Atomistic Simulations of Integranular Fracture in Symmetric-Tilt Grain Boundaries

Fracture experiments on symmetric-tilt grain boundaries in Cu are interpreted using the Peierls-Nabarro continuum model of dislocation nucleation as a starting point. Good agreement is found only when the continuum model is modified according to the results of atomistic simulations. The same experiments are also reproduced by direct Molecular Dynamics simulations of fracture propagation and dislocation emission from a microcrack placed in the interface plane of the symmetric-tilt (221)(221) grain boundary in fcc Cu. Direction-dependent fracture response is observed, namely the microcrack advancing by brittle fracture along the [11 ] direction and being blunted by dislocation emission along the opposite [ 4] direction. Moreover, the simulations allow us to establish important differences with respect to the continuum-model predictions due to the shielding of the stress field at the crack-tip and to the presence of the residual stress at the interface.     

Shell Analysis and Effective Disorder in a 2D Froth

The static and evolutionary properties of two-dimensional cellular structures, or froths, are discussed in the light of recent work on structuring of the froth into concentric shells. Of interest is the dual role of a topological dislocation (defect) in an otherwise uniform froth, considered both as a source of disorder and also as a source generating a shell-structured froth. We present simulations on an initially uniform hexagonal froth. A defect is introduced by forcing either a T1 or T2 process in the stable structure, after which the froth is allowed to evolve according to von Neumann's law. In the first case, topological inclusions are found in the first few layers early in the evolution. In the second case, no inclusions appear over the entire evolutionary period. The growing disorder (as measured by the second moment of the side distribution, ₂) is isotropic. For the special case of a T2-formed froth in a uniform network, the SSI structure is retained with ₂0 only for the zeroth, first, and second layers. The ratio between topological perimeter and radius of the shells is close to 6, the value for a hexagonal froth.     

石墨颗粒/CuAlMn形状记忆合金复合材料中的位错内耗峰

利用渗流技术制备出了以石墨颗粒为阻尼增强相、以Cu-11.9Al-2.5Mn(wt%)形状记忆合金为基体的复合材料，对该合金的内耗行为进行了研究. 在淬火态样品的内耗-温度曲线上观察到两个内耗峰，分别位于240 ℃和370 ℃附近. 对其中低温峰的变化规律和机理进行了研究. 实验发现，低温峰仅在复合材料中出现，峰位与频率无关，峰高随频率升高而上升；随升温速率增加，峰高增加，峰位移向高温；随石墨颗粒体积分数增加，峰高增加；经多次热循环后该内耗峰消失. 由以上特征和微观观察，可以证明该峰起因于外加交变应力与位错的相互作用.     

Singular potentials and double-force solutions for anisotropic laminates with coupled bending and stretching

Summary Exact solutions are obtained in the framework of the classical theory of laminates subjected to the action of normal moments, double forces, double moments or momentless double dipoles. Seven cases of such loads are considered and completed by considering the case of given transversal discontinuity of normal deflection. It is shown that, in contrast to the case of infinite straight dislocations in a pure in-plane problem, the energy of this eighth solution depends on the discontinuity orientation. Some numerical examples are presented. Besides the formal value, the obtained double-force and double-moment solutions, as well as dimensionless double dipoles, can be used to construct kernels of additional boundary integral equations (BIE). Due to the coupling phenomena in the BIE system for the region with a corner point, additional variable such as corner forces appear and require the mentioned equation. Received 22 June 1999; accepted for publication 6 March 2000     

一种新的熔化理论——位错运动的量子化模型

 本文提出了一种新的熔化理论，从位错运动的量子化模型出发导出了林德曼熔化定律，得到了与实验结果相符的熔点计算值。     

Anti-plane elastodynamic analysis of planes with multiple defects

The solution of a screw dislocation under time-harmonic condition is obtained in an infinite isotropic plane by means of the Fourier transform method. The stress components reveal the familiar Cauchy singularity at the location of dislocation. The solution is employed to derive integral equations for a plane weakened by cracks and cavities. Cavities are considered as closed curved cracks without singularity. Several examples are solved and the stress intensity factor of cracks and hoop stress on cavities are obtained.     

弱界面异质球形夹杂本征应变问题的解析解

本文研究了具有线弹簧弱界面的异质球形夹杂的本征应变问题，所采用的线弹簧界面模型既能界面的切线方向滑动，又能考虑界面的法线方向张开，根据叠加原理、原问题的弹性场可分成三部分；二部分由真实均匀本征应变所引起，另一部分由等效的非均匀本征应变所引起，后一部分则由虚拟的Ｓｏｍｉｇｌｉａｎａ位错场所产生。本文求得了等效非均匀本征应变和虚拟位错场的Ｂｕｒｇｅｒ矢量的解析表达式，进而确定的问题的弹性场。     

A survey of experimental heat transfer data for nucleate pool boiling of liquid metals and a new correlation

The experimental data for heat transfer during nucleate pool boiling of saturated liquid metals on plain surfaces are surveyed and a new correlation is presented. The correlation is h = Cq^0.7p_r^m, where C and m are, respectively, 13.7 and 0.22 p_r < 0.001 and 6.9 and 0.12 for p_r > 0.001 (h is in W/m² K and q in W/m²). This correlation has been verified with data for K, Na, Cs, Li, and Hg from 17 sources over the reduced pressure (p_r) range of 4.3 × 10⁻⁶ to 1.8 × 10⁻². The correlation of Subbotin et al. was found unsatisfactory, but a modified correlation was developed that also gives good agreement with most of the data.     

强流脉冲电子束作用下20钢的微观结构状态

 为了研究金属材料的超快变形行为,利用强流脉冲电子束（HCPEB）装置对20钢进行轰击,采用X射线衍射、光学显微镜及透射电镜等技术分析了受轰击样品的变形组织与结构。实验结果表明,强流脉冲电子束能够在材料表层诱发幅值为1 GPa量级的应力,快速的加热和冷却过程在近表层诱发了强烈的塑性变形,并在材料表层内形成了复杂的位错缠结结构和位错胞结构,同时还伴随位错圈等空位簇缺陷的形成,多次轰击导致局部区域形成纳米和非晶结构。HCPBE轰击诱发的幅值极大的应力和极高的应变速率而导致的整个原子平面的位移可能是非晶结构形成的关键原因。     

4H-SiC中基面位错发光特性研究

本文利用阴极荧光(CL)和选择性刻蚀的方法对4H-SiC同质外延材料中基面位错的发光特性进行了研究. 结果表明螺型基面位错(B_TSD)和混合型基面位错(B_MD)分别具有绿光和蓝绿光特性,其发光峰分别在530 nm附近和480 nm附近. 从测试结果中还发现B_MD 的发光位较B_TSD有所蓝移,分析认为B_TSD位错芯附近原子沿伯格斯 关键词： 4H-SiC 基面位错 发光特性 禁带宽度