fcc-,亚稳hcp-和bcc-Cu的原子状态及物理性质随温度的变化关系

结合纯金属单原子(OA)理论和Debye-Gruneisen模型,采用CALPHAD方法确定的晶格稳定参数,研究了 SGTE纯单质数据库中fcc-,hcp-和bcc-Cu的原子状态及物理性质(原子势能、原子动能、原子体积、体弹性模量和热膨胀系数等)随温度的变化关系．结果表明:电子结构计算结果与第一原理方法非常接近． 3种晶体结构的电子结构差别不大,单键半径非常接近．原子体积顺序为:Va(bcc)>Va(hcp)>Va(fcc);共价电子浓度顺序为:nc(fcc)>nc(hcp)>nc(bcc);原子势能大小顺序为:εp(fcc)<εp(hcp)<εp(bcc);晶格稳定性顺序为:△G(fcc)>△G(hcp)>△G(bcc)．原子动能随温度的增加幅度约为势能的3-4倍．     

Cu熔体中原子团簇在凝固过程中的演变规律分子动力学模拟

用分子动力学模拟研究了Cu熔体以不同速率冷却微观结构的演变规律.结果表明,冷却速率在10^12.6K/s到10^14.5K/s之间时,Cu熔体凝固后形成了非晶体与晶体的混合体;Cu熔体中的原子团簇、临界晶核及凝固后晶体的结构均是由hcp和fcc结构层状镶嵌排列构成,这说明Cu凝固后形成的层状镶嵌结构起源于形核阶段;冷却速率小于10^13.3K/s时,Cu层状镶嵌结构中具有fcc结构的原子数多于hcp结构的原子数,而冷却速率大于10^13.3K/s后,hcp结构的原子数多于fcc结构的原子数;Cu非晶基体中晶态结构原子团簇的尺寸小于临界晶核尺寸时,虽然用HA键型指数法能确定出一定数量晶态结构原子键对的存在,但径向分布函数反映不出其晶态结构的特征.     

过冷液态和非晶态金属Pb等温驰豫过程中bcc相的形成和演变特性(英文)

采用分子动力学模拟方法和团簇类型指数法,对过冷液态和非晶态金属Pb在等温驰豫过程中bcc相的形成和演变特性进行研究。结果表明:bcc相的形成和演变密切依赖等温驰豫过程的初始温度和初始结构,在过冷液态区,bcc相很容易形成并在模拟时间范围内保持稳定;而在非晶态区,bcc相先形成并随后部分转变为hcp相,当驰豫的初始温度在较低的153K和113K时,hcp和fcc相不经历亚稳bcc相而直接在非晶态结构中形成;这说明Ostwald的"步进原则"在过冷液态和非晶态Pb等温驰豫过程中是有效的,并且,亚稳bcc相起到重要的晶化前驱的作用。     

Fe基合金连续冷却相变的分子动力学模拟

采用分子动力学模拟了Fe99Cu1合金在1×1014 K/s冷速下从fcc奥氏体结构转变为bcc铁素体结构的相变过程.结果表明,Fe99 Cu1合金在900～800℃之间开始发生相变,600 ℃时相变明显,100℃时55％的原子转变为bcc结构.Cu元素阻碍合金相变,并且促进bcc孪晶形成.     

bcc-Fe等温压缩下相变的微观模拟与分析

使用嵌入原子势分子动力学方法,对bcc-Fe在等温压缩(沿[001]晶向)下的相变(bcc至hcp)的微观过程进行了数值模拟.结果表明,当应力超过相变阈值,hcp相开始形核并沿(011)面长大成片状体系,同时系统进入超应力松弛状态;平均应力及hcp相质量分数在初始形核时发生突变,之后与体系的体积变化近似呈线性关系;纵向偏应力与相变质量分数在整个相变过程保持线性关系;混合相中,hcp相的平均势能高于bcc相.     

晶体相场法模拟晶界预熔区液相熔池的演化

预熔是指晶体在低于熔点温度时,晶界处预先出现类似液体而块体仍为晶体状的现象。采用晶体相场法研究原子密度对晶界预熔的影响。结果表明:晶界处液相熔池的早期演化主要涉及4个形态特征:固-固状态→小液滴状态→较大的液相熔池→均质熔融层。通过对比不同平均原子密度下的微观结构和能量变化,表明平均原子密度对液相池的形态特征敏感。二维与三维模拟结果表明,平均原子密度的降低能抑制刃型位错聚集区域中原子的结晶相特征,这有助于液相熔池的形成。从热力学的角度验证平均原子密度与液相熔池宽度之间的关系,在一定程度上为后续施加高温应变提供前提条件。     

金属Pd的原子状态和物理性质

依据纯金属单原子理论(OA)确定了面心立方结构(fcc)金属Pd的原子状态为[Kr](4dn)5.98(4dc)2.23(5sc)1.56(5sf)0.23,并对Pd的密排六方结构(hcp)和体心立方结构(bcc)初态特征晶体及初态液体的原子状态进行了研究,并在此基础上解释了金属Pd的原子状态与晶体结构的关系,并通过计算得到了fcc-Pd的势能曲线及体弹性模量、线热膨胀系数、比热容和Gibbs自由能等热力学性质随温度变化的曲线,这些性质的理论值与实验值符合较好。     

Fe10Ni90和Fe90Ni10合金结晶过程微观结构的模拟

以101 K/s、1O10 K/s两种冷速对富Ni的Fe10Ni90合金和富Fe的Fe90Ni10合金进行快速凝固,并采用原子平均势能、双体分布函数、配位数、3D可视化、最大标准团簇等方法研究了快速凝固过程中两种合金的微观结构随温度快速降低的演变.结果表明:在1010 K/s冷速下,两种合金的结晶温度一致,但Fe10Ni90合金的原子平均势能要低于Fe90Ni10合金,使得其双体分布函数的第一峰值要高于Fe90Ni10合金,表明Ni原子之间比Fe原子之间成键概率更高;在两种冷速下,Fe10Ni90合金的fcc晶体数目变化较大,Fe90Ni10合金的bcc晶体数目变化较小,而且由于二十面体和截边十面体的出现,Fe10Ni90合金出现了五重孪晶结构.     

N对TWIP钢马氏体相变及力学性能的影响

以传统TWIP钢为对比,测试了含N TWIP钢的力学性能,并利用XRD进行物相分析和TEM进行做观结构表征.结果表明,在由fcc或hcp结构向bcc结构马氏体进行相变时,晶体结构中的最大间隙由0.1047 nm降低至0.0725 nm.间隙原子N的存在显著增大bcc结构的晶格畸变能,提高α马氏体切变的阻力,因而强烈抑制α马氏体相变,导致组织中hcp结构ε相含量大幅度增加,提高了TWIP钢的强度,但也降低了钢的塑性.另外,奥氏体平均和区域层错几率的计算及微观组织分析结果表明,形变增加层错的数量,而马氏体相变消耗层错,从而减少层错数量.     

生长铜膜中孪晶形成与出现几率的分子动力学模拟

运用分子动力学和静力学方法对(111)生长铜膜中孪晶形成的原子过程与能量进行了模拟研究.所用的原子间相互作用势为Finnis-Sinclair型镶嵌原子法(EAM)势.模拟和计算分析结果表明,(111)生长铜膜表面沉积原子在不同局部可形成正常排列的fcc畴或错排的hcp畴;沉积原子处于hcp位置时体系的能量比fcc位置时要高,其增量决定了孪晶面出现几率.沉积原子错排能还受相邻{111}孪晶面的影响,其间距小于3个原子层厚时,沉积原子错排能与不形成孪晶的Al晶体表面沉积原子错排能相当,此时形成孪晶面的几率极低;随间距的增加,表面沉积原子错排能迅速降低,在间距达到约12个原子层厚以后,降到略低于完整Cu晶体{111}表面的沉积原子错排能,这表明此时出现孪晶面的几率比在完整晶体表面形成一个新的孪晶面的几率要大.     

Study of the structure of cobalt single crystals during the β → α transformation

Methods of metallography, transmission electron microscopy, and X-ray diffraction have been used to study the specific features of the structure formation during the fcc → hcp transformation occurring under different thermokinetic conditions in a single crystal of cobalt. It is shown that only one of four possible versions of hcp-phase crystal orientations is predominantly realized upon the β → α transformation occurring in the single crystal at a low cooling rate (V _cool ～ 1–2 K/min). It has been established that several cycles of slow heating to temperatures of 600, 800 and 1000°C and subsequent cooling of the single crystal do not increase the number of α-phase orientations. The restoration of the initial fcc-phase single-crystal orientation was observed after each heating cycle of the oriented sample, while after cooling the restoration of the preferred hcp-martensite orientation was observed; in this case, the quantity of the retained β phase fixed in the structure at room temperature increases with increasing number of cycles. After rapid quenching from 530°C into salt water (V _cool ～ 600–700 K/min), α-phase crystals of all four possible orientations are formed in the structure. Upon high-temperature quenching from 1000°C, the volume of crystal is divided into packets each containing martensite plates of predominantly one orientation. The transformation-induced recrystallization of the cobalt pseudo-single crystal quenched in salt water has been observed during repeated heating to temperatures above the β → α transformation temperature.     

Crystallographic laws governing the formation of the structure of pseudo-single crystals of some 3<Emphasis Type="Italic">d</Emphasis> metals and related alloys

Methods of metallography, X-ray diffraction, transmission electron microscopy, and dilatometry have been used to study the formation of the structure of pseudo-single crystals of nitrogen-bearing steel Kh18AG20 upon the δ → γ (bcc → fcc) transformation, in pseudo-single crystals of pure cobalt and binary alloy Co-29.7% Ni upon the β → α (fcc → hcp) transformation, and in pseudo-single crystals of zirconium upon the β → α (bcc → hcp) transformation. It has been established that the precipitation of austenite from ferrite during the δ → γ transformation in a single crystal of the Kh18AG20 steel occurs via a crystallographically ordered mechanism with the fulfillment of orientation relationships close to the Kurdjumov-Sachs orientation relationships. In the volume of the pseudo-single crystal there were realized six orientations of austenite with the retention at room temperature of a significant fraction of residual δ (α) ferrite. It has been shown that in the process of cooling of the β single crystals of cobalt, Co-29.7% Ni alloy, and zirconium to below the temperature of the β → α transition there formed several crystallographic orientations of the α phase that are grouped into packets. In each packet, there exist crystals of the α phase of one orientation. In accordance with the Wassermann orientation relationships, in the pseudo-single crystals of cobalt and Co-29.7% Ni alloy there are realized packets of four variants. In the pseudo-single crystal of zirconium, six variants of packets based on the Burgers orientation relationships are realized.     

Atomic states and properties of Ru-electrocatalyst

1 Introduction In recent years, because the consumption of irreplaceable fossil fuels which meet the bulk of power needs nowadays is non-sustainable and worldwide attention focus on processes that convert these precious resources into electricity via eff…     

Thermodynamic Assessment of the Fe-Y System

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Structure of As-Cast Hafnium

The structure of a hafnium crystal grown by the method of floating zone melting which underwent upon cooling the β → α (bcc → hcp) polymorphic transformation has been studied using the metallography, EBSD analysis, and electron microscopy. It has been shown that the α-phase structure of as-cast hafnium consists of lath-shaped crystals grouped into packets. As a rule, the α-phase grains contain several packets of different orientations from 12 orientations that can arise according to the Burgers orientation relationships. The boundaries of the α-phase grains and packets differ significantly. The grains have smooth boundaries, whereas the boundaries of packets are wavy. The misorientations between separate laths in a packet are less than 1°. Extended twins with a \({\left\{ {10\;\bar 1\;2} \right\}_\alpha }\) twinning plane have been discovered in the structure of hafnium. The presence of twins in the hafnium crystal is due to the influence of the total internal stresses caused by the temperature gradient that arise upon zone melting and by the β → α phase transformation. The fracture of the hafnium crystal at room temperature occurs along the basal (0001) plane. The typical fracture is brittle-viscous with a predominance of the brittle component.     

Application of classical nucleation theory to phase selection and composition of nucleated nanocrystals during crystallization of Co-rich (Co,Fe)-based amorphous precursors

Classical steady-state nucleation theory is applied to Co-rich Fe,Co-based alloys to provide a rationale for experimental observations during the nanocrystallization of Co-rich (Co,Fe)₈₉Zr₇B₄ and (Co,Fe)₈₈Zr₇B₄Cu₁ amorphous precursors. The amorphous precursor free energy is estimated using density functional theory. This simple theory suggests: (i) strain or interface energy effects could explain a tendency for a body-centered cubic (bcc) phase to form during crystallization. Dissolved glass formers (Zr,B) in crystalline phases may also contribute; (ii) similar face-centered cubic (fcc) and hexagonal close-packed (hcp) free energies could explain the presence of some hcp phase after crystallization even though fcc is stable at the crystallization temperature; (iii) nanocrystal compositions vary monotonically with the Co:Fe ratio of the amorphous precursor even when multiple phases are nucleating because nucleation is not dictated by the common tangency condition governing bulk phase equilibria; and (iv) Fe-enrichment of the bcc phase can be attributed to a relatively small free energy difference between the amorphous and bcc phases for high Co-containing alloys.     

A STUDY OF TEMPERATURE-DEPENDENT VALENCE BOND STRUCTURE OF TITANIUM

On the basis of energy and shape method for the determination of the valence bond (VB) structures of crystal, the valence bond structure of titanium is redetermined at room temperature and calculated in the whole temperature range of 0-1943K. The outer shell electronic distribution of Ti is e_c~(2.9907) · (s_c~(0.4980) d_c~(2.4927)) ef1.0093 in crystal. The temperature dependences of the VB structures of hcp and bcc phases are the same. The VB structures of hcp and bcc phases monotonically increase or decrease with the increase in temperature, but show discontinuous changes at the phase-transformation temperature 1155K.     

EXAFS STUDY OF THE SHORT RANGE STRUCTURE OF NANOCRYSTALLINE BCC-Fe_(80)Cu_(20) SOLID SOLUTION

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