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     

Microstructural evolution and martensitic transformation mechanisms during solidification processes of liquid metal Pb

The microstructural evolution and the martensitic transformation (bcc–hcp and bcc–fcc) mechanisms during the solidification process of liquid metal Pb were studied by molecular dynamics simulation. Results indicate that, with the decrease of temperature, the system undergoes two phase transitions: from the liquid state into a metastable bcc phase first and then from the bcc phase into a coexisting crystal structure of hcp and fcc phases. Moreover, the complicated martensitic transformation processes are clearly observed by cluster type index method (CTIM) and the tracing method. The two transformation mechanisms are very analogous at the atomic level; the essential difference between them is that, in the bcc–hcp transformation, two adjacent layers shift in opposite directions, whereas in the bcc–fcc transformation, the top layer and bottom layer shift in opposite directions relative to the middle layer. The specific mechanisms for the bcc–hcp and bcc–fcc transformations are confirmed to correspond to the revised Burgers mechanism and Bain mechanism, respectively.     

Atomistic simulations to study crack tip behaviour in single crystal of bcc niobium and hcp zirconium

In this article, molecular dynamics based simulations were carried out to study the fracture toughness of single crystals of niobium (Nb) and zirconium (Zr). Separate set of simulations were performed with different orientations of crack plane in Nb and Zr. In each case, efforts were made to align the crack front with principal planes of corresponding crystal structure, that was bcc for Nb and hcp for Zr. Deformation in single crystal of Nb and Zr was governed either by twinning or emergence of dislocations from the crack tip and surfaces. The deformation mechanism in single crystal of Nb and Zr also helps in governing the overall toughness of the material, while deforming via twinning leads to higher change in the crack length, whereas dislocations emerging from the crack tip blunts the opening and improves the toughness.     

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     

An ab initio molecular dynamics study of iron phases at high pressure and temperature

The crystal structure of iron, the major component of the Earth's inner core (IC), is unknown for the IC high pressure (P; 3.3-3.6 Mbar) and temperature (T; 5000-7000 K). There is mounting evidence that the hexagonal close-packed (hcp) phase of iron, stable at the high P of the IC and a low T, might be unstable under the IC conditions due to the impact of high T and impurities. Experiments at the IC P and T are difficult and do not provide a conclusive answer as regards the iron stability at the pressure of the IC and temperatures close to the iron melting curve. Recent theory provides contradictory results regarding the nature of the stable Fe phase. We investigated the possibility of body-centered cubic (bcc) phase stabilization at the P and T in the vicinity of the Fe melting curve by using ab initio molecular dynamics. Thermodynamic calculations, relying on the model of uncorrelated harmonic oscillators, provide nearly identical free energies within the error bars of our calculations. However, direct simulation of iron crystallization demonstrates that liquid iron freezes in the bcc structure at the P of the IC and T = 6000 K. All attempts to grow the hcp phase from the liquid failed. The mechanism of bcc stabilization is explained. This resolves most of the earlier confusion.     

Reconstructive structural phase transitions in dense Mg

The question raised recently about whether the high-pressure phase transitions of Mg follow a hexagonal close-packed (hcp) → body centered cubic (bcc) or hcp → double hexagonal close-packed (dhcp) → bcc sequence at room temperature is examined by the use of first principles density functional methods. Enthalpy calculations show that the bcc structure replaces the hcp structure to become the most stable structure near 48 GPa, whereas the dhcp structure is never the most stable structure in the pressure range of interest. The characterized phase-transition mechanisms indicate that the hcp → dhcp transition is also associated with a higher enthalpy barrier. At room temperature, the structural sequence hcp → bcc is therefore more energetically favorable for Mg. The same conclusion is also reached from the simulations of the phase transitions using metadynamics methods. At room temperature, the metadynamics simulations predict the onset of a hcp → bcc transition at 40 GPa and the transition becomes more prominent upon further compression. At high temperatures, the metadynamics simulations reveal a structural fluctuation among the hcp, dhcp, and bcc structures at 15 GPa. With increasing pressure, the structural evolution at high temperatures becomes more unambiguous and eventually settles to a bcc structure once sufficient pressure is applied.     

金属Zr中自间隙子扩散的分子动力学模拟

本文采用分子动力学方法,对HCP结构的金属锆中的自间隙子扩散进行研究。通过检查不同时刻维格纳原胞(Wigner-Seitz cell)中的原子个数的方法得到不同时刻缺陷的位置,应用Rahman-Parrinello应力控制方法计算出了外应力为零时α-Zr的4种不同扩散机制在不同温度下的扩散系数,通过扩散系数随温度变化的关系得到各种扩散机制的势垒。     

Crystal structure analysis in (3+d) dimensions

Inelastic neutron scattering on in situ grown bcc single crystals of the group 4 metals Ti, Zr and Hf show a band of low energy and strongly damped phonons. Geometrical considerations show how these damped lattice vibrations achieve the displacements necessary for the two martensitic phase transitions from bcc to ω (under pressure) and from bcc to hcp (upon lowering the temperature). The low energy and temperature dependent phonons are precursor fluctuations of the hcp or ω phase within the bcc phase.     

Computer simulation of vacancy migration in a fcc tilt boundary

Vacancy jumps in a bicrystal model of Σ = 5 (36.9°) [001] tilt boundary in aluminum have been observed at temperatures between 700 and 800 K by means of molecular dynamics simulation. Preliminary results indicated considerable structure dependence and yielded an activation energy for vacancy migration of 0.38 eV. In contrast to a previous study of vacancy migration in a bcc tilt boundary, thermal activation of a boundary interstitial by Frenkel pair production was not observed.     

平均键能Em的物理内涵探讨

在面心立方(fcc)、体心立方(bcc)和六角密堆积(hcp)3种不同结构晶体的自由电子能带模型中,发现4个最低能带与5个次低能带本征值的平均能量(称为平均键能,E_m)与费米能级(E_F)相当接近;并进一步在hcp结构的钛(Ti)、锆(Zr)和铪(Hf)以及bcc结构的铁(Fe)等金属中,采用从头赝势能带计算方法和平均键能计算方法,证实在这些金属的实际能带中,平均键能(E_m)值仍然非常接近于费米能级(E_F)值.该发现有助于进一步了解平均键能(E_m)的物理内涵. 关键词： 平均键能 费米能级 能带结构     

Self-evolving atomistic kinetic Monte?Carlo: fundamentals and applications

The fundamentals of the framework and the details of each component of the self-evolving atomistic kinetic Monte Carlo (SEAKMC) are presented. The strength of this new technique is the ability to simulate dynamic processes with atomistic fidelity that is comparable to molecular dynamics (MD) but on a much longer time scale. The observation that the dimer method preferentially finds the saddle point (SP) with the lowest energy is investigated and found to be true only for defects with high symmetry. In order to estimate the fidelity of dynamics and accuracy of the simulation time, a general criterion is proposed and applied to two representative problems. Applications of SEAKMC for investigating the diffusion of interstitials and vacancies in bcc iron are presented and compared directly with MD simulations, demonstrating that SEAKMC provides results that formerly could be obtained only through MD. The correlation factor for interstitial diffusion in the dumbbell configuration, which is extremely difficult to obtain using MD, is predicted using SEAKMC. The limitations of SEAKMC are also discussed. The paper presents a comprehensive picture of the SEAKMC method in both its unique predictive capabilities and technically important details.     

冷速对Cu10Ag90合金凝固结构的影响

通过分子动力学对液态Cu10Ag90合金在四种冷速条件下进行快速凝固模拟。结果显示,1 × 1010和1 × 1011 K/s下系统的平均原子能量分别在750 K和650 K发生突变,冷速越低最终平均原子能量越低;1 × 1012和1 × 1013 K/s的双体分布函数第二峰出现分裂,表明结构处于非晶态。从1 × 1011K/s开始出现尖锐小峰,表明此冷速开始出现晶化现象,1 × 1010 K/s下分裂的峰更加尖锐明显,说明体系形成结晶度较高的晶体结构;1 × 1010 和1 × 1011 K/s下系统凝固后晶体结构含量由高到低分别为fcc, hcp, bcc。冷速越低晶体结构数目越多,系统的有序度更高,结构熵越低。     

Atomic mechanism of homogeneous melting of bcc Fe at the limit of superheating

Atomic mechanism of homogeneous melting of bcc Fe is studied via monitoring spatiotemporal arrangements of the liquid-like atoms, which are detected by the Lindemann criterion of melting, during the heating process. Calculations are performed by molecular dynamics (MD) simulations. Calculations show that liquid-like atoms occur randomly in the crystalline matrix at temperature far below the melting point due to local instability of the crystalline lattice. Number of liquid-like atoms increases with increasing temperature and they have a tendency to form clusters. Subsequently, a single percolated liquid-like cluster is formed in the crystalline model and at the melting point 99% atoms in the model become liquid-like to form a liquid phase. Melting is also accompanied by the sudden changes in various static and thermodynamic quantities. However, total melting is reached just at the point above the melting one. Three characteristic temperatures of the homogeneous melting of bcc Fe are determined.     

A simple theoretical model for evaluating the ability to form a single crystal

A simple theoretical model proposed recently to evaluate the ability of bulk materials to form single crystals is further tested via vast molecular dynamics simulations of growth for fcc (Ni,Cu,Al,Ar) and hcp (Mg) crystals,especially applied to the growth of bcc (Fe) crystal,showing that the validity of the model is independent of crystal types and the interaction potentials of the constitute atoms.     

非晶Ag晶化过程中不同类型晶核结构的识别与跟踪

采用分子动力学模拟研究了非晶Ag的等温晶化过程,通过原子轨迹逆向追踪法分析了不同类型晶体团簇的结构遗传与组态演化.在团簇类型指数法的基础上,根据基本团簇种类与联结方式不同,提出了一种可区分fcc单晶、多晶与混晶团簇的分析方法.在非晶Ag等温晶化过程中,基于团簇结构的连续遗传性特征,发展了一种可区分fcc单晶、多晶与混晶晶胚与晶核的结构分析技术.结果发现:不论临界尺寸还是几何构型,不同类型的晶核结构都存在差异,其中fcc单晶临界尺寸最小,多晶次之,混晶最大; fcc单晶与多晶壳层原子中有少量hcp和bcc原子,而混晶壳层则全部为非晶类原子,并且fcc单晶、多晶与混晶的临界晶核都不是球型结构.     

Cold brittleness and fracture of metals with various crystal lattices: Dislocation mechanisms

The dislocation mechanisms of formation of the ductile–brittle transition temperature and the low-temperature brittle fracture of metals (single crystals, polycrystals) with various crystal lattices (bcc, fcc, hcp) are considered. The conditions of appearance of cold shortness and intracrystalline crack propagation (brittle fracture) are determined. These conditions can be met in bcc and some hcp metals and cannot be met in fcc and many hcp metals. The nondestructive internal friction (at 100 kHz) method is used to determine the temperature ranges of cold shortness (ductile–brittle transition temperatures) in bcc metals (ferritic–martensitic EK-181 steel, V–4Ti–4Cr alloy), which depend on their structure–phase state and strength (yield strength).     

Assembly of body-centered cubic crystals in hard spheres

We investigate the crystallization of monodisperse hard spheres confined by two square patterned substrates (possessing the basic character of the body-centered cubic (bcc) crystal structure) at varying substrate separations via molecular dynamics simulation. Through slowly increasing the density of the system, we find that crystallization under the influence of square patterned substrates can set in at lower densities compared with the homogeneous crystallization. As the substrate separation decreases, the density, where crystallization occurs (i.e., pressure drops), becomes small. Moreover, two distinct regimes are identified in the plane of bcc particle fraction and density for the separation range investigated. For large substrate separations, the bcc particle fraction displays a local maximum as the density is increased, and the resulting formed crystals have a polycrystalline structure. However, and more importantly, another situation emerges for small substrate separations: the capillary effects (stemming from the presence of two substrates) overwhelm the bulk driving forces (stemming from the spontaneous thermal fluctuations in the bulk) during the densification, eventually resulting in the formation of a defect-free bcc crystal (unstable with respect to the bulk hard-sphere crystals) by using two square patterned substrates.     

Theory of the crystal structures of cerium and the light actinides

First-principles theory, based on the density-functional approach, is used to study the crystal structures of Ce and the light actinides (Th-Pu) at low temperatures as a function of hydrostatic pressure. Calculated ground-state properties, such as crystal structure, atomic volume and bulk modulus, are shown to be very well described within this theory. We present the following pressureinduced phase transitions: Ce, fcc -> bct -> hcp; Th, fcc -> bct -> hcp; Pa, bct -> alphaU bct -> hcp; U, alpha-U -> bct -> bcc; Np, alpha-Np -> beta-Np -> bcc; Pu, alpha-Pu -> alphaNp -> beta-Np -> bcc. We explain the occurrence of low-symmetry (complex) structures in these metals as a consequence of a symmetry-breaking mechanism that shows similarities to a Peierls distortion. The ultimate high-pressure phases are well accounted for in a canonical model for the f bands for these metals.     

Evidence of a medium-range ordered phase and mechanical instabilities in strontium under high pressure

We provided the first theoretical evidence for a medium-range ordered phase in high pressure strontium from the first-principles calculations. At the absolute zero temperature, the enthalpy–pressure relation shows that the bcc and hcp are energetically more favorable than the other experimentally observed phases between 24 and 27 GPa. In the present work, we concentrate on the bcc phase because we found a link to a medium-range ordered phase. Our results reveal that the bcc phonon dispersion at the N and H points starts softening at around 24.1 GPa. The ab initio molecular dynamics at 300 K and 27 GPa showed that the bcc is quickly transformed into a lower energy structure with R3c symmetry and distorted basis. The simulated diffraction patterns showed that the R3c structure has only a single major peak at low angle. The R3c peak locates near the first peak of the bcc structure. This is the evidence of the so-called medium-range ordered phase. This structure is a strong candidate for the unsolved S-phase reported by experiments.     

Molybdenum at high pressure and temperature: melting from another solid phase

The Gibbs free energies of bcc and fcc Mo are calculated from first principles in the quasiharmonic approximation in the pressure range from 350 to 850 GPa at room temperatures up to 7500 K. It is found that Mo, stable in the bcc phase at low temperatures, has lower free energy in the fcc structure than in the bcc phase at elevated temperatures. Our density-functional-theory-based molecular dynamics simulations demonstrate that fcc melts at higher than bcc temperatures above 1.5 Mbar. Our calculated melting temperatures and bcc-fcc boundary are consistent with the Mo Hugoniot sound speed measurements. We find that melting occurs at temperatures significantly above the bcc-fcc boundary. This suggests an explanation of the recent diamond anvil cell experiments, which find a phase boundary in the vicinity of our extrapolated bcc-fcc boundary.