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).     

非静水压条件下铁从α到ε结构相变的第一性原理计算

采用基于密度泛函理论的平面波赝势方法,研究了三轴加载的非静水压力和静水压力对铁从体心立方结构（bcc,α相）到六角密排结构（hcp,ε相）相变压力和磁性的影响,结果发现：在0—18 GPa压力范围内,相对静水压力条件,随着压力的升高,bcc结构的原子磁矩在非静水压力下降低得更快;在非静水压力下,相变更容易发生,相变压力随着非静水压力程度的增加而降低;并且对非静水压力对相变压力影响的物理机理进行了讨论. 关键词： 相变 非静水压力 第一性原理 铁     

Importance of shear in the bcc-to-hcp transformation in iron

Iron shows a pressure-induced martensitic phase transformation from the ground state ferromagnetic bcc phase to a nonmagnetic hcp phase at approximately 13 GPa. The exact transformation pressure (TP) and pathway are not known. Here we present a multiscale model containing a quantum-mechanics-based multiwell energy function accounting for the bcc and hcp phases of Fe and a construction of kinematically compatible and equilibrated mixed phases. This model suggests that shear stresses have a significant influence on the bcc<-->hcp transformation. In particular, the presence of modest shear accounts for the scatter in measured TPs. The formation of mixed phases also provides an explanation for the observed hysteresis in TP.     

动态压缩下马氏体相变力学性质的微观研究

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     

Order-disorder phase transition in CoPd thin films

The thin films of a CoPd alloy in the equiatomic composition region are prepared by condensation at different substrate temperatures. The substrate temperature is varied from the liquid nitrogen temperature to +280°C. At low substrate temperatures, the crystal structure of the condensed films is the single-crystal blocks of the hexagonal close-packed (hcp) phase. As the substrate temperature is further increased, the domains characterized in the initial state by the microdiffraction patterns in the form of a diffuse halo appear in the films, and these domains have a clear-cut boundary with the regions indicated by point reflections in the electron diffraction patterns. At substrate temperatures from +150 to 160°C, the CoPd alloy films in the equiatomic composition region are fully amorphous. The given state is a polymorphic transformation of the martensitic type. It arises in the martensitic transformation of the low-temperature hcp phase to the high-temperature fcc phase.     

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.     

Structure of heterogeneous states in nanocrystalline CoPd films

Thin films of equiatomic alloy CoPd have been produced by condensation at high substrate temperatures. The films obtained are characterized by microdiffraction patterns in the form of diffuse halo in the initial state. This is polymorphic martensitic transient state, formed during martensitic transformation of the low-temperature hcp phase to the high-temperature fcc phase. The metastable crystal structures arising in these alloy films are identified.     

Mechanisms of deformation localization and mechanical twinning under the conditions of phase instability of a crystal in stress fields

A new mechanism of the deformation and reorientation of a crystal has been studied experimentally and theoretically. This mechanism, which is effective in strain localization mechanical twinning bands of metal alloys and intermetallides, is a mechanism of dynamic phase (direct plus reverse martensitic) transformations in fields of high local stresses. The features of the reorientation and the defect substructure in these bands are discussed using electron microscopy data. With models of martensitic transformations based on the concept of cooperative thermal vibrations of extended coherent objects in crystals, the atomic mechanisms of direct plus reverse transformations are analyzed and the reorientation matrices (vectors) and distortion tensors are calculated for some (fcc bcc fcc, bcc hcp bcc) variants of these transformations. The carriers and nature of the above deformation mechanism and the principal physical effects underlying this mechanism are discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 28–48, August, 2004.     

单轴应变条件下Fe从α到ε结构相变机制的第一性原理计算

采用基于密度泛函理论的平面波赝势方法,研究了沿［001］方向单轴应变条件下Fe从体心立方结构(bcc,α相)到六角密排结构(hcp,ε相)相变的临界压力、相变路径、相变势垒以及相变过程中原子磁性的变化.结果发现:单轴应变条件下Fe从α到ε结构的相变路径与以前理论计算模拟给出的静水压力条件下的相变路径明显不同;原子磁矩沿着相变路径突然降低,同时伴随着能量和体积的突然变化,是典型的一阶磁性相转变,表明原子磁性的丧失导致了bcc结构不稳定而向hcp结构转变.对单轴应变下吉布斯自由能的计算表明,相变势垒随着单轴应 关键词： 相变 单轴应变 第一性原理 铁     

面心立方铁纳米粒子的相变与并合行为的分子动力学研究

本文采用分子动力学模拟结合Finnis-Sinclair多体势研究了面心立方铁纳米粒子在加温过程中的相变与并合行为. 模拟结果表明: 纳米粒子在熔化之前均发生了由面心立方至体心立方的马氏体相变; 大小相等的两纳米粒子在并合之前发生了相对转动; 而大小不等的两纳米粒子在并合过程中并未出现转动, 小纳米粒子倾向于吸附在大纳米粒子上, 并随着温度的升高而熔化, 最终形成更大的纳米粒子. 关键词： 纳米粒子 相变 并合 分子动力学     

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     

Lattice dynamics and phase diagram of aluminum at high temperatures

The dispersion of phonons in the fcc, hcp, and bcc phases of aluminum is calculated at ultrahigh pressures by the method of small displacements in a supercell. The stability of the phonon subsystem is studied. The thermodynamic characteristics are calculated in the quasi-harmonic approximation, and a phase diagram of aluminum is plotted. As compared to the Debye model, the use of a phonon spectrum calculated in the quasi-harmonic approximation significantly broadens the hcp phase field and strongly shifts the phase boundary between the fcc and bcc phases. The normal isentrope is calculated at megabar pressures. It is shown to intersect the fcc-hcp and hcp-bcc phase boundaries. The sound velocity along the normal isentrope is calculated. It is shown to have a nonmonotonic character.     

Prediction of a bcc-hcp phase transition for Sn: A first-principles study

The high-pressure structural transformation of elemental Sn is studied using an ab initio density functional theory implementation of the metadynamics method that predicts with sufficient compression, Sn will transform from the bcc structure into an hcp structure. The low-free-energy pathway associated with this phase transition is characterized as the Burgers transition mechanism. The superconducting properties of Sn under pressure are also investigated. Both bcc and hcp structures of Sn exhibit very weak electron-phonon coupling and therefore would not sustain superconductivity at high pressure.     

Modelling of dislocation-induced martensitic transformation in anisotropic crystals

The structural transformation caused by dislocation-induced heterogeneous nucleation in the fcc?→?bcc martensitic transformation in elastically anisotropic crystals is investigated by using the phase field microelasticity model. The three-dimensional microstructure of the dislocation-induced martensitic embryos is obtained. It is found that the embryos are not single-domain particles as is usually assumed but rather a complex self-organized assemblage of stress-accommodating twin-related microdomains. Sessile metastable martensitic embryos around the dislocation loops form in the prototype Fe–Ni alloy system above the temperatures of the martensitic transformation. A possibility that the presence of these pre-existing embryos could be responsible, at least, for a part of the elastic modulus softening with the temperature decrease observed in many martensitic systems is discussed. The effects of elastic anisotropy, the “chemical” energy barrier and structural anisotropy of the Landau free energy on the formation and growth of martensitic embryos are investigated. The assumptions of elastic isotropy and a choice of the anisotropic term in Landau polynomial do not significantly affect the microstructure of martensitic embryos but may appreciably change the undercooling that is necessary to eliminate the total nucleation barrier and start the athermal martensitic transformation.     

第一性原理计算Fe从bcc到hcp结构的相变路径及其磁性边界

本文采用基于密度泛函理论的第一性原理方法,计算了压力作用下Fe从bcc到hcp结构相变的势能面、相变路径以及相变过程中的磁性相边界.结果表明:与Burgers路径不同,相变过程中bcc结构(110)bcc面的剪切和相对滑移相互耦合,并伴随有(110)bcc面间距的减小;这一相变机制可以解释Fe的静高压实验中在相变初期观察到的hcp结构异常.因此,并不需要像Wang和Ingalls提出的那样,在相变过程中引入一个亚稳定的fcc相来解释这些实验结果.对相变势能面的计算表明剪切对相变的发生有激活作用.此外,分析表明相变过程中涉及复杂的磁性转变,相变过渡态位置正好位于磁性相边界上,并对原子磁性对结构转变影响的物理机制进行了讨论.     

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.     

冷速对液态金属Mg凝固过程中微观结构演变的影响

采用分子动力学方法对不同冷速下液态金属镁(Mg)快速凝固过程中的微观结构演变进行了模拟研究.并采用能量-温度(E-T)曲线、双体分布函数、Honeycutt-Andersen键型指数法、原子团簇类型指数法(CTIM-3)以及三维可视化等方法系统地考察了凝固过程中微观结构演变与相转变过程.结果发现:在以冷速为1×10~(11)K/s的凝固过程中,亚稳态bcc相优先形成,随后大量解体,其变化规律符合Ostwald规则,系统最终形成以hcp结构为主体与fcc结构共存,中间还夹杂部分bcc结构的致密晶体结构.在1×10~(12)K/s冷速下,结晶过程呈现迟缓现象,形成bcc结构的初始温度降低,系统形成以hcp居多、与bcc和fcc三相共存的结构,且因相互竞争、相互制约而导致不易形成粗大的晶粒结构.而在1×10~(13)K/s冷速下,系统则形成以1551,1541,1431键型为主的多种非晶态基本原子团组成的非晶态结构.此外,在冷速1×10~(12)与1×10~(13)K/s之间的确存在一个形成非晶态结构的临界冷速.     

Microstructural aspects of the hcp-fcc allotropic phase transformation induced in cobalt by ball milling

A detailed correlation between microstructure evolution and allotropic phase transformations occurring in Co when subjected to ball milling has been carried out. After short-term milling, the starting mixture of hcp + fcc Co develops into an almost pure hcp phase. However, for longer milling times, plastic deformation introduces large amounts of stacking faults, especially of twin type, in the hcp structure. As a consequence, some of the hcp Co is converted back into fcc and the hcp unit cell is progressively anisotropically distorted. After long-term milling, a steady 'pseudo-equilibrium' state is observed, where all microstructural parameters, including the fcc percentage, tend to level off. However, the milling intensity can still be adjusted to increase further the stacking-fault probability and, consequently, the amount of fcc Co in the milled powders. The results imply that the stacking-fault formation, rather than the local temperature rise or crystallite size reduction associated with the milling process, is the main mechanism governing the hcp-fcc transformation.     

Order-disorder phase transition in CoPd thin films

The thin films of a CoPd alloy in the equiatomic composition region are prepared by condensation at different substrate temperatures. The substrate temperature is varied from the liquid nitrogen temperature to +280°C. At low substrate temperatures, the crystal structure of the condensed films is the single-crystal blocks of the hexagonal close-packed (hcp) phase. As the substrate temperature is further increased, the domains characterized in the initial state by the microdiffraction patterns in the form of a diffuse halo appear in the films, and these domains have a clear-cut boundary with the regions indicated by point reflections in the electron diffraction patterns. At substrate temperatures from +150 to 160°C, the CoPd alloy films in the equiatomic composition region are fully amorphous. The given state is a polymorphic transformation of the martensitic type. It arises in the martensitic transformation of the low-temperature hcp phase to the high-temperature fcc phase. Original Russian Text ? E.M. Artem’ev, M.E. Artem’ev, 2007, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2007, Vol. 86, No. 11, pp. 838–840.