一维应变加载下单晶铁结构转变的微观研究

采用嵌入原子势和分子动力学方法,模拟了单晶铁在一维应变条件下由体心立方（bcc）转变为六角密排（hcp）结构的微观过程. 当应变加载至相变临界值时,hcp相开始均匀形核并沿{011}晶面长大为薄片状体系.弹性常数C₃₁和C₃₂在相变前被逐渐硬化,C₃₃则在相变前出现软化行为;当体系完全相变后,上述各弹性常数显示开始随体积压缩而迅速硬化,温度效应对晶格具有软化作用,可削弱C₃₃的硬化和软化过程;样品在压缩过程可出现孪晶结构,孪晶结构使晶格发生剪切变形.混合相中,hcp相势能比bcc相高,最大剪应力方向与bcc相反向;系统的偏应力与hcp相质量分数近似呈线性关系. 关键词： 结构转变 分子动力学 一维应变     

纳米多晶铁的冲击相变研究

利用分子动力学方法研究了不同晶粒度的纳米多晶铁在冲击压缩下的结构相变过程,模拟结果表明:纳米多晶铁的冲击结构相变(由体心立方(bcc)结构 α 相到六角密排(hcp)结构 ε 相)发生的临界冲击应力在15 GPa左右.纳米多晶铁在经过弹性压缩变形后,晶界导致的塑性变形开始发生,然后大多数相变从晶界成核并最终发展为大规模相变.不同变形过程在应力和粒子速度剖面上能得到清晰的体现,并通过微观原子结构分析分辨.冲击压缩后的微观结构以晶界原子和以fcc结构原子充当孪晶界的hcp原子为主.晶粒度明显影响晶界变形及相变 关键词： 冲击相变 纳米多晶铁 冲击波 分子动力学     

单晶铁沿[101]晶向冲击过程中面心立方相的形成机制

铁的冲击相变过程是科研工作者们关注的热点领域之一.铁沿[100]晶向冲击时会发生体心立方相到密排六方相的转变;而沿[101]晶向冲击时,相变产物除了密排六方相之外还出现一定量的面心立方相.人们已经明确了体心立方到密排六方相的转变机制,然而对于面心立方相的形成机制问题至今还在探索.本文通过分子动力学方法模拟了体心立方单晶铁沿[101]晶向的冲击过程,模拟结果显示体心立方相将转变为高压密排结构(密排六方相和面心立方相);并分析了面心立方相的形成机制:在冲击过程中,单晶铁沿[101]和101]晶向突然收缩,同时沿[010]晶向突然扩张,从而导致体心立方到面心立方相的转变.此外,本文进一步研究了不同应力状态下单晶铁的相变机制,发现沿[101]晶向单轴压缩以及沿[101]和[101]晶向双轴压缩时铁将发生体心立方到面心立方相的转变;而沿[101]和[010]晶向双轴以及三轴压缩时将会发生体心立方到密排六方相的转变.最后进一步计算了三个相的吉布斯自由能随压力的变化,并对冲击模拟结果进行了能量分析,给出了沿[101]晶向冲击条件下高压密排相产生的原因.     

强激光辐照下孔洞尺寸对铁相变过程的影响

 利用分子动力学模拟研究了完美单晶铁以及含不同尺寸孔洞的单晶铁相变过程,分析了孔洞尺寸对相变过程的影响。模拟结果表明：孔洞的存在降低了相变的阈值应力,加速了相变区域成核速率和相变传播速率;随着孔洞直径的增大,相变的阈值应力逐渐降低;孔洞也改变了相变的初始成核区域,使相变区域呈现出一个蝴蝶状的形貌;孔洞反射的稀疏波对相变成核区域的影响随孔洞体积增大而增大,导致孔洞周围出现大量的无序结构原子;孔洞体积对相变的影响也体现在了粒子速度空间分布上,压缩过程中孔洞周围出现的大量“热点”导致了更低的粒子速度空间分布。     

冲击波压缩下含纳米孔洞单晶铁的结构相变研究

利用分子动力学模拟方法对含纳米孔洞的单晶铁在冲击波压缩下的结构相变(由体心立方结构α到六角密排结构ε)进行了研究,单晶铁样品的尺寸为17.2nm×17.2nm×17.2nm，总原子数428341个，在样品的中央预置一个直径为1.12nm的孔洞，利用一活塞分别以350，500，1087m/s的速度撞击样品产生冲击波，对应的冲击波压缩应力分别为12，17，35GPa.撞击方向沿单晶铁的［100］晶向.计算结果表明，在冲击波压缩下，孔洞对铁中的相变起了诱导作用，伴随着孔洞的塌陷，相变首先出现在孔洞周围的(011)面和(011)面上，然后扩展到整个样品.通过分析冲击压缩下原子的位移历史，解释了相变的微观机制，发现孔洞周围的原子在{011}面上沿〈011〉晶向滑移，离孔洞中心距离越近的{011}面上的原子容易滑移，间隔一层的{011}面与相邻层原子的移动位移幅度不同，这种相对滑移导致出现了新的结构(hcp结构). 关键词： 相变 分子动力学 冲击波 纳米孔洞     

单轴应变驱动铁bcc—hcp相转变的微观模拟

用分子动力学方法模拟了沿〈001〉晶向应变加载和卸载情况下单晶铁中体心立方(bcc)与六方密排(hcp)结构的相互转变,分析了相变的可逆性和微结构演化特征.微观应力的变化显示样品具有超弹性性质,而温度变化表明在相变和逆相变过程中均出现放热现象.相变起始于爆发式均匀形核,晶核由块状颗粒迅速生长为沿{011}晶面的片状分层结构; 而卸载逆相变则从形核开始就呈现片状形态,且相界面晶面指数与加载相变完全一致,表现出形态记忆效应.在两hcp晶核生长的交界面易形成面心立方(fcc)堆垛层错. fcc通过在hcp晶粒内     

应力诱发NiAl单晶马氏体相变的分子动力学模拟

利用嵌入原子势（EAM）,对NiAl单晶在外应力作用下的动态拉伸过程进行了分子动力学模拟.应力-应变曲线分析以及原子构型分析表明外应力诱发NiAl合金发生了马氏体相变,原子结构由B2相转变为L1₀相.通过研究原子构型的演化过程,发现马氏体相变是通过多个{110}孪晶面的扩展和湮灭作用来完成的.同时探讨了马氏体相变的微观机理. 关键词： 马氏体相变 NiAl 分子动力学模拟     

铁冲击相变的晶向效应

采用基于火炮加载的三样品精细波剖面对比测量,研究了晶向效应对铁弹-塑性转变及体心立方结构(bcc,α相)至六角密排结构(hcp,ε相)相变特性的影响.观测到单晶铁异常的弹-塑性转变行为,这与基于位错密度描述的黏塑性本构模型计算结果相符,对应的Hugoniot弹性极限δ_(HEL)均大于6 GPa,且具有晶向相关性,即δ(111)/(HEL)δ(110)/(HEL)δ(100)/(HEL);系统获取了相变起始压力P_(PT)晶向相关性的实验数据,[100],[110]和[111]晶向的PPT实测值分别为13.89±0.57 GPa,14.53±0.53 GPa,16.05±0.67 GPa,其变化规律与非平衡分子动力学计算结果相符.上述结果揭示出冲击压缩下单晶铁存在塑性与相变微观机理的强耦合,为完善用于冲击实验描述的相场动力学模型提供了重要的实验支撑.     

单晶钨纳米线拉伸变形机理的分子动力学研究

利用分子动力学方法, 对本课题组率先采用金属催化的气相合成法制备出的高纯度单晶钨纳米线进行拉伸变形数值模拟, 通过分析拉伸应力-应变全曲线及其微观变形结构, 揭示出单晶钨纳米线的拉伸变形特征及微观破坏机理. 结果表明: 单晶钨纳米线的应力-应变全曲线可分为弹性阶段、损伤阶段、相变阶段、强化阶段、 破坏阶段等五个阶段, 其中相变是单晶钨纳米线材料强化的重要原因; 首次应力突降是由于局部原子产生了位错、孪生等不可逆变化所致; 第二次应力突降是发生相变的材料得到强化后, 当局部原子再次产生位错导致原子晶格结构彻底破坏而形成裂口、且裂口不断发展成颈缩区时, 材料最终失去承载能力而断裂. 计算模拟得到的单晶钨纳米线弹性模量值与实测值符合较好. 关键词： 分子动力学 应力应变曲线 微观机理 单晶钨纳米线     

铁的冲击相变机制的分子动力学研究

 用分子动力学方法模拟计算了在冲击波加载条件下,单晶铁中的结构相变（由体心立方结构α相到六角密排结构ε相）,相互作用势采用铁的嵌入式原子势（EAM）,单晶铁样品的尺寸为28.7 nm×22.9 nm×22.9 nm,总原子数为1.28×10⁶个。通过推动一个运动活塞对静止靶的作用来产生冲击压缩,加载方向沿单晶铁的[100]晶向。通过对原子位置的追踪,揭示了铁的冲击相变机制,计算结果表明相变机制包括两步：首先是在{011}面上的原子受到沿〈100〉晶向的压缩,使{011}面转化成正六角形密排面;然后是在{011}面上原子沿〈0-11〉晶向的滑移,完成由bcc结构到hcp结构的相变。同时发现滑移面只出现在与冲击波加载方向平行的(011)和(0-11)面上。     

Anomalies in the variation of elastic properties of cesium during phase transformations under a pressure up to 5 GPa

The pulsed ultrasonic method is used for studying polycrystalline cesium under a pressure up to 5 GPa. Elastic parameters and compression ratio of cesium are determined and anomalies in the elastic properties of cesium during phase transitions CsI-CsII-CsIII-CsIV are revealed. It is found that the CsII-CsIII electron-structure transformation is preceded by anomalous compressibility of the fcc phase of cesium and by softening of longitudinal and transverse acoustic modes of the cesium phonon spectrum.     

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.     

Ductile titanium alloy with low Poisson's ratio

We report a ductile beta-type titanium alloy with body-centered cubic (bcc) crystal structure having a low Poisson's ratio of 0.14. The almost identical ultralow bulk and shear moduli of approximately 24 GPa combined with an ultrahigh strength of approximately 0.9 GPa contribute to easy crystal distortion due to much-weakened chemical bonding of atoms in the crystal, leading to significant elastic softening in tension and elastic hardening in compression. The peculiar elastic and plastic deformation behaviors of the alloy are interpreted as a result of approaching the elastic limit of the bcc crystal under applied stress.     

Molecular dynamics simulation of nanoscale mechanical behaviour of ZnO under nanoscratching and nanoindentation

Molecular dynamics is used to simulate the mechanical behaviour of zinc oxide under nanoscratching and nanoindentation. The effects of indenter speed and substrate temperature on the structure-phase formation, slip vector, radial distribution function, and residual stresses are investigated. Simulation results show that the dislocation loops nucleate and propagate, forming a body-centred tetragonal lattice structure along the slip direction due to high local stress. Furthermore, the dislocation loops nucleate and propagate due to the resolved shear stress along the 45° slip direction under nanoscratching. The average mean biaxial stress and the normal stress of the O layers are –9.35 and –4.36 GPa, respectively, and those of the Zn layers are –0.80 and –0.30 GPa, respectively. This may be attributed to the energetic O atoms, with which unstable atoms have high activation.     

冲击加载下Zr51Ti5Ni10Cu25Al9金属玻璃的塑性行为

进行了10—27 GPa应力范围内Zr₅₁Ti₅Ni₁₀Cu₂₅Al₉金属玻璃的平面冲击实验以研究其高压-高应变率加载下的塑性行为.由样品自由面粒子速度剖面的分析获得了冲击加载过程的轴向应力,并通过轴向应力与静水压线的比较获得剪应力.实验结果表明,尽管存在明显的松弛效应,但Zr基金属玻璃的Hugoniot弹性极限随着冲击应力的增加而增加.然而,塑性波阵面上的剪应力则显示先硬化而后软化现象,而且软化的幅度随冲击应力的增加而增加.冲击加载下Zr基金属玻璃的上述剪应力变化特征与分子动力学模拟结果比较一致,但与压剪实验结果和一维应力冲击实验结果明显不同.     

Face-centred cubic to body-centred cubic phase transformation under [1 0 0] tensile loading

Molecular dynamics simulation was used to verify a speculation of the existence of a certain face-centred cubic (FCC) to body-centred cubic (BCC) phase transformation pathway. Four FCC metals, Ni, Cu, Au and Ag, were stretched along the [1?0?0] direction at various strain rates and temperatures. Under high strain rate and low temperature, and beyond the elastic limit, the bifurcation of the FCC phase occurred with sudden contraction along one lateral direction and expansion along the other lateral direction. When the lattice constant along the expansion direction converged with that of the stretched direction, the FCC phase transformed into an unstressed BCC phase. By reducing the strain rate or increasing the temperature, dislocation or ‘momentum-induced melting’ mechanisms began to control the plastic deformation of the FCC metals, respectively.     

Investigation of the radial compression of carbon nanotubes with a scanning probe microscope

Efforts have been made to characterize the mechanical properties of carbon nanotubes. Previous work has concentrated on the tubes' longitudinal properties, and studies of their radial properties have lagged behind. We have used a scanning probe microscope with an indentation/scratch function to investigate the radial compression of multiwalled carbon nanotubes under an asymmetric stress. In particular, we have determined the radial compressive elastic modulus at different compression levels and have estimated the compressive strength to be well beyond 5.3 GPa.