用于中子衍射的原位应力加载装置

一种小型化的应力加载系统,配合中子衍射应力谱仪可实现对多晶材料的原位测量,为进一步获得材料内部相、织构与应力演化的原位中子衍射实验结果,建立基于微观机制的材料宏观本构模型提供可能。该系统利用伺服电机提供动力,机架使用7050铝合金材料制造,系统的拉伸强度可达10 kN,运动速度可调（1 m/s~1 mm/s）。试样拉伸（压缩）时,S型传感器内部应变片变形产生电压信号,再经PLC处理后得到试样应力与应变之间关系。通过与英斯特朗5967实验拉伸机针对同一钢质试样进行对比实验发现,应力-应变曲线一致性良好。     

中子衍射分析时效处理对镍基单晶高温合金相结构的影响

采用φ振荡和φ固定两种数据采集模式的中子衍射实验结果表明较高的时效温度对消除枝晶最有效,微应变(晶粒区域间的变形不协调性)主要存在于γ'相.利用中子衍射结合扫描电子显微镜对合金的微观组织形貌进行了细致观察,给出了时效温度和时间对γ'相的影响状况,超晶格测量发现了γ'相晶粒之间出现的独特取向差.由不同晶面的中子衍射结果判断时效后合金出现了轻微的四方对称性(a < c)畸变,对这种畸变起主要作用的是基体相.实验结果同时证实了不同方向的应变差异,因而为筏化驱动模型的定量 关键词： 单晶高温合金 中子衍射 超晶格 时效处理     

冲击诱发NiTi形状记忆合金相变行为研究

NiTi形状记忆合金的高应变动态响应特性在军事、航空等领域具有重要应用.为研究NiTi合金在动态力学诱导下的相变行为,在不同温区不同冲击速率下,通过轻气炮装置对NiTi合金进行了动态加载实验.利用差示扫描量热仪(DSC),综合物性测量系统分析了冲击波残余效应对NiTi合金相变行为的影响.研究发现:受冲击的样品在第一次DSC热循环中观察到了三个马氏体吸热峰,表现为三步逆马氏体相变,而在第二次热循环中其中两个应力诱发马氏体吸热峰因变形恢复消失.形成两个应力诱发马氏体吸热峰的原因可能是晶粒内部与晶界处的相变过程不同步.受冲击后样品DSC放热峰上出现了一小肩峰,表明可能因中间相(R相)的出现而发生了两步相变,结合电阻测量曲线进一步确认R相的存在,且发现奥氏体相向R相转变以及R相向马氏体相转变这两种相变过程在某一温度范围内可同时进行.同时,文中也具体讨论了不同的冲击加载条件对相变过程的影响.     

用于形状记忆材料研究的中子衍射原位温度加载系统

配合中国工程物理研究院的中子衍射应力谱仪开展形状记忆材料的相关研究,设计了一套原位温度加载系统。该系统可为测量样品提供25~800 ℃温度环境,温度控制器采用大林改进算法,有效地消除了温度过冲问题。该系统已开展了多次带束测试,结果表明其结构设计适用于中子衍射实验,温度控制在上升阶段无超调、稳态误差为1 ℃,满足形状记忆材料的温度加载实验要求。     

碳化硅压腔和高压下的中子衍射

使用宝石级碳化硅晶体作为压砧材料,成功研制出了碳化硅压腔（MAC）,并应用全景式MAC进行了高压下物质的中子衍射实验研究.结果表明,MAC是一种既能产生高的压力又具有大的高压样品室的装置,特别适合于高压下的中子衍射研究.     

碳化硅压腔和高压下的中子衍射

使用宝石级碳化硅晶体作为压砧材料，成功研制出了碳化硅压腔（MAC），并应用全景式MAC进行了高压下物质的中子衍射实验研究。结果表明，MAC是一种既能产生高的压力又具有大的高压样品室的装置，特别适合于高压下的中子衍射研究。     

中子与α相互作用的两体等效可分离势

本文提供了一种n与α相互作用的等效两体可分离势,并给出了n-α散射的S_1/2,P_1/2,P_3/2分波相移对入射中子动能的解析表达式.用它计算各种动能的中子微分截面和极化都比较方便,计算结果与20MeV以下的中子散射实验数据作了比较,它们符合得很好.     

NiTi形状记忆合金的价电子结构分析与马氏体相变

应用固体与分子经验电子理论，对ＮｉＴｉ合金的母相和马氏体相进行了价电子结构分析，计算了合金在两种状态下的结合能。计算结果与实验相符很好；在此基础上对ＮｉＴｉ合金的马氏体相变前后的宏观杨氏模量的变化从电子结构角度进行了解释。     

FeMnNi合金冲击相变与卸载逆相变历程研究

采用VISAR测试技术,将正向加载和逆向加载实验相结合,研究了不同加载压力下FeMnNi合金的冲击相变和卸载逆相变历程.结果表明:冲击加载压力与冲击相变阈值之差小于逆相变阈值时,即0＜σp-σt＜σn,FeMnNi合金发生α→ε相变,加载段呈现弹性波、塑性波和相变波三波结构,塑性波在自由面反射稀疏波使FeMnNi合金完...     

Cu-Al-Be合金逆马氏体相变过程中内耗的等温变化研究

本文研究了Cu-Al-Be合金等温逆马氏体相变过程中内耗的变化规律.研究发现,在相变温区,样品内耗值随等温时间的延长而有规律地下降.我们认为,这一现象是由于晶体内部缺陷运动引起的.在等温过程中,点缺陷向界面偏聚,逐渐限制界面移动,引起内耗的下降.     

Softening micromechanical constitutive model of stress induced martensite transformation for NiTi single crystal shape memory alloy

Based on the assumption of laminated microstructure, a micromechanical model of stress induced martensite transformation for NiTi shape memory alloys single crystal is proposed. Elasticity anisotropy and different proper-ties for two phases are considered. Martensite volume fraction is chosen as the internal variable that controls the phase transformation quantitatively. An effective macroscopic elasticity matrix based on the different elasticity characteristics of each phase and the martensite volume fraction are obtained with the help of the perfect interfa-cial relationships. A phase transformation driving force is derived to construct the transformation criterion. The model corresponds to a non-convexity free energy function during phase transformation, so softening behavior can be well simulated by the model. A numerical simulation is implemented for the uniaxial loading of NiTi single crystal alloy according to the model, and simulation results are proved by experimental results of polycrystal with strong {111} texture. Superelasticity, Anisotropy, the evolution of microstructure and softening behavior can be well simulated.     

Atomistic study on shock behaviour of NiTi shape memory alloy

Abstract

The shock behaviour of NiTi shape memory alloy is investigated by using molecular dynamics simulation. The nano-pillar samples of the alloy are subjected to the impact of a piston with a velocity of 350 m/s at initial environment temperatures of 325 and 500 K. At 325 K, we observe two different pathways of the formation of BCO phase, the gradient twins, and the detwinning phenomena, strongly depending on the local stress and the deformation state. As the initial temperature increases to 500 K, the plasticity is dominated by the dislocation movements rather than the twinning at 325 K. The phase transformation and plasticity result in stress attenuation when the stress wave propagates through the nano-pillar. Furthermore, it is interesting to note that multiple stress peaks occur due to the formation of local complex atomic structures with various wave speeds, leading to the catch up and overlap of the stress waves.     

NiTi合金薄膜厚度对相变温度影响的X射线光电子能谱分析

采用X射线衍射和X射线光电子能谱实验手段对不同厚度的NiTi薄膜相变温度的变化进行了分析.结果表明在相同衬底温度和退火条件下,3?μm厚度的薄膜晶化温度高于18?μm厚度的薄膜.衬底温度越高,薄膜越易晶化,退火后薄膜奥氏体相转变温度As越低.薄膜的表面有TiO₂氧化层形成,氧化层阻止了Ni原子渗出;膜与基片的界面存在Ti₂O₃和NiO.由于表面和界面氧化层的存在,不同厚度的薄膜内层的厚度也不同,因而薄膜越薄,Ni原子的含量就越高.Ni原子的含量的不同会影响薄膜的相变温度. 关键词： NiTi合金薄膜 X射线衍射 相变 X射线光电子能谱     

Atomistic study of temperature and strain rate-dependent phase transformation behaviour of NiTi shape memory alloy under uniaxial compression

Molecular dynamics simulation was conducted to investigate the phase transformation behaviour of nickel–titanium (NiTi, 50%-50% at.%) nanopillar under uniaxial compression at loading rates varying from 3.30 × 10⁷ to 3.30 × 10⁹ s^?1 and at temperatures varying from 325 to 600 K. The phase transformation of NiTi was observed to be sensitive to loading rates and temperatures. The phase transformation stress of B2 → B19 increased with increasing temperature while it was insensitive to loading rate. The phase transformation stress of B19 → B19′ → BCO increased with increasing strain rate and decreasing temperature. In addition, reverse phase transformation was observed during compression due to the interaction between the phase transformation of B19 → B19′ → BCO and the deformation twinning/dislocation slide-induced plasticity of the BCO phase, leading to different residual crystal structures after loading. Moreover, a diagram for the phase transformation behaviour of NiTi in the simulated ranges of strain rate and temperature was obtained, from which the contrary experimental observations on the phase transformation behaviour of NiTi from the studies of Nemat-Nasser et al. (Mech. Mater. 37 (2005) p.287) and Liao et al. (J. Appl. Phys. 112 (2012) p.033515) at various strain rates could be well explained.     

First-principles study of martensitic phase transformation of TiRh alloy

Geometric structures and atomic positions were studied with plane wave pseudo-potential method based on density functional theory for cubic, tetragonal, and monoclinic phases of TiRh alloy. Their phonon dispersion curves were obtained with frozen phonon method at harmonic approximation using density-functional perturbation theory. Our calculations revealed that both B2 and L1₀ phases are thermodynamically unstable. Jahn-Teller effect triggers the occurrence of Bain transformation from cubic to tetragonal phase, and then soft-mode phonon further leads to the transition from tetragonal to monoclinic phase on cooling. The monoclinic phase was predicted to be P2/m space group through atomic vibrational movement along [001] direction of virtual frequency modes of L1₀ phase. The temperature from B2 to L1₀ and then to P2/m were predicted to be about T=1100.53 K and T=324.48 K through free energy calculations with the electronic plus vibrational energy of formation, respectively, which is in good agreement with experimental observations.     

Magnetic correlations in oxides: Neutron diffraction and neutron depolarization study

We have studied magnetic correlations in several oxide materials that belong to colossal magnetoresistive, naturally occurring layered oxide showing low-dimensional magnetic ordering, solid oxide fuel cell interconnect materials, and magnetic nanoparticles using neutron diffraction and neutron depolarization techniques. In this paper, an overview of some of these results is given.      

Face-centered-cubic to body-centered-cubic phase transformation of Cu nanoplate under [100] tensile loading

ABSTRACT

Molecular dynamics simulation was used to stretch Cu nanoplates along its [100] direction at various strain rates and temperatures. Under high strain rate and beyond the elastic limit, the Cu nanoplates underwent an unusual deformation mechanism with expansion along free surface lateral direction and contraction along the other lateral direction, which leaded to the face-centred-cubic phase transforming into unstressed body-centred-cubic phase. Under low strain rate, the deformation of the nanoplate went back to well-known dislocation mechanism. The face-centred-cubic to body-centred-cubic phase transformation mechanism was further discussed in terms of elastic stability theory and free surface stress effect.     

Role of thermal vibrations in alloy phase transitions

We resolve the fundamental contradiction that has existed from the earliest studies on order–disorder transitions between theoretical models that ignore the effect of thermal vibrations on the chemical interaction parameter, J(0), and the analysis of diffraction data that always incorporates the role of vibrations. New analysis of diffraction data shows that the temperature dependence of the order parameter, the central feature of order–disorder transitions, predicted by existing models is scientifically invalid. All models are constrained by the diffraction data to represent the interaction parameter as J(T) and the ordering energy as temperature dependent. The discrepancy between the experimental and theoretical ordering energies in Ni₃V and Pd₃V is direct evidence of their temperature dependence. Thermal vibrations influence order–disorder transitions through both the configurational and vibrational terms and not just the vibrational terms as hitherto believed. A modified Bragg–Williams model is proposed, which is the simplest model whose predicted order parameter can be compared with experiments.     

Anomalous phase transformation in magnetostrictive Fe81Ga19 alloy

In this work, we have investigated the room-temperature phase constitution of heat-treated Fe₈₁Ga₁₉ alloys cooled from 800 °C at different rates. Results show that at cooling rates in the range from 0.43 to 0.26 °C/min, in addition to the A2 matrix, an fcc phase also can be observed in Fe₈₁Ga₁₉ samples at room temperature. To investigate the precipitation of the fcc phase out of A2 matrix, a systematic study of phase constitution was carried out on the samples quenched from different temperatures during cooling from 800 °C at 0.32 °C/min, which reveals an anomalous phase transformation between A2 and fcc. Precipitation of the fcc phase from A2 matrix occurs at 500 °C and its volume fraction exhibits a sharp increase at 400 °C. However, it begins to dissolve when further decreasing the temperature and only a minor fcc phase can be retained at room temperature, which suggests that the fcc phase is metastable below 400 °C. Magnetic measurements indicate that the precipitation of fcc phase deteriorates the saturation magnetization of Fe₈₁Ga₁₉.