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《物理学报》2020,(13)
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. 相似文献
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《物理学报》2020,(11)
金属Ce在室温条件下当压力达到约0.7 GPa时会发生一阶相变,体积突变减小14%—17%,相变前后两相分别为γ-Ce和α-Ce,均为面心立方结构.实验中发现冲击波在Ce中传播,其波形存在明显的多波结构依次为γ-Ce弹性前驱波、γ-Ce塑性波、γ-Ce→α-Ce相变波.基于新发展的金属Ce的嵌入原子势,对单晶Ce的冲击相变行为进行了分子动力学模拟.模拟结果表明,在一定强度下,单晶Ce中的冲击波阵面分裂为多波结构,波形结构与加载晶向明显相关:在[001]和[011]晶向加载下表现为双波结构,依次为前驱波和相变波;在[111]晶向加载下波阵面分裂为弹性前驱波、γ-Ce塑性波、γ→α相变波,与已有的实验观察相一致.冲击波速的Hugoniot关系在低强度加载下与实验符合得较好.同时在此冲击相变过程中,应力偏量对相变起促进作用,相较于静水压加载,冲击加载的相变压力条件更低一些. 相似文献
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钯作为典型高压标定材料,研究其在极端条件下的结构变化以及热力学性质具有广泛需求并充满了挑战,特别是冲击加载下钯的固-固相变过程研究仍然匮乏.本文基于嵌入原子势,使用经典分子动力学方法从原子角度揭示了冲击载荷加载下钯的结构相变路径,在0—375 GPa的压力区间观察到一系列复杂的结构转变特征,从初始的面心立方(FCC)结构,至带密排六方(HCP)结构的层错体心立方(BCC)结构,直至完全熔化.在沿<100>晶向冲击下,在70.0 GPa发现了FCC-BCC相变过程,远低于之前研究中静高压的结果.此外,还发现了冲击方向依赖的相变点,在沿着<110>及<111>晶向冲击时FCC-BCC相变压力分别增加至135.8和165.4 GPa,同时相比完美晶体,引入缺陷会使FCC-BCC相变压强值有20—30 GPa的增幅,并通过势能分布的分析予以验证.本文发现冲击加载下钯的FCC-BCC相变压力大大降低的特殊现象,为钯在高压实验等极端条件下的应用提供了新的理论认识. 相似文献
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获取动态压缩条件下结构演化过程是冲击相变及其动力学机理研究最为关注的基础问题之一.对此,基于激光驱动瞬态X射线衍射技术,通过系列实验的物理状态关联和抽运-探测时序控制,实现了静态与动态晶格衍射信号的同时获取,消除了不同实验的装置结构和样品差异带来的测量误差,建立了一种基于原位X射线衍射技术的动态晶格响应测量方法.利用上述实验方法,成功实现了激光冲击加载下[111]单晶铁晶格压缩过程的原位测量,获取弹性及塑性响应的晶格压缩度与宏观雨贡纽测量结果完全符合,从晶格层面证实了超快激光加载下的高屈服强度(雨贡纽弹性极限值大于6 GPa),以及可能与晶向效应或加载率效应相关的相变迟滞现象(至终态压力23.9 GPa仍为体心立方结构),相关物理机制仍有待进一步研究.上述测量方法的建立为后续开展相变动力学机理研究提供了可行的技术途径和重要的参考价值. 相似文献
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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。 相似文献
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用分子动力学方法模拟了冲击加载(沿[001]向)下单晶Fe中孔洞诱导相变形核及生长过程,并分析了初始温度对这一生长过程的影响.数值模拟显示:1) 相变形核首先出现在孔洞周围的(110)和(110)面上,并分别沿[110],[110]向和[110],[110]向生长成片状,之后核的生长方向则变为沿〈111〉向,形成“V”形板条状新相颗粒;2) 在相同冲击压力下,初始温度为300 K时在新相晶核边缘出现许多核胚,生成的新相颗粒比60 K时明显减小.这些现象表明,孔洞诱导相变形核及生长过程沿着特定晶向进行,而初
关键词:
相变
孔洞
分子动力学 相似文献
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使用分子动力学方法,模拟了活塞以恒定加速运动从一端压缩单晶铁(沿[001]晶向)发生马氏体相变的微观过程.根据模拟结果将上述压缩过程分为弹性压缩、晶格软化、相变(bcc至hcp)、超应力松弛和高压相弹性压缩五个阶段,对各阶段的原子滑移规律和应力变化特征做了详细分析.分析得出应力超过约10 GPa时,开始出现弹性常数软化行为;层错结构(fcc)和孪晶界为新相形核的两种缺陷,前者更为稳定;相变后粒子首先进入超应力松弛状态(即沿加载方向的偏应力呈现负值),在应力超过约36 GPa粒子转变为高压相弹性压缩状态.
关键词:
分子动力学
单晶铁
相变
动态压缩 相似文献
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A general approach is given for the nonlinear excitations in the compressible Heisenberg ferromagnetic chain in coherent-state representation. It is shown that the relative ratio of ε to δ is very important for determining the nonlinear modified terms of the equations of motion. ε = 1√S (S is the spin length) and δ = α/λ0 (α is the lattice constant and λ0 is the characteristic wavelength of the excitations) are two small dimensionless parameters used in the semiclassical approximation and in the long wavelength approximation, respectively. In the case of δ = O(ε), the reduced equations of motion are solved exactly and spin soliton and elastic kink solutions are obtained. The other cases of the relation between δ and ε are also discussed and the solitary wave solutions are also given. The result obtained by Ferrer, as a special case, has been included in our approach. 相似文献
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富铁釉瓷在中国古陶瓷史上具有重要地位,综合利用光学相干层析成像技术(OCT)、光学显微镜(OM)、激光拉曼光谱技术(LRS)和X射线荧光光谱分析技术(XRF) 对河南省宝丰清凉寺窑址出土宋金时期富铁釉瓷样品进行无损分析研究,获取了富铁釉瓷的釉层结构、釉面析晶的物相信息及胎釉化学成分等信息,探讨了釉层断面OCT灰度图像特征与其显微结构之间的关系以及富铁釉瓷的烧制工艺。结果表明,酱釉瓷和柿叶红釉瓷表面及黑釉红斑区存在富铁析晶层,黑釉瓷釉层表面未发现富铁析晶层的存在。酱釉瓷釉层富铁析晶层中铁的氧化物以ε-Fe2O3晶体为主,而柿叶红釉层则主要是α-Fe2O3晶体,黑釉红斑层同时存在ε-Fe2O3,α-Fe2O3晶体以及磁铁矿。上述差异形成的原因与釉层化学成和烧制工艺存在联系。釉层中高含量碱土金属氧化物及氧化铁区域更容易析出氧化铁晶体。黑釉瓷烧成气氛以还原气氛为主,而酱釉瓷和柿叶红釉瓷则倾向于氧化烧成气氛,且柿叶红釉瓷的烧成温度高于酱釉瓷。该研究将OCT,OM,LRS与XRF相结合,有助于加强对富铁釉瓷的整体认知,为富铁釉瓷的断源断代提供更为丰富的线索。 相似文献
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Using the bosonic numerical renormalization group method, we studied the equilibrium dynamical correlation function C(ω) of the spin operator σ_z for the biased sub-Ohmic spin-boson model. The small-ω behavior C(ω) ∝ω~s is found to be universal and independent of the bias ε and the coupling strength α(except at the quantum critical point α = αc and ε = 0). Our NRG data also show C(ω) ∝χ~2ω~s for a wide range of parameters, including the biased strong coupling regime(ε = 0 and α α_c), supporting the general validity of the Shiba relation. Close to the quantum critical point αc,the dependence of C(ω) on α and ε is understood in terms of the competition between ε and the crossover energy scale ω_0~*of the unbiased case. C(ω) is stable with respect to ε for ε《ε~*. For ε》ε~*, it is suppressed by ε in the low frequency regime. We establish that ε~*∝(ω_0~*)~(1/θ)holds for all sub-Ohmic regime 0≤s 1, with θ = 2/(3s) for 0 s≤1/2 and θ = 2/(1 + s) for 1/2 s 1. The variation of C(ω) with α and ε is summarized into a crossover phase diagram on the α–ε plane. 相似文献
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A new model of Bknard-Rayleigh convection has been put forward from mantle convection and liquid crystal, where the external force deviates from constant to a lineal function of vertical dimension z. The dimensionless parameter ε is induced to describe the scale of this kind of deviations. Through linear stability analyses, we find that even-odd symmetry broken appears. By numerical calculation under rigid boundary conditions, we find that αc is almost α constant but Rc decreases nonlinearly as ε increases, so the increase of ε benefits convection. When ε=0, all results reduce to the former works automatically. 相似文献
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通过分子动力学模拟,观察到[001]取向的四方氧化锆纳米柱在拉伸载荷下具有两个线弹性变形的应力-应变关系.这一现象是四方结构向单斜结构相变的结果 .为了进一步阐明应力-应变曲线,进行了包括晶体结构分析和原子应变计算在内的详细研究.晶格取向强烈影响塑性变形机制,即[001]和[111]取向的纳米柱在拉伸载荷下经历相变,而沿[110]取向的纳米柱导致脆性断裂.观察到显著的温度效应,随着温度从300K升高到1500K,弹性模量从573.45GPa线性降低到482.65GPa.此外,还用轻推弹性带(NEB)理论估算了相变能垒,观察到相变能垒随温度的升高而降低.这一工作将有助于加深对氧化锆的四方相到单斜相转变和纳米尺度力学行为的理解. 相似文献
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Orientation dependence of structural transition in fcc Al driven under uniaxial compression by atomistic simulations 下载免费PDF全文
By molecular dynamics simulations employing an embedded atom method potential,we have investigated structural transformations in single crystal Al caused by uniaxial strain loading along the [001],[011] and [111] directions. We find that the structural transition is strongly dependent on the crystal orientations. The entire structure phase transition only occurs when loading along the [001] direction,and the increased amplitude of temperature for [001] loading is evidently lower than that for other orientations. The morphology evolutions of the structural transition for [011] and [111] loadings are analysed in detail. The results indicate that only 20% of atoms transit to the hcp phase for [011] and [111] loadings,and the appearance of the hcp phase is due to the partial dislocation moving forward on {111} fcc family. For [011] loading,the hcp phase grows to form laminar morphology in four planes,which belong to the {111} fcc family; while for [111] loading,the hcp phase grows into a laminar structure in three planes,which belong to the {111} fcc family except for the (111) plane. In addition,the phase transition is evaluated by using the radial distribution functions. 相似文献
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In contrast to Gaussian or Woods-Saxon potential a two-term four parameter nuclear Hulth′en type interaction is considered to describe the α-α, α-~3He and α-~3H systems. By exploiting the phase function method, scattering phase shifts are computed up to ELab = 100 MeV for the α-α system and ELab = 15 Me V for α-~3He and α-~3H systems.The S-wave phase shift δ_0 for the α-α system tends to 2π and δ_(3/2)-for the α-~3He system tends to π, in the limit of zero energy. Reasonable agreements in phase shifts with the standard data are obtained with this simple potential model except for the 5/2~- states of α-~3He and α-~3H systems. With an additional energy-dependent correction factor to our potential, a good agreement with experimental data is obtained for 5/2~- states. We have also compared our results with the convenient Born approximations. 相似文献
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Employing the ab initio total energy method based on the density functional theory with the generalized gradient approximation, we have investigated the theoretical mechanical properties of copper (Cu) systematically. The theoretical tensile strengths are calculated to be 25.3 GPa, 5.9 GPa, and 37.6 GPa for the fcc Cu single crystal in the [001], [110], and [111] directions, respectively. Among the three directions, the [110] direction is the weakest one due to the occurrence of structure transition at the lower strain and the weakest interaction of atoms between the (110) planes, while the [111] direction is the strongest direction because of the strongest interaction of atoms between the (111) planes. In terms of the elastic constants of Cu single crystal, we also estimate some mechanical quantities of polycrystalline Cu, including bulk modulus B, shear modulus G, Young's modulus Ep, and Poisson's ratio ν. 相似文献