%Ni alloy was investigated by optical microscope, scanning electronic microscope (SEM) X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The results show the microstructure of Ti-50.2Ni-1Y alloy is different with that of Ti-50.7at.%Ni alloy, and its microstructure consists of B19′martensite and the Y-rich phase, which may be YNi phase. One-step martensitic transformation occurs in Ti-50.2Ni-1Y ternary alloy. The phase transformation temperatures increase rapidly with Y addition, and the increase of Ms is about 85°C.     

Mechanical behavior and shape memory effect of an aged NiAl–Fe alloy

The mechanical behavior and shape memory effect of an aged NiAl–Fe alloy has been investigated. It was found that the first yielding stress of NiAl–Fe alloy in a compression test was decreased with the precipitation of a Ni₅Al₃ phase after aging at 473–673 K, and increased as the aging temperature increased higher. The one-way shape recovery of NiAl–Fe alloy increased as the aging temperature increased from 473 to 673 K, and decreased as the aging temperature increased higher than 673 K. The morphology of precipitates in the NiAl–Fe alloy aged at 473–873 K was investigated, and the mechanism of its effect on the mechanical properties is discussed.     

A comparative study of Ni–Ti and Ni–Ti–Cu shape memory alloy processed by plasma melting and injection molding

Shape memory alloys (SMA) are smart materials that present potential applications in such diverse areas as aeronautics, automotive, electronics, biomedicine and others. This work aimed at comparing some physical and functional properties of a Ni–Ti–Cu and equiatomic Ni–Ti SMA. Therefore, Ni–50Ti and Ni–50Ti–5Cu (at.%) were manufactured using plasma melting followed by injection in metallic mold, named Plasma Skull Push–Pull (PSPP) process. Afterwards, samples of both Ni–Ti based SMA were annealed at 1113 K during 2400 s and water quenched. The obtained specimens were analyzed by optical microscopy, microhardness, differential scanning calorimetry, electrical resistance as a function of temperature, and force generation tests. The results showed that Ni–Ti alloy presented higher levels of hardness and lower generated recover forces during heating when compared to the Ni–Ti–Cu SMA. Moreover, the Ni–Ti alloy holds hysteresis larger than the Ni–Ti–Cu SMA as a result of the presence of the R-phase transformation. There was also a better stability under thermal cycling of NiTiCu SMA compared with the equiatomic NiTi.     

TiNiHfCu高温形状记忆合金的马氏体相变研究

通过差热扫描分析、X射线衍射和透射电镜等方法研究了Cu的添加对Ti36 Ni49-x Hf15 Cux(x=1,3,5,8)合金马氏体相变行为的影响.研究表明:Cu的加入可稍降低合金的相变温度(约15 ℃),但其Ms仍高于150 ℃,可作为高温形状记忆合金使用;随着Cu含量的升高,合金的B19'马氏体晶格常数a、b和c不发生明显变化,但单斜角β逐渐降低;Ti36 Ni49-x Hf15 Cux(x=1,3,5,8)合金的相组成为B19'单斜马氏体和(Ti,Hf,Cu)2Ni相,马氏体的亚结构为(001)复合孪晶.     

Short communication Effects of tantalum addition on transformation behaviour of (Ni51 Ti49 )1-x Tax and Ni50 Ti50-y Tay shape memory alloys

Abstract

The effect of Ta content on the transformation characteristics of Ni–Ti–Ta ternary alloys has been studied. In (Ni₅₁ Ti₄₉ )_1-x Ta_x type alloys, the phase transformation temperatures increase with Ta content, especially when the Ta content is less than 4 at.-%. In Ni₅₀ Ti_50-x Ta_x type alloys, the phase transformation temperatures decrease as Ta content increases. The martensite start temperature is less sensitive to changes in Ni content in ternary Ni–Ti–Ta alloys than that in Ni–Ti binary alloys. The phase transformation temperatures of Ni–Ti–Ta ternary alloys are mainly controlled by the Ni/Ti ratio in the Ni–Ti matrix.     

Phenomenological description of thermomechanical behavior of shape memory alloy

On the basis of a thermomechanical phenomenological model, we analyze the thermomechanical behavior of polycrystalline NiTi. Pseudoelastic response and strain-temperature response under fixed stress are studied by using finite element simulation. Calculated mechanical and thermal hysteresis behaviors of NiTi sheet are in good agreement with those observed experimentally. Hardening in stress–strain hysteresis loop and sharp change of strain in strain-temperature hysteresis loop are described by numerical simulation. The result from thermomechanically coupled calculation shows the phenomenon that phase transition occurs by nucleation and propagation of transformation fronts.