晶体相场模拟晶界预熔及熔化

利用晶体相场模拟晶界的预熔以及熔化现象,研究不同取向差角度时,预熔及熔化的微观组织形貌,并且采用过剩质量方法定量计算预熔及熔化时晶界处液相薄膜宽度.研究表明,在接近熔点时,晶界处预先出现一层液相薄膜,液相膜的形态与晶界处的取向差角度有关.当取向差角为大角晶界时,液相膜沿晶界均匀稳定分布;当取向差角为小角晶界时,若干由液相区包围的独立位错均匀分布在晶界处,随着温度逐渐接近熔点,晶界处发生结构转变:独立位错两两合并,原来小的液相区也相应合并成为较大的"液相池".这种结构转变不仅出现在预熔时,而且发生在过热状态下,在液相宽度曲线图上表现为宽度"跳跃"性增大.同时,晶体相场模型计算得到的临界润湿角θ_c为12°,较Read-Shockley理论所得的数值更接近实际结果.     

锌铝合金中稀土及铁的晶界偏聚电子理论

根据分子动力学理论建立液态ZA27合金的原子集团,结合计算机编程构造出ZA27合金α相与液相共存时的原子构形及α相大角度重位点阵晶界模型.利用递归法计算铁、稀土元素固溶于晶粒内、游离于固液相界面及其在α相晶界处的环境敏感镶嵌能.结果表明:铁、稀土处于固液相界区比在晶内更稳定,这解释了铁、稀土在α相内溶解度很小,结晶时富集于固液相界前沿液体中,从而导致凝固结束后铁、稀土元素偏聚于晶界,并形成成分复杂的稀土化合物的事实.     

预熔二元合金异质固液界面的液膜分离势能

金属液相界面间的短程相互作用——分离势——是诸多失效过程的关键因素.此热力学参量是晶界基因组工程中的重要组成部分,精确测量分离势对合金微观界面结构与热力学性质的调控具有重要意义.基于预熔铝(100)-铅异质固-液界面体系为研究对象的分子动力学模拟,测量了6种不同温度固相铝-预熔铝膜和预熔铝膜-液相铅界面位置的涨落概率密...     

铌和钨(001)面熔化的分子动力学模拟

采用改进分析型嵌入原子方法(MAEAM),对铌、钨晶体的(001)面预熔和熔化过程进行分子动力学模拟。通过计算不同温度的原子位置、层原子密度、层结构因子、径向分布函数等物理量,研究(001)面的微观结构随温度的变化。结果表明,(001)面在低于其完整晶体的熔化温度出现预熔。另外,通过将该面预熔摩尔分数对温度进行拟合发现,可以近似地预测(001)面的预熔过程。     

微区熔覆锡青铜凝固组织与热流密度的关系

金属表面熔覆处理是改善其耐磨性的有效手段,但熔覆过程温度变化较快,不易于直接测量。基于ANSYS模拟计算获得熔覆层温度变化,并进行组织分析,构建工艺-温度-与组织性能之间的关系规律。利用氩弧作为热源在铜基体上微区熔凝制备锡青铜熔覆层。通过金相显微镜、电子探针等对熔覆过程温度场不同位置处的微观组织观察分析,确定微观组织和热流密度之间的定性关系。通过对微小熔池内部温度分布的数值模拟和微观组织的观察分析,建立熔池内部晶体形态、大小、分布和过冷度与冷却速率的关系模型,重点对锡青铜熔覆层中柱状晶向等轴晶的转变进行研究。结果表明:晶粒尺寸、枝晶臂间距、析出相δ相和单质Pb大小与热流密度负相关,确定了柱状晶/等轴晶转变(CET转变)与热流密度的数学模型。通过对模拟参数的调整可将该模型扩展应用至不同材料微区熔覆制备工艺。     

预变形及液固两相区等温处理对ZA27合金铸态组织的影响

研究了固液两相区等温处理对铸态ZA27合金初生相形貌的影响,结果表明：在固液两相区进行等温处理时,枝晶熔断受固液界面处原子的散控制,提高等温温度、延长等温时间因有利于Zn和Al原子的扩散而促进初生相由枝晶向颗粒状晶演变,预变形促进初生相形貌演变的根本原因在于等温处理时液相沿预变形产生的亚晶界的熔渗。     

不同热处理工艺下6111铝合金的组织及抗晶间腐蚀

采用金相显微镜、透射电镜、X射线衍射仪等研究6111铝合金经不同温度固溶、预时效后的组织和晶间腐蚀情况。结果表明，在不同温度固溶和预时效后，晶粒均发生了再结晶，部分晶粒沿着轧制方向拉长，520℃和540℃固溶后的平均晶粒尺寸为60～70μm, 560℃固溶后的平均晶粒尺寸略大于520℃和540℃,为80μm左右。520℃固溶、80℃预时效后的抗晶间腐蚀最差，腐蚀深度为191.59μm, Q相在晶界处连续分布，表面晶粒尺寸较小；540℃固溶、80℃预时效后的抗晶间腐蚀最好，腐蚀深度为59.63μm, Q相在晶界处断续分布，并未发现尺寸较小的表面晶粒。     

保温时间、冷却速率以及预变形对钴金属γ→ε相变行为的影响

研究保温时间、冷却速度以及预变形对金属钴γ→ε相变行为的影响。重点使用电子背向散射衍射技术表征γ→ε转变的微观组织及织构演变行为。结果表明,经过γ→ε转变后金属钴的微观组织结构为双相组织(γ相和ε相):γ相中存在Σ3特殊晶界,ε相中存在70.5°/1120特殊晶界;且由于这些特殊晶界的存在,导致晶粒细化和基面织构弱化。保温时间和冷却速度对残余奥氏体(γ相),Σ3晶界以及70.5°/1120晶界含量的影响,不存在明显的规律。然而,随着预变形程度的增加,奥氏体含量、Σ3晶界和70.5°/1120晶界的含量都随之增加,且晶粒细化及织构弱化效应显著。对γ→ε相变后板条组织的表征,发现不同形态特征的板条组织可由块状和三角形为基本单元所构成。     

半固态加工变形温度对Ti14微观组织及变形机制的影响

研究了不同半固态加工变形温度下Ti14合金组织的演变规律,探讨了变形机制.结果表明:温度影响了液相的析出,随着温度的升高,液相形成量增加,并集中在晶界处,使得晶界宽化,由不连续转变为连续分布;由于晶界液相的增加,产生了润滑作用,使得晶界的摩擦力减小,滑移易于开动,加工变形机制由单一的固相塑变转变为以晶界滑移为主伴随少量固相塑变.     

钢铁材料晶界性质和冲击断裂特征的电子结构表征

采用电子能量损失谱(EELS)研究了不同商用钢铁材料的晶界,计算了晶界处和晶粒内铁原子的3d电子占据态密度,并将其和晶界性质以及材料的宏观断裂性能相联系．结果表明：当样品晶界处铁原子的3d电子占据态密度高于晶粒内时,晶界结合强度低于晶内,晶界表现出脆性,材料的冲击断裂方式主要为脆性的沿晶断裂;反之,如果晶界处铁原子的3d电子占有态密度与基体没有明显的差异,则晶界结合强度与晶内相当,晶界表现出韧性,材料的断裂方式主要为韧性的穿晶断裂．     

连续升温、降温过程中纯Fe的微观结构分析

通过分子动力学模拟,采用较先进的键型指数法HA及原子团类型指数法CTIM-2,对Fe连续升温、降温过程中微观结构进行模拟研究.结果表明:连续升温过程,Fe的微观结构变化是bcc→fcc\hcp→bcc→液体；连续降温过程,Fe的微观结构变化是液体→fcc\hcp.Fe凝固结束没有形成大量的高温bcc晶体,原因是在高温液态中bcc结构原子稳定性较差,fcc和hcp结构原子更易稳定存在.此外,温度变化速率过快,可诱导晶体生长过程中发生层错,促使Fe在升温、降温过程出现fcc和hcp晶体的交替分层分布,这与fcc和hcp晶体的原子能量相近、晶体的致密度相同、原子空间堆垛方式局部相同有关.     

铝合金液态填充焊的工艺特性分析

采用高速摄像和工艺试验相结合的方法分析了液态填充焊的影响因素,进行了焊接接头质量分析.结果表明,激光液态填充焊有两种填充模式:焊丝半熔态、焊丝全熔态.焊接电流较小时,焊丝为半熔态,液态填材顺焊丝流向熔池;焊接电流较大时,焊丝为全熔态,依靠间隙的毛细作用,能稳定的流入熔池中.随着焊接速度的提高匙孔距熔池边缘的距离变小.为了使液态填材稳定的过渡到熔池中,光丝间距应控制在-0.5~2.0 mm范围内.经过接头质量分析发现:与激光填丝焊相比,液态填充焊接焊缝中的气孔率明显降低.     

激光液态填充焊的填材熔化与过渡稳定性

针对激光填丝焊过程中焊丝对入射激光能量的反射、匙孔不稳定等问题,提出了激光液态填充焊方法,采用电弧提前预熔焊丝,让填材以液态方式流入熔池中.分析了激光液态填充焊的熔化、过渡模式,以及工艺参数对过渡模式的影响规律.结果表明,在电弧热作用下焊丝有液桥过渡和滴状过渡两种模式,焊接电流较小时,焊丝的上部熔化,下部依然保持固态,形成稳定的液桥过渡;焊接电流较大时,焊丝端部全部熔化易回缩成球为滴状过渡,过渡稳定性变差.为改善滴状过渡的稳定性,可依靠间隙的毛细作用稳定流入熔池中.激光液态填充焊可降低对匙孔的冲击,提高匙孔的稳定性.     

等温处理对挤压态AZ80A镁合金半固态组织的影响

采用应变诱发熔化激活法(SIMA)工艺制备了AZ80A镁合金半固态坯料,研究了保温温度和保温时间对半固态组织的影响。结果表明:随着保温温度的升高和保温时间的增加,AZ80A镁合金的平均晶粒尺寸与液相率都呈上升趋势,形状因子呈先增大后减小的趋势。半固态组织由α-Mg晶粒、Al、Zn元素富集形成的晶界处液相和晶内“小液池”组成,其组织演变分为初始晶粒合并长大,晶粒球化、彼此分离,最终合并粗化3个阶段。采用该种方法制备AZ80A镁合金半固态坯料时合适的保温温度为550 ℃、保温时间为45 min,此时半固态组织的平均晶粒尺寸、形状因子和液相率分别为89 μm、0.795和26.7%。     

Models of melting and liquid structure based on rotationally free atomic clusters

Atomistic models, describing the melting process of crystalline materials and the resultant liquid state, have been developed. With an increase in temperature, the inter-atomic distance of a crystal is suggested to fluctuate by the wave-like behavior of lattice vibration. At the melting point, the crystal is divided into numerous atomic clusters and the rotational symmetry of the atoms is broken. The model is adequate for describing the first-order phase transition characteristic of the melting process, which shows an abrupt increase in volume. The liquid state can, therefore, be defined as an assemblage of temporary atomic clusters. The atoms have no long-range order due to the rotational freedom of the clusters.     

Our thoughts are ours,their ends none of our own: Are there ways to synthesize materials beyond the limitations of today?

Historically, the following three approaches to synthesize materials with new atomic structures and, thus, new properties may be distinguished. In the first period – beginning with the discovery of metals about 5000 years ago – the atomic structure of crystalline metallic materials was modified by introducing crystal defects, e.g. by hammering or rolling. However, even at the maximum defect density achievable by this approach (about 10¹² dislocations per cm²) only about 1% of the atoms are located in the cores of the crystal defects where the atomic structure deviates significantly from the one in the perfect crystal lattice. In other words, this approach does not permit the generation of crystalline materials, the atomic arrangements of which deviate significantly (i.e. in a large volume fraction of the material) from the atomic arrangement in perfect crystals with the same chemical composition. Many crystalline materials used commercially today are based on this approach. In the second approach, the way to a class of materials with new atomic structures was opened about 35 years ago by having up to 50% of the atoms located in cores of grain boundaries and/or interphase boundaries. Materials of this kind were obtained by reducing the crystal size of polycrystals to a few nanometers. These materials were called nanocrystalline or nanostructured materials. As the atomic arrangements in the cores of grain and/or interphase boundaries differ from the ones in perfect crystals, this approach led to materials with new properties (in comparison to the chemically identical single crystals). In the most recent, third approach, a new class of materials with a glassy structure is synthesized. Their novel feature is that the atomic structure throughout the entire volume of the material as well as the density of the entire material can be tuned. Materials of this kind are called nanoglasses. They are generated by introducing interfaces into metallic glasses on a nanometer scale. These interfaces delocalize upon annealing, so that the free volume associated with the interfaces spreads throughout the volume of the glass. This delocalization changes the atomic structure and density of the glass throughout the volume. In fact, by controlling the spacing between the interfaces introduced into the glass as well as their degree of delocalization (by modifying the annealing time and/or annealing temperature), the atomic structures as well as the density (and hence all structure/density-dependent properties) of nanoglasses may be controlled. A reduction of the density by up to 15% seems to be possible. A comparable tuning of the atomic structure/density of crystalline materials is not conceivable because defects in crystals (grain boundaries, dislocations, etc.) do not delocalize upon annealing.     

GH2132高温合金熔敷金属结晶裂纹敏感性

针对GH2132高温合金熔敷金属热裂纹敏感性问题,通过采用试制焊丝、熔敷金属焊接试验、组织及断口分析、凝固计算等手段对熔敷金属组织、凝固行为、开裂机制等进行了研究. 结果表明,试验熔敷金属金相组织主要由柱状树枝晶γ相(NiCrFe固溶体)、枝晶间富Ti的Laves相(Cr,Fe,Ni)₂ (Ti,Mo)、MC碳化物与共晶组织组成,凝固路径为L→L + γ→L + γ + MC→L + γ + MC + Laves→γ + MC + Laves,裂纹断口呈典型的鹅卵石共晶花样,整个断口形貌被呈自由表面的液膜所覆盖,属于发生在高温段的结晶裂纹. 结晶裂纹开裂机理为在凝固过程的终了阶段,发生了L→γ + Laves的低熔点共晶反应,在凝固收缩应力作用下,残余液相未及时补充而形成. Laves相的形成主要与凝固过程中Ti元素的偏析有关,理论计算结果表明,GH2132结晶裂纹指数(solidification cracking index, SCI)值为1944 ℃,(solidification temperature range, STR)为258 ℃,在结晶裂纹敏感性评价方面,相比STR,SCI指标能相对更为合理地实现结晶裂纹敏感性的量化评价,但仍存在考虑因素不全等问题.     

选区激光熔化成形316L不锈钢微观组织及拉伸性能分析

采用选区激光熔化技术制备了316L不锈钢的拉伸试样,分析了试样不同区域的组织特征,测试了其拉伸力学性能.结果表明,其组织形貌主要为胞状晶,但在某些“微熔池”内晶粒生长方向不相同,而近乎相互垂直,从而在同一视野中显示出典型的细小柱状晶（亚晶）和近似六边形“胞晶”共存的组织特征.试样的抗拉强度与传统工艺制备的相比有较大提高,但断后收缩率有所降低.这主要由于选区激光熔化是快速熔化与冷却凝固的过程,其选区熔化的特征使得不同区域的激光入射角度、选区熔化扫描方式、“熔池”散热条件各不相同,导致不同区域呈现复杂的结晶过程,形成不同特征的微区组织.由于冷却速度较快所得的细小柱状晶的直径为亚微米级,致密分布,显著提高了材料的抗拉强度.但由于晶粒生长明显的方向性,使得拉伸过程中晶粒在不同方向的塑性变形不均匀,相互牵制,加之大量熔合线界面处不可避免的内应力,导致断后收缩率有所降低.     

Features of forming structure of titanium pseudosingle crystal during β → α (bcc → hcp) polymorphic transformation

The structure of a titanium iodide single crystal obtained by zone melting has been studied by metallography, X-ray diffraction, and electron microscopy. It has been shown that the initial bcc titanium single crystal becomes a pseudosingle crystal upon cooling below the temperature of the β → α polymorphic transition. The pseudosingle crystal consists of macroscopic packets, i.e., crystals of lath morphology with a size of 0.1–0.5 cm² in different crystal sections. Each packet consists of α-phase laths of the same orientation, which are separated by dislocation boundaries. A total of six different types of packets in the pseudosingle crystal volume is realized in accordance with the Burgers orientation relationships. The structural heredity in the titanium pseudosingle crystal after the cycle of the α → β → α transformations is confirmed.     

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.