超弹性NiTi合金循环相变诱发塑性本构模型

为定量描述镍钛形状记忆合金循环相变诱发塑性导致的超弹性退化行为,在广义粘塑性框架下对Graesser模型进行了拓展,考虑了奥氏体和马氏体弹性模量的差异以及马氏体非线性硬化行为,引入循环相变过程中相变应力和残余应变的演化方程,建立了超弹性Ni Ti合金循环相变诱发塑性本构模型,总结了模型参数确定方法.通过镍钛形状记忆合金微管的循环相变试验结果和模拟结果的对比表明,提出的模型能够很好地预测镍钛形状记忆合金的循环相变诱发塑性行为。     

Ni-Mn-Ga-Fe合金纤维的马氏体相变与超弹性

通过高真空电弧熔炼炉和熔体抽拉液态成形设备制备Ni-Mn-Ga和Ni-Mn-Ga-Fe纤维,并对纤维进行阶梯式有序化热处理。利用场发射扫描电子显微镜(SEM)、透射电子显微镜(TEM)、射线衍射仪(XRD)对其微结构和相结构进行表征;利用差示扫描量热仪(DSC)对纤维的马氏体相变过程进行分析;利用动态机械分析仪对纤维的超弹性进行测试。结果表明：有序化热处理后纤维内部原子排列有序性提高,相邻孪晶间沿近90°大角度晶界方向生长,晶界处平直。Fe掺杂使得纤维内部晶粒细化,晶格体积缩小,纤维整体致密性提高。Fe掺杂使得晶格内自由电子数量增多,电子浓度升高,致使马氏体相变(martensitic transformation, MT)的相变温度(M_s)明显提高;通过有序化热处理,高温下的自由电子自由排列,形成有差异性的新布里渊区,构成新的孪晶界,进一步提高晶格内部的致密程度。同时Fe具备耐高温、抗拉强度高的特性,Fe掺杂的Ni-Mn-Ga纤维降低了其本征脆性。超弹性曲线显示了热弹性马氏体相变的两个基本特征：完全超弹性(superelasticity, SE)和低温恢复特性。...     

FeMnAlNi系超弹性合金的研究进展

FeMnAlNi系合金因极大的超弹性温度范围(-196～240℃)与马氏体相变临界应力极低的温度依赖性(Clausius-Clapeyron斜率压缩时小于0.2 MPa/℃,拉伸时小于0.5 MPa/℃),在航空航天、空间探索、减振抗震等方面展现出了良好的应用前景,成为近年来超弹性合金研究的热点。FeMnAlNi系合金的超弹性受到多种因素的影响,提高其超弹性的关键在于控制析出相合理的析出状态提高相变的热弹性,增大晶粒尺寸提高相变的协调性,选择合适的晶粒取向激活更多的马氏体变体。首先介绍了马氏体相变与超弹性的关系,然后从FeMnAlNi系合金特殊的马氏体相变出发,对影响其超弹性的主要因素,包括组织因素(析出相、晶粒尺寸、晶粒取向)以及环境因素(磁场、温度)目前的研究进展作出了归纳与总结,最后对Fe-MnAlNi系超弹性合金未来的研究发展方向进行了展望。     

TiNiAl合金形状记忆及超弹性效应研究

研究了不同热处理制度对TiNiAl合金的形状记忆恢复率的影响,得出了适宜的时效处理温度和时间.通过对合金相转变温度的测定发现Al的加入大大降低了TiNi合金的相转变温度,合金的成分对其相转变温度具有重要的影响.对合金的应力-应变曲线的测试表明其具有良好的超弹性效应.     

生物医用Ti-Nb-Sn合金的超弹性

通过室温下拉伸实验研究了Ti-14Nb-4Sn和Ti-16Nb-4Sn（at．％）合金的超弹性。发现锻造态和400℃冰水淬火态的Ti-16Nb-4Sn合金超弹性良好,通过4％变形量循环拉伸三次即可获得完全的超弹性;而400℃冰水淬火态的Ti-14Nb-4Sn合金通过3％变形量循环拉伸两次即可完全回复。Ti-14Nb-4Sn合金和Ti-16Nb-4Sn合金均以700℃冰水淬火态断裂延伸率为最大,分别为14．42％和12．02％。锻造态Ti-14Nb-4Sn合金的马氏体逆相变回复温度As为134．8℃。XRD分析结果表明室温下Ti-16Nb-4Sn合金的组织为β相和＋α″马氏体相;而Ti-14Nb-4Sn合金的室温组织除β和α″外,还存在α相。     

NiTi合金形状记忆效应的微观机制研究进展

NiTi合金具有优异的形状记忆功能和良好的生物体兼容性,近年来对它的应用研究受到工程界和医学界的重视,同时对NiTi合金形状记忆效应的微观机制的研究也在逐步深入.介绍了NiTi合金的主要特性及影响其形状记忆功能的主要因素,总结了NiTi合金的形状记忆效应和超弹性的微观机制研究现状,并指出了需对该合金进一步研究的一些问题.     

温度和应变率对NiTi记忆合金相变转化率的影响

为了探讨温度和应变率对NiTi记忆合金相变转换率的影响,本文利用准静态试验机和分离式Hopkinson压杆的恒应变率技术对板条状和圆柱状试样进行了不同应变率的试验,同时通过改变温度检查相变转换的速度,结果表明:在较低的10-2/s应变率下,NiTi记忆合金伪弹性变形过程自身就会有6.5℃温升,这意味着在高应变率下其性能是温度与率的耦合效应;随外界环境温度的增加,NiTi记忆合金的相变恢复速率由递增趋于一稳态值,当温度超过365K时,恢复速率趋缓至约0.014mm/s;随应变率增加,相变转换率会趋于极限,即应力诱发的奥氏体向马氏体相转化阶段在应变率超过6000/s时基本不变,表明更高的应变率已不能引起NiTi相变转换。     

基于TiNi合金的超弹性对其相关磨损机制的FEM研究

本文在重点考虑TiNi合金高弹性变形量的前提下，采用等向强化模型，对不锈钢和超弹TiNi合金在法向接触载荷作用下的六种模型进行了有限元(FEM)分析。结果表明：在相同载荷条件下，超弹TiNi合金产生的von Mises弹性应变要高于不锈钢，但其von Mises应力和塑性应变却恰恰相反，在同一载荷下该合金发生塑性变形的区域要小于不锈钢；此外，超弹TiNi合金发生塑性变形要比不锈钢困难，所需的临界载荷值随其最大弹性变形量(屈服点处的应变值)的增加而增加。最后，基于本文的有限元计算结果对超弹TiNi合金的蘑粒磨损和疲劳磨损机制进行了讨论。     

形状记忆合金材料相变伪弹性有限元分析

形状记忆合金(SMA)的相变伪弹性是其智能功能的体现之一,对它的分析研究,有利于把SMA材料应用于土木工程并实现其指定的功能。针对形状记忆合金的Brinson本构模型中的相变伪弹性的热力学方程和求解特点,提出了一种新的求解方法,先将热力学方程变化为差分形式,然后采用对分法对相互嵌套的热力学方程和控制方程进行求解,最后利用MATLAB软件编制了SMA材料相变伪弹性恢复力本构关系的有限元分析程序,得出了能够反映其本构特征的相变伪弹性恢复力曲线。     

外加载荷对热弹性马氏体正-逆相变影响机制的相场模拟研究

本工作建立了一种新的马氏体逆相变的相场模型,以Cu-Al-Ni合金为例,研究了热弹性马氏体正相变和逆相变的演化规律,揭示了热弹性马氏体的形状记忆效应。同时模拟了拉伸释放弹性应变能这种机制对热弹性马氏体相变和热弹性马氏体逆相变的作用,研究了外加载荷对马氏体逆相变温度As的影响。模拟结果表明：应变能是形状记忆合金马氏体相变的阻力,是其逆相变的驱动力。在马氏体正相变过程中,拉伸载荷释放了应变能,降低了相变阻力,从而对马氏体相变起促进作用;在马氏体的逆相变过程中,由于拉伸载荷降低了马氏体所储存的应变能,因而降低了逆相变过程的驱动力,使合金逆相变As温度升高,进而提高了热弹性马氏体的低温稳定性。模拟结果与实验结果相一致。     

Non-proportional multiaxial transformation ratchetting of super-elastic NiTi shape memory alloy: Experimental observations

Experimental observations are reported for the non-proportional multiaxial transformation ratchetting of super-elastic NiTi shape memory alloy performed under the stress-controlled cyclic tension–torsion loading conditions and at room temperature. The effect of axial mean stress on the evolutions of transformation ratchetting strain and dissipation energy per cycle during the cyclic tests is discussed firstly; and then the dependence of multiaxial transformation ratchetting on the different non-proportionally loading paths (e.g., linear, square, hourglass-typed, butterfly-typed, rhombic and octagonal paths) is investigated. The results show that the multiaxial transformation ratchetting occurs mainly in the axial direction because only the non-zero axial mean stress is used and the mean shear stress is set to be zero in the all prescribed multiaxial loading paths; and the axial peak and valley strains increase with the increasing axial mean stress and also depend significantly on the shapes of loading paths. Comparison with the corresponding uniaxial ones illustrates that the multiaxial stress states are more helpful to promote the development of transformation ratchetting, especially for the non-proportional ones.     

Non-proportional multiaxial whole-life transformation ratchetting and fatigue failure of super-elastic NiTi shape memory alloy micro-tubes

Experimental observations are performed to investigate the non-proportional multiaxial whole-life transformation ratchetting and fatigue failure of superelastic NiTi SMA micro-tubes in stress-controlled loadings at human-body temperature (310 K). The effects of axial mean stress and stress hold on the whole-life transformation ratchetting and fatigue life are investigated with uniaxial, torsional and five different multiaxial loading paths. The results show that the stress holds on the upper or lower transformation plateaus will both promote forward and reverse transformation, and lead to shorter fatigue life. The multiaxial fatigue lives of NiTi shape memory alloy depend significantly on loading paths and applied stress levels.     

Phase transformation behavior of porous NiTi alloy fabricated by powder metallurgical method

Nickel titanium shape memory alloys (NiTi-SMAs) were successfully produced from elemental Ni/Ti powders by powder metallurgical method and then subjected to age treatment. Microstructure was examined by SEM and XRD and phase transformation temperatures were measured by dilatometric method. The phase transformation temperatures increased with both duration and temperature of the age treatment. The porous product exhibited desirable shape memory effect.     

Effects of plate thickness on reverse martensitic transformation of prestrained NiTi/NiTi alloy

In this paper, differential scanning calorimeter (DSC) was used to study the effects of predeformation and plate thickness on the reverse martensitic transformation of explosively welded NiTi/NiTi alloy. Results showed that there was a constraint between Ni_50.4Ti (NiTi-1) and Ni_49.8Ti (NiTi-2), which led to that the thickness of NiTi-1 or NiTi-2 strongly affected the reverse martensitic transformation behavior because residual stress variations in thickness wound enable bias force to be built inside the composite. The DSC measurements showed that after deformation, the reverse martensitic transformation temperature of the composite was increased with the increasing thickness of NiTi-2. Also, the XRD results revealed that the microstructure of NiTi/NiTi alloy changed from B2 phase to B19’ phase along the thickness direction.     

热循环对NiTi合金焊接接头相变的影响

为了研究热循环对NiTi合金激光焊接接头相变行为的影响,用激光点焊了φ0.5 mmTi-50.6%Ni合金细丝,利用金相显微镜观察了母材和接头的显微组织,通过示差热分析仪研究了热循环对点焊接头相变行为的影响.结果表明,NiTi合金激光点焊接头,熔化区为树枝晶,热影响区由粗大等轴晶和细小等轴晶组成.热循环可改变接头的相变行为,使马氏体相变由B2→B19'转变为B2→R→B19'.随着热循环次数的增加,正相变过程中Rs点升高,R相变温度范围变宽,Ms点降低;逆相变的相变温度都降低.     

钛合金超声扭振铣削强度优化设计仿真

钛合金具有比强度大、热传导率低等特点,对其进行强度优化设计仿真,可以提高其切削加工性能。运用Abaqus软件建立了二维超声扭振铣削模型,并采用John-cook分离准则、任意的拉格朗日-欧拉(ArbitraryLagrange-Euler,ALE)网格划分方法和施加相应边界等条件,根据刀-屑的周期分离原理,确立了接触率与角速度、振动频率和超声振幅之间的关系。此外,采用单因素法依次改变角速度、频率和振幅,以此进行强度优化设计仿真。结果表明,切削力和温度均随角速度的增加而增加;振幅的增大使得切削力有所降低,而温度却有所上升;频率的变化对两者影响不大。并得出结论:给定参数范围内,影响切削力的权重依次为振幅、角速度和频率;影响温度的权重依次为角速度、振幅和频率。     

NiTi合金主动脉支架弯曲变形行为的研究

利用Pro/E和Ansys软件建立了胸主动脉支架轴向柔顺性分析模型,对27种NiTi合金支架的柔顺性进行了模拟分析与实验验证。在此基础上,又进一步系统地分析了弯曲变形时支架结构尺寸变化对中央部位横截面变形行为的影响。结果显示,减少圆周方向上的连接体的数量,增加支撑体的长度或数量,都能够提高支架的柔顺性,但是减少连接体的数量比增加支撑体的长度或数量对支架柔顺性的影响程度要大。支架弯曲变形时连接体的数量对支架中央部位横截面的变形具有显著的影响。     

Effect of martensite transformation on fracture behavior of shape memory alloy NiTi in a notched specimen

In this paper, the finite element calculation of the stress–strain distribution in front of a notch tip were carried out for two materials. One is a shape memory alloy NiTi with the stress-induced martensite transformation, and another is a fully transformed martensite NiTi without the transformation. Based on the results obtained, and combining a model of the fracture process zone, effect of martensite transformation on the fracture behavior of the shape memory alloy NiTi in a notched specimen of plane stress state is comparably analyzed. The results show that the martensite transformation increases the load to produce plastic deformation in the transformed martensite at the notch tip and decreases the maximum normal stress and plastic strain near the notch tip, and tends to suspend the crack nucleation and propagation in the fully transformed martensite in front of the notch tip, and thus increases the fracture load and improves the toughness. A quantitative analysis based on the model of the fracture process zone shows that the martensite transformation in the SMA NiTi causes about 47% increase in the apparent fracture toughness.     

Effect of graphite addition on martensitic transformation and damping behavior of NiTi shape memory alloy

In this investigation, the effect of graphite addition on martensitic transformation and damping behavior of Ni₅₀Ti₅₀ (at.%) shape memory alloy has been studied. It is found that martensitic transformation temperature decreases obviously with the addition of graphite. Microstructural observation shows that TiC precipitates and forms whiskers when the carbon content is increased beyond ~ 0.6%. With the increase of graphite content, the damping capacity during reverse transformation increases initially and then decreases while the damping capacity of full martensite is remarkably improved by the addition of graphite particles. It is proposed that the enhancement of damping capacity can be ascribed to the high damping capacity of graphite itself, as well as, the increase of the amount of interface between martensite and austenite can be beneficial to the damping capacity.