<p>The shape memory effect exists in the temperature range between martensitic phase transformation temperature and reverse martensitic phase transformation temperature, thus the control of martensitic phase transformation temperature is a key issue for the application of shape memory alloys. Valence electrons have been thought to dominate phase stability and phase transformation temperature in TiNi alloy. Inconsistent with the valence electron theory, Ti-site Ni could lead to a significant decrease of phase transformation temperature in TiNi alloy. To deeply understand the effect of Ti-site Ni, a point-defect-perturbation strategy was proposed to prove that Ti-site Ni indeed induced a local lattice instability in B2 austenite. It is the structural feature of instability final phase that one-dimensional <100>_B2 atomic column compression and <111>_B2 column expansion from the perturbation site. The final phase is energetic lower than B2 structure, and the lowest energy of final phase is 20 meV/atom lower than B2 structure, when the perturbing Ti-site Ni content reaches 2%~4%. In contrast to the case in austenite, Ti-site Ni did not induce the lattice instability in B19′ martensite. The difference between austenite and martensite is to some extent the origin of the significant decrease of phase transformation temperature brought by Ti-site Ni in TiNi alloy.</p>

提出了点缺陷扰动策略,并利用此策略证实Ti位Ni实际上引起了B2奥氏体局域晶格失稳。失稳终态相的结构特征是从扰动位出发的一维方向上的_B2原子列收缩和_B2原子列膨胀。失稳终态相的能量低于B2相,最低能量比B2相低20 meV/atom,在Ti位Ni浓度达到2%~4%时出现。与奥氏体情况相反,Ti位Ni无法令B19′马氏体失稳。Ti位Ni显著降低TiNi合金相变温度的现象一定程度上来源于此。

Room-temperature brittleness and strain-softening during deformation of bulk metallic glasses, and limited processability of shape memory alloys have been stumbling blocks for their advanced functional structural applications. To solve the key scientific problems, a new shape memory bulk metallic glass based composite, through the approach using transformation-induced plasticity (TRIP) effect of shape memory alloys to enhance both ductility and work-hardening capability of metallic glasses, and superplasticity of bulk metallic glass in supercooled liquid region to realize near net forming, was developed in this work. And the Ti-Ni base bulk metallic glass composites (BMGCs) rods were prepared by the levitation suspend melting-water cooled Cu mold process. Microstructure, thermal behavior, mechanical properties and high temperature deformation behavior of the alloy were investigated. The results show that the as-cast alloy microstructure consists of amorphous matrix, undercooled austenite and thermally-induced martensite. Besides, the size of the crystal phase precipitated on the amorphous matrix increases from the surface to the inside. The alloy exhibits excellent comprehensive mechanical properties at room temperature. The yield strength, fracture strength and the plastic strain of alloy are up to 1286 MPa, 2256 MPa and 12.2%, respectively. Under compressive loading in the supercooled liquid region, the composite exhibits approximate Newtonian behavior at lower strain rate in higher deformation temperature, and the optimum deformation temperature is T>480 ℃ and the intersection part with supercooled liquid region (SLR). When the temperature is 560 ℃ and the strain rate is 5×10-4 s-1, the stress sensitivity index m and the energy dissipation rate ψ are 0.81 and 0.895, respectively. Furthermore, the volume of activation is quantified to characterize the rheological behavior.

采用水冷Cu坩埚悬浮熔炼-Cu模吸铸法制备了 Ti-Ni基块体金属玻璃复合材料(BMGCs)试棒,研究了合金的微观组织、热力学行为以及室温和高温力学性能。结果表明,该铸态合金组织由非晶基体和过冷奥氏体及热致马氏体组成,且晶体相尺寸由表及里增大。在室温压应力加载时,合金表现出优异的综合力学性能,其屈服强度为1286 MPa,断裂强度为2256 MPa,且塑性应变为12.2%。在过冷液相区压应力加载时,合金在高的变形温度和低应变速率下,表现出近Newtonian流变特征,其最佳变形温度为T>480 ℃且与过冷液相区(SLR)的交集部分。温度为560 ℃、应变速率为5×10^-4 s^-1时,合金应力敏感指数m和能量耗散率ψ分别为0.81和0.895。

运用拓扑模型研究了等原子比NiTi合金B2-B19'马氏体相变晶体学,根据最优扭转角(ω_o)准则计算得到ω_o=-0.969°,并获得了马氏体惯习面指数及母相-马氏体相界面位错结构特征,计算结果与实验测量值非常接近。NiTi合金马氏体相变所产生的相变应变包含一个平行于惯习面的剪切应变和一个垂直于惯习面的轴向应变,轴向应变量表示B2-B19'相变所引起的宏观体积变化为$ε_{33}^{HP}$=6.6879×10^-3,表明NiTi合金的负热膨胀现象来源于合金中马氏体相变所产生的相变应变。

The micro-actuater materials are needed urgently in micro-electro-mechanical systems (MEMS) which are developing rapidly. The melt-spun Ti-Ni shape memory alloy ribbons have become candidate materials since their fast heat response and large acting density. The bulk Ti-47Ni (atom fraction, %) shape memory alloy is an ideal material to make thermosensitive actuators since its one-stage martensitic transformation and small temperature hysteresis. In order to develop the micro-actuator materials with fast response using in MEMS, the chilled Ti-47Ni alloy ribbons were fabricated by melt-spinning in this research. The effects of the roller speed and the annealing processes on microstructure, phase composition, phase transformation behaviors and shape memory effect of Ti-47Ni alloy ribbons were investigated by CLSM, XRD, DSC and bending test. The results show that the microstructure of as-cast and 300~800 ℃ annealed Ti-47Ni alloy ribbons fabricated under different roller speeds is vertically and horizontally arrayed columnar. The higher the roller speed, the finer the grain is. The annealing processes do nearly affect the microstructure of the alloy ribbons. The composition phases of Ti-47Ni alloy ribbons are martensite (B19' phase, monoclinic structure) and parent phase (B2 phase, CsCl-type structure). The B2→B19'/B19'→B2 type one-stage martensitic transformation occurs in Ti-47Ni alloy ribbons upon cooling and heating, the martensitic transformation temperature and the reverse martensitic transformation temperature are about 54 and 81 ℃, respectively, and the temperature hysteresis is about 27 ℃. With increasing the roller speed, the martensitic transformation temperatures of the alloy ribbons decrease, and the recovery rate of shape memory of the alloy ribbons increases. With increasing the annealing temperature, the transformation behaviors of the alloy ribbons change a little, and the recovery rate of shape memory changes in the range of 93%~98%. The as-cast and annealed Ti-47Ni alloy ribbons are all of excellent shape memory effect.

为了开发微机电系统用快响应微执行器材料,采用熔体快淬法制备了激冷Ti-47Ni (原子分数,%)形状记忆合金薄带,利用CLSM、XRD、DSC和弯曲实验研究了铜辊速率和退火工艺对Ti-47Ni合金薄带显微组织、相组成、相变行为和形状记忆行为的影响。结果表明,不同辊速制备的铸态和300~800 ℃退火态Ti-47Ni合金薄带的显微组织均呈纵横排列的柱状,辊速越高合金薄带的晶粒越细,退火工艺对合金薄带显微组织影响不大。Ti-47Ni合金薄带的组成相为马氏体(B19'相,单斜结构)+母相(B2相,CsCl型结构),冷却/加热时发生B2→B19'/B19'→B2一阶段马氏体相变,正、逆马氏体相变温度分别约为54和81 ℃,相变热滞约为27 ℃。随辊速增加,合金薄带马氏体相变温度降低,形状记忆恢复率提高。随退火温度升高,合金薄带相变行为变化不大,形状记忆恢复率在93%~98%之间变化。铸态和退火态Ti-47Ni合金薄带皆具有优异的形状记忆效应。

<p>When NiTi shape memory alloys thin strips under the uniaxial tensile deformation, the stress-induced martensitic transformation tends to exhibit strain localization and instability, and the sample shows reversible transformation bands evolution on macroscopic. The displacement, strain and temperature fields were investigated with in situ optical method, and the full-field strain and temperature information about martensitic localization were quantitatively obtained during uniaxial loading&mdash;unloading conditions. Strain field is calculated by digital image correlation (DIC) method, and the temperature field is captured by infrared thermograph, the thickness field and in-plane rotation angle are also calculated by DIC data.The strain, temperature variation, thickness, in-plane rotation both inside and outside of the transformation bands were studied when it nucleation, expansion, combination, reduction and disappear. The results show that transformation strain of the samples are mainly concentrated inside the transformation bands but small outside, and temperature variation mainly concentrated in the transformation fronts, the thickness field in transformation bands is 2% smaller than out of bands. In-plane rotation angle is not only concentrated in the transformation fronts, but also heterogeneous in the transformation bands. In addition, the maximum in-plane rotation angle during tension is 1.5 &deg;. The whole loading-unloading progress is full thermal coupling, transformation localization, martensite and austenitic critical nucleation stress are greatly influenced by temperature variation.</p>

采用数字图像相关(DIC)方法原位测定了NiTi形状记忆合金薄带在单轴拉伸下变形时的应变场,采用红外热成像法(IR)原位测定其温度 场, 并定量获得了单轴拉伸加载-卸载条件下马氏体带局部化的全场应变与温度信息. 同时研究了相变带的形核、扩展、合并、缩减、 分裂及消失等过程中相变带内和带外应变场、温度场、厚度场及折曲角场的变化规律. 结果表明, 相变应变主要集中在相变带内部, 而带外应变量很小, 温度场和厚度场变化主要集中在相界面移动的前端,折曲角场变化集中在相变前端且带内分布不均匀.

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(A_s)和结束温度(A_f)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A_s和A_f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.

建立了适用于含孔隙NiTi合金中B2-R相变的相场模型, 并用该相场模型研究了多孔NiTi合金中B2-R转变的微观组织演化过程以及孔隙率和孔尺寸对R相变体生长动力学行为的影响. 多孔NiTi合金中R相变体以相互协调的方式形成&ldquo;带状&rdquo;的三维结构和&ldquo;鱼骨&rdquo;状的二维组织, 变体之间形成的孪晶面包括{101}_B2和{001}_B22种, 4组变体相交于_B2; 这些结果与致密NiTi合金B2-R相变过程相同. 多孔NiTi合金中 R相优先在孔周围形核, 且较大的孔周围有较多的变体形核; R相变体的平均尺寸随孔隙率增大而逐渐减小, 随孔径增大而增加; 相对于规则圆孔, 不规则孔隙可导致R相变体尺寸略微减小; 变体的尺寸均匀性则随孔隙率增大而提高, 但对孔径大小和孔形状不敏感; 孔隙数量越多且孔径越小, 则B2-R转变越倾向于产生均匀而细密的R相组织.

The effect of Cr addition on the transformation and the cyclic deformation characteristics in 550℃ annealed Ti-50.8Ni shape memory alloy (SMA) were investigated by differential scanning calorimetry (DSC) and tensile test. The B2&rarr;R&rarr;M/M&rarr;B2 type reversiable transformation occurred in Ti-50.8Ni alloy upon cooling/heating. After the addition of 0.3%Cr in Ti-50.8Ni alloy, the transformation type for Ti-50.8Ni-0.3Cr alloy was B2&rarr;R&rarr;M/M&rarr;R&rarr;B2, and the transformation temperatures decreased significantly. At room temperature, the phase composition was parent phase B2 for both Ti-50.8Ni and Ti-50.8Ni-0.3Cr alloys, while the former showed shape memory effect; the latter showed superelasticity. With increasing cyclic deformation number <em>n</em>, Ti-50.8Ni alloy evolved from shape memory effect to linear superelasticity, and its stress inducing martensitic critical stress and the accumulation residual strain increased rapidly; while Ti-50.8Ni-0.3Cr alloy evolved from incomplete superelastic to complete superelastic, and the stress-strain curve shape was stable. With increasing <em>n</em>, the superelastic residual strain <em>&epsilon;</em>_r decreased and the superelastic strain recovery ratio <em>&eta;</em>_s of Ti-50.8Ni alloy increased, while the <em>&epsilon;</em>_r and <em>&eta;</em>_s in Ti-50.8Ni-0.3Cr alloy kept at lower and higher values, respectively. The superelastic of Ti-50.8-0.3Cr alloy was stable.

运用相场模型模拟了外加应力对NiTi形状记忆合金中Ni4Ti3沉淀相组织形貌及长大行为的影响。无外加应力作用的时效过程中，4组Ni4Ti3变体均沿着各自对应的(111)B2惯习面从NiTi基体相中析出；而在B2方向施加压应力时，则只有一组Ni4Ti3变体从NiTi基体相中析出，其惯习面法线方向平行于外加应力方向；单轴压应力作用下时效时， Ni4Ti3形核数量明显增加。外加单轴压应力可促进Ni4Ti3变体的形核和长大，但并不改变变体的面积分数、长度、宽度以及长度-宽度比等参数随时效时间变化的趋势；相同时刻，压应力增大使变体长度及宽度均略微增加，但均小于无应力作用下的情形；而相同时刻，变体的面积分数随外加应力的增大而增加。相场模拟结果与实验观察能较好地吻合。

用拉伸实验研究了退火温度(&theta;_an)和应力-应变循环对Ti-50.8Ni-0.3Cr合金超弹性(SE)的影响. 结果表明, 随&theta;_an升高, 应力诱发马氏体(M)相变的临界应力&sigma;_M先降低后升高, 随后趋于稳定, 分别在450和600℃达到最小值(311 MPa)\linebreak 和最大值(583 MPa). 残余应变在&theta;_an=350-590℃之间变化不大, 始终保持在较低的水平; 当&theta;_an高于590℃时, 残余应变则急剧升高. 随着应力-应变循环次数(N)增加, 350和590℃退火态合金由部分非线性超弹性逐渐转变为完全非线性超弹性, 而650℃退火态合金由部分非线性超弹性逐渐转变为线性超弹性. 此外, 随N增加, 590和650℃退火态合金的能耗作用逐渐下降, 并趋于稳定, 而350℃退火态合金的能耗作用基本保持不变. 要使Ti-50.8Ni-0.3Cr合金获得良好的SE, 退火温度应低于再结晶温度.

A Fe/NiTi composite wire fabricated by simple mechanical processing exhibits a singular deformation feature of strain recovery ratio increase with increasing tensile strain, which is diametrically opposite to that of conventional metal materials. Furthermore, the composite possesses a recoverable strain of over 7% and a higher mechanical damping capability than that of the well-known high damping NiTi shape memory alloy. In-situ synchrotron X-ray diffraction reveals that these extraordinary features originate from the strong interaction between the soft Fe core and the superelastic NiTi tube during tensile cycling. This study offers in-depth understanding of the deformation behavior of the composites composed of soft metal and superelastic shape memory alloy.

Freestanding nanowires have ultrahigh elastic strain limits (4 to 7%) and yield strengths, but exploiting their intrinsic mechanical properties in bulk composites has proven to be difficult. We exploited the intrinsic mechanical properties of nanowires in a phase-transforming matrix based on the concept of elastic and transformation strain matching. By engineering the microstructure and residual stress to couple the true elasticity of Nb nanowires with the pseudoelasticity of a NiTi shape-memory alloy, we developed an in situ composite that possesses a large quasi-linear elastic strain of over 6%, a low Young's modulus of ~28 gigapascals, and a high yield strength of ~1.65 gigapascals. Our elastic strain-matching approach allows the exceptional mechanical properties of nanowires to be exploited in bulk materials.

This study investigated the elastic deformation behaviour of Nb nanowires embedded in a NiTi matrix. The Nb nanowires exhibited an ultra-large elastic deformation, which is found to be dictated by the martensitic transformation of the NiTi matrix, thus exhibiting unique characteristics of locality and rapidity. These are in clear contrast to our conventional observation of elastic deformations of crystalline solids, which is a homogeneous lattice distortion with a strain rate controlled by the applied strain. The Nb nanowires are also found to exhibit elastic-plastic deformation accompanying the martensitic transformation of the NiTi matrix in the case when the transformation strain of the matrix over-matches the elastic strain limit of the nanowires, or exhibit only elastic deformation in the case of under-matching. Such insight provides an important opportunity for elastic strain engineering and composite design.

纳米线具有超高强度及超大弹性应变,以其增强的复合材料被预期具有超常力学性能,然而,已有研究结果一直“令人失望”。其原因是纳米线的超常力学性能未能在复合材料中体现。基于对位错滑移型金属基体中的位错与纳米线界面交互作用是“失望”之源的猜想,提出采用点阵切变型金属的切应变与纳米线的弹性应变相匹配的设计概念,选择纳米线Nb/NiTi 记忆合金等体系,证实了纳米线(带、粒子)的超常力学性能得以在复合材料中体现,使复合材料具备超常力学性能成为现实。此外,还发现上述突破可引发复合材料呈现超常力学性能的新机制(大应力耦合效应等)。据此设计的Nb纳米线/NiTi 记忆合金复合材料兼具：弹性应变极限大于6%,弹性模量低于28 GPa,屈服强度高达1.65 GPa。

An open question is the underlying mechanisms for a recent discovered nanocomposite, which composed of shape memory alloy (SMA) matrix with embedded metallic nanowires (NWs), demonstrating novel mechanical properties, such as large quasi-linear elastic strain, low Young's modulus and high yield strength. We use finite element simulations to investigate the interplay between the superelasticity of SMA matrix and the elastic-plastic deformation of embedded NWs. Our results show that stress transfer plays a dominated role in determining the quasi-linear behavior of the nanocomposite. The corresponding microstructure evolution indicate that the transfer is due to the coupling between plastic deformation within the NWs and martensitic transformation in the matrix, i.e., the martensitic transformation of the SMA matrix promotes local plastic deformation nearby, and the high plastic strain region of NWs retains considerable martensite in the surrounding SMA matrix, thus facilitating continues martensitic transformation in subsequent loading. Based on these findings, we propose a general criterion for achieving quasi-linear elasticity.

Measurements of the Young’s modulus are reported for in-situ formed Cu-Nb wire composites containing 7.5 and 15.0 vol.% niobium filaments. Comparison of the experimental values for the as-drawn and annealed wires shows rapidly diverging trends with decreasing wire diameter. In composites with the smallest filaments (50–200 Å) the modulus increases by almost 100% upon annealing and exceeds the maximum values calculated from bulk elastic constants. However, plastic strain (in elongation) of as little as 0.02% results in a substantial (∼25%) reduction in modulus, suggesting that most of the enhancment in the annealed composites originates from the thermally induced elastic strains at the matrix-filament interfaces.