Direct measurement of large reversible magnetic-field-induced strain in Ni–Co–Mn–In metamagnetic shape memory alloys

Direct measurements of reversible magnetic-field-induced strain (MFIS) on a single crystalline Ni₄₅Co₅Mn_36.5In_13.5 metamagnetic shape memory alloy were attained via magnetic-field-induced martensitic transformation under different stress levels and at various temperatures. This was achieved using a custom-designed micro-magneto-thermo-mechanical testing system capable of applying constant stress while measuring strain and magnetization simultaneously on the samples, which can fit into conventional superconducting magnets. MFIS levels are reported as a function of temperature, magnetic field and external bias stress. It was necessary to apply an external bias stress in these materials to detect a notable MFIS because a magnetic field does not favor a specific martensite variant resulting in no shape change even though magnetic field leads to reversible martensitic transformation. Fully recoverable transformation strains up to 3.10% were detected under repeated field applications in the presence of different compressive stress levels up to 125 MPa. The bias stress opposes the field-induced martensite-to-austenite phase transformation and causes the critical field for the transformation to increase at a given temperature in accordance with the Clausius Clapeyron relationship. The effect of the bias stress on the kinetic arrest of austenite is also explored.     

单晶Ni_(52)Mn_(24)Ga_(24)的磁感生应变和磁增强双向形状记忆效应

铁磁性Heusler合金Ni2 MnGa是近年开发的磁控制功能材料 ,已发现该材料结合马氏体相变可以产生大磁致伸缩 (磁感生应变 )和磁控制形状记忆效应两种应用功能。用磁悬浮冷坩埚提拉设备沿 [0 0 1]方向生长了组分为Ni52 Mn2 4 Ga2 4 的单晶。室温时沿该单晶样品 [0 0 1]方向加磁场 ,在该方向获得了 - 0 6 %的大磁感生应变。当磁场方向垂直于 [0 0 1]方向时 ,样品在 [0 0 1]方向的磁感生应变值为 0 5 %。同时该单晶样品在室温附近还具有可由磁场增强和控制的双向形状记忆效应。无磁场作用时 ,降低温度 ,样品在发生马氏体相变时 ,在 [0 0 1]方向产生1 2 %的收缩形变。随后升高温度 ,反马氏体相变时样品以同样的应变量膨胀 ,恢复到原来的形状 ,显示了特有的无需外应力协助的自发的双向形状记忆效应。其温度滞后只有 10℃。如果在样品的 [0 0 1]方向加一个偏磁场 ,其形状记忆的应变量随磁场的增强而增大。在磁场为 1 2T时可达 4%。而当磁场转向 [10 0 ]方向时 ,形状记忆的应变可以改变符号。本文指出产生大磁感生应变和磁增强双向形状记忆效应的关键是马氏体变体的择优取向。     

Ni_(52)Mn_(24)Ga_(24)金属间化合物的单晶生长和磁性功能

研究了 Ｎｉ５２Ｍｎ２４Ｇａ２４合金单晶在磁场作用下能产生的目前最大的磁致伸缩应变．发现了该材料的双向相变应变效应以及磁场对此的增强现象．在 １．２ Ｔ磁场的作用下,可逆的相变应变达 ４％以上实现这一结果的关键条件是马氏体变体的择优取向报道了获得马氏体变体的择优取向样品的单晶生长特性和后处理方法根据前人报道的理论模型分析了实验结果,指出磁感生应变的物理机制是磁场提供的 Ｚｅｅｍａｎ能驱动变体间孪晶界的移动     

相场法模拟弹性应变能对Ti-Al-Nb合金α2→O相变粗化动力学的影响

采用相场方法模拟了Ti-Al-Nb合金α2→O相转变,探讨了有无外力场作用下弹性应变能对O相颗粒形貌、取向、数目、体积分数及平均尺寸的影响.结果表明:在弹性应变能作用下,颗粒形貌为长方块状,且沿弹性软方向分布;无外力场时,弹性应变能越大,沉淀相越易形核,稳定时颗粒数目越多,体积分数及平均尺寸越小;在外力场作用下,取向有利的变体优先长大,取向不利的变体长大受到抑制;在较小压应力作用下,沉淀相易于形核,稳定时颗粒数目增多,体积分数减小;在拉应力或较大压应力作用下,应力越大,越难于形核,稳定时颗粒数目越少,体积分数越大.     

应力/应变场对插销试验焊接接头氢扩散的影响

采用ABAQUS有限元分析软件及有关焊接接头中氢扩散与集聚的计算模拟技术计算分析了不同外加载荷条件下插销试件的能力、应变场及其对氢扩散与聚集过程的影响。计算结果表明，静载拉伸条件下插销缺口前沿有明显的应力应变集中，随着外加载荷的增加，缺口尖端应力逐渐增大，但应力集中系数逐渐减小；在相同初始氢条件下，随着外加载荷的增加，缺口前沿应力应变集中部位氢的峰值浓度逐渐提高，氢浓度达到峰值的时间也逐渐增长。分析认为，插销缺口附近产生的应力应变集中是导致氢在该区域产生聚集的重要原因。     

应力和晶粒组织对磁畴动态特性的影响

近年新兴的磁光成像技术,特别是磁畴动态特性可视化技术,为评估铁磁材料的应力状态提供了一种新的磁检测方法。为研究应力和晶粒组织/晶界对磁畴动态特性的影响,通过磁光成像装置,观测取向硅钢片在施加不同的应力和外加磁场时表面磁畴结构的变化。结果表明:应力作用下,附加磁畴数量减少,从点状附加磁畴转变为长条状主磁畴,且应力增加会使主磁畴平均宽度减小,主磁畴完全翻转所需要的磁场减小;外加磁场作用下,可使磁畴向磁场方向转动,磁场强度的增加,使与外加磁场同向的磁畴面积增加。同时,晶粒组织结构和内部的各向异性,以及晶界处的能量和钉扎作用,都会影响磁畴结构和磁畴壁的运动状态。研究结果有助于理清铁磁构件的磁弹耦合效应和磁检测技术的微观物理机制,为提高应力评估的灵敏度和可靠性提供重要的科学依据。     

Kinetic arrest behavior in martensitic transformation of NiCoMnSn metamagnetic shape memory alloy

High-field magnetic measurements were carried out in order to investigate behaviors of field-induced reverse martensitic transformation and kinetic arrest of NiCoMnSn metamagnetic shape memory alloy. In the thermomagnetization curves, it was confirmed that the reverse martensitic transformation temperature decreases 67 K by applying magnetic field of 5 T, while in the magnetic field cooling process under 5 T, martensitic transformation does not occur down to low temperatures. Equilibrium magnetic field, defined from the critical magnetic fields of the metamagnetic evidence in the magnetization curves, exhibits almost constant below about 100 K, suggesting that the entropy change becomes zero, which is considered to cause kinetic arrest behavior.     

Fe81Ga19磁致伸缩合金的动态磁导率研究

研究了Fe81Ga19磁致伸缩合金在不同的偏置磁场和频率下的动态磁导率。在低频或准静态下,该合金的磁导率能够达到160以上,但随频率增加,磁导率逐渐下降,频率大于6 KHz时,磁导率的下降减缓,并逐渐趋于稳定。当施加平行偏置磁场时,磁导率明显降低,而施加垂直偏置磁场时,与零偏置磁场相比,磁导率除较低频率段有少量的下降外,基本保持不变。     

强磁场对钢中马氏体相变的影响及其研究展望

固态相变中,由于两相之间的磁性差异,外加磁场将改变奥氏体向马氏体的相变过程,同时对马氏体的微观组织产生影响。主要阐述了马氏体相变过程中,强磁场对其产生的影响。相关研究表明,磁场可以提高马氏体的开始转变温度,这主要是由磁场的静磁能效应、高场磁化率效应和磁致伸缩效应导致的。磁场促进了马氏体的形核和长大,从而使其含量增加,但磁场对马氏体的形貌影响不显著。同时介绍了强磁场作用下马氏体相变的最新进展与未来展望。     

Mechanical Behavior of Shape Memory Alloys Under Complex Loading Conditions of Stress, Strain, and Temperature

The transformation behavior of shape memory alloys is simulated for complex loadings of stress, strain, and temperature. Calculations are made by using the ??Accommodation Model?? which is a constitutive model for shape memory alloys considering the accommodation behavior of the transformation strain. Calculated results are given for the superelastic behavior, the shape memory effect, the transformation behavior under temperature change with stress or strain holding, the structural behavior of a shape memory wire with a bias spring, etc. The effect of the plastic strain on the transformation strain is also investigated.     

Magnetoelastic interactions in a cracked ferromagnetic body

The stress–strain state associated with a moving crack in a ferromagnetic material is investigated. The model considers a soft magnetic ferroelastic body and incorporates a realistic (nonlinear) susceptibility. The moving crack propagates in a direction perpendicular to the magnetic field. A closed-form solution yields the magnetoelastic stresses and a stress intensity factor. An applied magnetic field makes the stress intensity factor depend upon the crack velocity. The nonlinear susceptibility produces a completely different magnetoelastic stress state than a constant susceptibility, and the stress intensity is highly sensitive to material properties. The stresses around the crack are largely insensitive to the external magnetic field and crack speed except at critical combinations for which the stresses are singular. Some combinations of magnetic field and crack velocity cause the stress components ahead of the crack tip to change sign.     

NiTi形状记忆合金的热电行为

研究了形状记忆合金线的热电行为。当合金线被加热到温度高于其相变温度时,由于相的转变而产生一个大的机械力。将 SMA 线用作驱动器,并研究了不同的参数及它们之间的关系。在不同的压力水平下,这些变化的参数分别是弹性应变(位移)、温度磁滞和电阻。通过传递热模型得到安全的加热电流并预测了相变温度。在自然空气对流的条件下,采用680 mA的电流加热、冷却合金线796 s。在43 MPa的应力水平下,应变恢复率为4.33%,相应的电阻变化为11.2%。在加热、冷却循环过程中,电阻变化与位移和电流分别呈线性关系。该研究有助于精确控制有、无外部传感器反馈的SMA线驱动器。     

Giant magneto-impedance effect of two paralleled amorphous microwires

Giant magneto-impedance（GMI）is effectively enhanced by the mutual magnetic interaction between two amorphous microwires.A comparative study on GMI properties of a single wire and two wires arranged in parallel mode was reported in this work.Two-peak（TP）of impedance characteristic is presented when the dc external field changed from 0 to 320 A m^-1in two-wire system,which is attributed to successive magnetization process in two wires induced by their magnetic interaction.And the evolution of single peak to TP phenomenon,when the distance between two wires is upto 8 mm,evidences a distance dependence of transformation from successive magnetization to simultaneous via a corresponding distance dependence of magnetostatic interactions.It is proposed that the recombination of magnetic interaction and the shielding effect results in a distance dependency of GMI response.When the distance is 8 mm,the magnetization process is close to synchronous between two wires,which give rises to higher circular permeability and better GMI response.The impedance ratio DZ/Z increases from74.5 % of single wire to 172.4 % at 10 MHz.However,when the distance is upto 12 mm,the magnetic interaction is weak and magnetization process is completely independent,and GMI response decreases,relatively.This indicates that the GMI response could be effectively improved in a two-wire connection with an opticaldistance,which is promising and useful for the application of high-performance GMI sensors.     

相场法模拟弹性应变能对Ti-Al-Nb合金α2→O相变粗化动力学的影响

采用相场方法模拟了Ti-Al-Nb合金α₂→O相转变,探讨了有无外力场作用下弹性应变能对O相颗粒形貌、取向、数目、体积分数及平均尺寸的影响.结果表明:在弹性应变能作用下,颗粒形貌为长方块状,且沿弹性软方向分布;无外力场时,弹性应变能越大,沉淀相越易形核,稳定时颗粒数目越多,体积分数及平均尺寸越小;在外力场作用下,取向有利的变体优先长大,取向不利的变体长大受到抑制;在较小压应力作用下,沉淀相易于形核,稳定时颗粒数目增多,体积分数减小;在拉应力或较大压应力作用下,应力越大,越难于形核,稳定时颗粒数目越少,体积分数越大.     

超细晶铁素体基体Q&P钢恒应力下的变形行为

采用预淬火热处理工艺得到了超细晶Q&P钢,通过力保载程序控制试验研究了试验钢在某一恒定应力作用下的组织演变和变形行为。结果表明,随着保载应力不断增大,应变量逐渐增加,应变随保载时间的延长略微增加;当加载应力低于屈服强度时,随保载时间延长,应变无变化;加载应力高于屈服强度时,变形量剧烈增加,在变形过程中应变诱导的残留奥氏体向马氏体转变;同时随着应变量不断增加,残留奥氏体的稳定性呈先降低再升高趋势。     

环境磁场对磁记忆信号的影响

金属磁记忆检测技术是一种可早期准确判断构件的应力集中位置和评估疲劳损伤程度的无损检测技术新方法,而磁记忆信号的存在离不开环境磁场。为探讨环境磁场对磁记忆信号的具体影响,在不同环境磁场下,对45钢进行静载拉伸试验,测量在相同环境磁场下不同应力作用下的磁记忆信号。试验结果表明:环境磁场不能改变磁记忆信号曲线的形状,但可以改变磁记忆信号值的大小;在一定的磁场范围内,磁记忆信号值随环境磁场的增加而增加,但当环境磁场超过某一临界值时,磁记忆信号值反而随环境磁场的增加而减少;若环境磁场为零或完全被抵消,应力则不能产生磁记忆信号。故在磁记忆检测实践中,特别是在定量检测应用中必须考虑环境磁场的影响。     

Martensitic transformation and mechanical properties of Ti-rich Ti-Ni-Cu melt-spun ribbon

Martensitic transformation, mechanical and thermomechanical properties of a Ti-rich Ti₅₂Ni₂₃Cu₂₅ melt spun ribbon annealed at a temperature below the crystallization temperature were studied by XRD, DSC and DMA. After annealing the initially amorphous ribbon at 400 °C for 10 h, the ribbon is fully crystallized and exhibits one-stage B2?B19 phase transformation with the temperature hysteresis of 14 °C. The annealed ribbon is composed of B2, B19 and B11-TiCu phase with (001) preferential orientation. On the stress―strain curves, the rearrangement of the martensite variants and stress-induced martensitic transformation are observed below the M_f temperature and above the A_f temperature, respectively. The annealed ribbon exhibits up to 1.6% superelastic shape recovery with small stress hysteresis of 25 MPa. No flat stress-plateau is associated with the superelasticity. The annealed ribbon shows a well-defined shape memory effect during thermal cycling from ?60 to 100 °C. The transformation strain and recovery strain increase with increasing the applied external stress. Under the external stress above 150 MPa, the shape recovery strain is not sensitive to it and keeps stable at about 1.74%.     

电磁耦合能作用下2A02叶片锻件的残余应力松弛

针对航空大尺寸薄壁件因残余应力释放导致加工变形的行业问题，提出了一种电磁耦合处理的新型材料处理方法，并将其应用于调控压气机2A02铝合金叶片锻件的残余应力。研究了电、磁场工艺对锻件残余应力和力学性能的影响；并对材料组织开展准原位EBSD分析。结果表明，相较于单一物理场，电磁耦合处理对残余应力的松弛效果最显著，在磁场强度为1.5 T、电场强度为750 V/m时，叶片锻件残余应力最大降幅为53%。电磁耦合处理可以在不损害强度的情况下，改善铝合金塑性，在上述电磁场参数下，其伸长率提高14.3%，电阻下降4.9%。EBSD准原位分析表明，电磁耦合处理后，锻件的几何位错密度降低58.2%，小角度晶界减少，塞积的位错分散、湮灭，局部应变降低，宏观应力松弛。     

NiMnGa合金磁控形状记忆效应及外特性

系统地阐述了铁磁NiMnGa合金的微结构、相变机理、形状记忆效应，并对NiMnGa磁控形状记忆合金的外特性进行了实验研究，得出了磁场、温度、预压力与NiMnGa合金形变率之间的静态关系，绘制了不同温度和预压力下的动态曲线，得比了静态形变率和动态形变率的变化规律。实验结果证明，NiMnGa合金变形率随预压力的增大而减小；随磁场强度的增大而增大；当温度接近向奥氏体转变温度时，形变率最大；这为NiMnGa合金的实际应用打下了理论和实验基础。     

The energetics of magnetoelastic actuators is analogous to phase transformations in materials

A ferrogel is a composite system comprised of a polymeric matrix and magnetic filler particles. The elastic properties of the polymer can be coupled with the magnetic properties of the particles to create novel soft actuators. Understanding the mechanical behavior of ferrogels in an external magnetic field is essential to optimize actuator performance. The energetics of the mechanical behavior of cylindrical ferrogel specimens was found to be analogous to the energetics of chemical phase transformations in materials. Depending on the sample geometry, the elongation mechanism of ferrogel cylinders in an external magnetic field was identified as either a continuous or discontinuous deformation, analogous to a second- or first-order phase transformation, respectively. In analyzing mechanical strain as a function of magnetic field, the first and second derivatives of energy can be used to predict metastability and transitions in ferrogel deformation behavior.