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。     

NiTi形状记忆合金丝在Al基复合材料中的约束态马氏体逆相变特征

利用DSC,XRD,SEM等实验手段研究了NiTi记忆合金丝在铝基体约束状态下的马氏体逆相变特征。研究结果表明,预应变NiTi记忆合金丝在加热过程中发生两种马氏体逆相变：热致马氏体逆转变(MT→P)与应力诱发马氏体逆转变(Md→P),其中热致马氏体逆相变温度与未预应变样品基本相同,且不受预应变影响;而应力诱发马氏体向母相转变温度随预应变增加而升高;随预应变增加,热致马氏体与应力诱发马氏体的逆转变量减少。文中引入马氏体变形度概念,并对马氏体在逆转变过程中的变形度变化进行了讨论。     

不同温度下纯Ni/NiTi合金的摩擦特性研究

NiTi合金具有形状记忆效应及超弹性特性,使得它与其他一般材料的摩擦特性有很大不同。为从微观角度揭示其摩擦特性,利用分子动力学研究了不同温度下纯Ni和NiTi合金的压/划痕过程,并进一步通过对比分析不同温度下纯Ni和NiTi合金在压/划痕过程中原子结构、表面形貌、摩擦力和摩擦系数的变化,研究了温度对NiTi合金摩擦系数的影响。结果表明,温度对NiTi合金摩擦性能的影响显著,在300～500 K范围内,温度越低,摩擦力与摩擦系数越小,这是由于在刻划过程中NiTi合金发生马氏体相变,NiTi合金表面向下凹陷,减少了NiTi表面与压头的接触,降低了对压头的阻碍,使摩擦力与摩擦系数大幅降低;当温度升高时,马氏体相变减少,NiTi合金的表面凹陷减少,使压头与NiTi合金的接触面积增大,阻碍增大,从而使得摩擦力和摩擦系数变大。而没有相变机制的金属Ni在刻划过程中,主要产生塑性形变,温度对其摩擦性能无显著影响。可见,温度对NiTi形状记忆合金的摩擦性能具备一定调控能力,可以通过控制温度达到减少磨损的目的,这可为延长NiTi合金元件的使用寿命提供理论基础和指导。     

TiNi合金在恒应力约束下的不完全相变

利用DSC对预应变TiNi形状记忆合金丝在恒应力约束下的马氏体不完全逆相变进行了研究,发现不完全相变热循环样品在第二次自由态加热过程中出现两步马氏体逆转变和两段应变回复现象.分析认为:经过恒应力约束下的不完全逆相变后,TiNi样品中存在不同的马氏体,在随后的加热过程中先后逆转变,产生两段回复应变.     

热处理对Ni50.3Mn24.5Ga25.2单晶预相变特性的影响

用提拉法生长的Ni50.3Mn24.5Ga25.2单晶样品,在真空中进行多种形式的热处理。通过测量交流磁化率曲线,研究了退火和淬火对Ni50.3Mn24.5Ga25.2单晶预马氏体和马氏体相变温度的影响。实验表明退火会明显降低预马氏体相变的温度,而对马氏体相变影响较小。还发现在1023K以下,预马氏体相变会随淬火温度的升高而逐步变得明显,而在1023K以上,则呈相反的趋势。另外,实验还表明,经过高温淬火后的样品,再次通过一定的退火处理,样品将恢复明显的预马氏体相变征候。分析表明,热处理是通过改变晶体的内应力来影响预马氏体相变征候的。     

约束态相变中TiNi形状记忆合金的马氏体自拉伸过程

研究了约束态加热过程中产生的回复力对TiNi形状记忆合金剩余马氏体的影响．结果表明，产生的回复力使剩余马氏体发生了塑性变形，使逆相变温度升高，剩余马氏体分数和其逆相变温度之间存在特定的函数关系．但是，外部约束条件的变化对马氏体的自拉伸过程所造成的剩余马氏体分数与其逆相变温度之间的关系影响很小．     

织构取向Ni_(54)Mn_(21)Ga_(25)合金的相变和磁致应变

用定向凝固方法制备Ni54Mn21Ga25取向多晶合金。在292K下测量了样品的磁致伸缩应变回线,结果表明:加正反方向磁场时,磁致伸缩应变回线基本对称,并存在磁致伸缩跳跃现象,饱和磁致应变约为-1.06×10-3,对应的饱和磁场为0.4T。差示扫描量热仪测量显示,马氏体相变起始温度Ms为334K,结束温度Mf为320K;逆马氏体相变起始温度As为333K,结束温度Af为353K。阻温特性测量给出样品的居里温度约350K左右。     

NiTi形状记忆合金丝在Al基复合材料的约束态马氏逆相变特征

利用DSC,XRD,SEM等实验手段研究了NiTi记忆合金丝在铝合金丝在铝基体约束状上的马氏体逆相变特征,研究结果表明,预应变NiTi记忆合金丝在加热过程中发生两种马氏体逆相变,热致马氏体转变（M^T→P) 应力诱发马氏体逆转变（M^d→P）,其中热致马氏体相变温度与未预应变样品基本相同,且不受参应变影响,而应力诱发马氏体向母相转变温度随预应变增加而升高,随预应变增加,热致马体与应力诱发马氏体的转变时减少,文中引入马氏体变形度概念,并对马氏体在逆转变过程中的变形度变化进行了讨论。     

掺杂元素对Ni-Mn-Ga合金马氏体相变和磁性能的影响

研究了添加微量的Fe、Co、Al、Si合金元素对Ni-Mn-Ga合金的马氏体相变和磁性能的影响作用.结果表明微量Fe的掺杂可显著提高合金的马氏体相变温度(Ms)而不降低居里温度(Tc);掺Co后Ms基本不变而Tc明显升高;Al使Ms升高,Si使Ms降低,而对Tc均影响不大.微量的掺杂元素并不改变合金的晶体结构,掺杂合金仍然具有热弹性形状记忆效应,并且在磁场作用下可以产生增强的形状记忆效应.     

外磁场对Ni-Mn-Ga磁性形状记忆合金相变应变及显微组织的影响

研究了外磁场对Ni52Mn24.6Ga23.4(%,原子分数)单晶马氏体相变及其相变应变的影响,并对磁场增强相变应变的微观机制进行了探讨。研究结果表明无外加磁场时,NiMnGa合金发生马氏体相变时可产生约0.3%的收缩形变,沿单晶[100]方向施加外磁场,其相变应变随磁场的增加而呈近线性增加。当外磁场强度为6.37×105A/m时,应变量达到最大值(3.5%)。磁场作用下冷却形成的马氏体虽然孪晶亚结构不变,但自协作组态消失,并伴随有孪晶板条的增厚。磁场对马氏体相变应变的增强效应来自于磁场作用下的马氏体变体的择优取向。     

Phenomenological analysis on subloops and cyclic behavior in shape memory alloys under mechanical and/or thermal loads

A theoretical framework is presented, from the phenomenological point of view, for the cyclic uniaxial deformation in shape memory alloys subjected to the thermal and/or mechanical loads by introducing three internal variables; the local residual stress and strain and the volume fraction of the martensic phase accumulated during cyclic forward and reverse martensitic transformations. The cyclic effect on the stress-strain and strain-temperature hysteresis loops is discussed. The subloops due to incomplete transformations are also analyzed by assuming the transformation starting stress or temperature which depends on the preloading. Numerical results explain qualitatively well the observations on the thermomechanical behaviors of shape memory alloys.     

Zirconia nanoparticles: a martensitic phase transition at low temperature

The low temperature (450–600°C) amorphous to tetragonal and tetragonal to monoclinic phase transformations in zirconia nanoparticles, produced by an aqueous sol–gel route, are analysed in terms of their changes of lattice parameters and by using the phenomenological theory of martensitic transformation (Bowles–Mackenzie theory) to explore whether or not those crystallographic changes may be considered a martensitic transformation within the zirconia nanoparticles.     

Martensitic transformation and shape memory properties of Ti-Ta-Sn high temperature shape memory alloys

The effects of Ta and Sn contents on the martensitic transformation temperature, crystal structure and thermal stability of Ti-Ta-Sn alloys are investigated in order to develop novel high temperature shape memory alloys. The martensitic transformation temperature significantly decreases by aging or thermal cycling due to the formation of ω phase in the Ti-Ta binary alloys. The addition of Sn is effective for suppressing the formation of ω phase and improves stability of shape memory effect during thermal cycling. The amount of Sn content necessary for suppressing aging effect increases with decreasing Ta content. High martensitic transformation temperature with good thermal stability can be achieved by adjustment of the Ta and Sn contents. Furthermore, the addition of Sn as a substitute of Ta with keeping the transformation temperature same increases the transformation strain in the Ti-Ta-Sn alloys. A Ti-20Ta-3.5Sn alloy reveals stable shape memory effect with a martensitic transformation start temperature about 440 K and a larger recovery strain when compared with a Ti-Ta binary alloy showing similar martensitic transformation temperature.     

Dilatometric study of martensitic transformation in NiTiCu and NiTi shape memory alloys

Dilatometric measurements have been carried out for the study of nature of martensitic transformation in the NiTiCu and NiTi shape memory alloy wire samples. Investigation has been done in the heat-treat temperature range 300–800°C. NiTiCu exhibits only single stage A M martensitic transformation in the entire heat-treat temperature range indicating the suppression of R-phase by Cu substitution. NiTi shows the two-stage A R M martensitic transformation in the heat-treat temperature range 340–410°C and the single-stage A M martensitic transformation above heat-treat temperature 410°C. The extent of dilation during phase transformation decreases with increasing heat-treat temperature in both the alloys. Effect of first 15 thermal cycles on transformation temperatures in both the alloys has been studied. It is found that transformation temperatures are unaffected with thermal cycles in NiTiCu whereas considerable decrease in transformation temperatures has been observed in the case of NiTi. The stability of transformation temperatures in NiTiCu during M A transformation against thermal cycling may be attributed to the associated smaller thermal hysteresis compared to NiTi.     

铁基合金中马氏体形核动力学探讨

根据马氏体相变的经典形核理论,重新估算了铁基合工中马氏体均匀形核激活能,研究了激活能与界面能和相变驱动力的关系。结果表明,热激活形核具有很低的形核激活能,热激活能形核动力学与马氏体形核动力学实验研究结果一致,在动力学研究的基础上,提出了马氏体相变的形核机制,在形核机制中,除了热激活菜核,还考虑了应力对马氏体形核的作用,特别是在低温时由于急应内应力对形核的贡献。     

Ni24.7Ti50.3Pd25.0 high temperature shape memory alloy with narrow thermal hysteresis and high thermal stability

High temperature shape memory alloys with operating temperatures above 100 °C are in demand for use as solid-state thermal actuators in aerospace, automobile and other engineering applications. The present study deals with transformation behaviour and thermal stability of Ni_24.7Ti_50.3Pd_25.0 (at.%) high temperature shape memory alloy, in cast and homogenized condition. The martensite finish temperature and transformation hysteresis of the alloy were determined to be 181.0 °C and ∼8.5 °C respectively. The alloy showed high stability upon stress-free thermal cycling, variation in transformation temperatures being ±1 °C. The narrow thermal hysteresis and high thermal stability of the alloy upon transformation cycling has been discussed and correlated with its microstructural features, activation energy and elastic strain energy of thermoelastic martensitic transformation. The alloy exhibited modulus of ∼82 GPa and hardness of ∼4.7 GPa in martensite phase.     

Stress induced martensite transformation texture in AISI 304 austenitic stainless steel

Abstract

The phenomenological theory of martensitic transformation was applied to a tension induced martensitic transformation in an AISI 304 austenitic stainless steel in order to estimate the transformation texture. Input data were obtained from the published literature. Calculated pole figures were constructed assuming a variant selection process based on Patel and Cohen’s theory, which emphasises that a mechanical component of free energy is the driving force for martensitic transformation at temperatures above martensite start M_s. The results showed a remarkably good match between the calculated and published measured data.     

Solution of an Inverse Problem of Heat Conduction of 45 Steel with Martensite Phase Transformation in High Pressure during Gas Quenching

In order to simulate thermal strains,thermal stresses,residual stresses and microstructure of the steel during gas quenching by means of the numerical method,it is necessary to obtain an accurate boundary condition of temperature field.The surface heat transfer coefficient is a key parameter.The explicit finite difference method,nonlinear estimation method and the experimental relation between temperature and time during gas quenching have been used to solve the inverse problem of heat conduction.The relationship between surface temperature and surface heat transfer coefficient of a cylinder has been given.The nonlinear surface heat transfer coefficients include the coupled effects between martensitic phase transformation and temperature.     

Simulation of martensitic microstructure

The defects appearing during the process of martensitic transformation from bcc austenite to a microtextured hcp martensite are examined, with reference to molecular dynamics simulations in zirconium. The simulations involve cooling from the high temperature austenitic bcc phase. A variety of geometric defects are identified, with the structure and dynamics being understood in terms of vicinal twin boundaries and dislocations. These defects are classified as either geometrically necessary or nanoscale effects, and the evolution of the material in response to annealing and external stress is described. The observed mechanism for twin boundary motion under applied strees involves a combination of stress concentration by preexisting sessile dislocation, causing nucleation, motion and absorption of defects in the boundary plane.     

Thermoelastic martensitic transformation in Cu-Zn-Al alloy

Thermoelastic martensitic transformation in a Cu-29% Zn-3% Al alloy has been observed with optical and electron microscopy, including 1.0–1.2 MV high-voltage transmission electron microscopy (TEM), and treated with the phenomenological theory suggested before. The observation showed that the transformation was conducted in three stages: parallel plates growing, self-accommodating variants advancing, and plates merging and/or tiny plates forming in carved-up parent phase areas. TEM showed that the martensite consisted of a huge number of packets with constant size and distinct interfaces. By using the phenomenological theory, the free-energy function as well as the friction quasi-enthalpy and friction quasi-entropy are obtained in SI units. Comparing with classical theory, the free energy can be broken into three parts: the chemical free energy, the interfacial energy and the elastic strain energy. In the range of 20–70% martensite in the alloy, the interfacial energy per unit of martensite formation is constant and the corresponding elastic strain energy is a linear function of martensite percentage. Some possible explanations for this energetics relating to the observation are given.