Effect of microstructure on the nucleation and initiation of adiabatic shear bands (ASBs) during impact
Effect of microstructure on the nucleation and initiation of adiabatic shear bands (ASBs) during impact
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DOI：
10.1016/j.msea.2014.07.095
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发表时间：
2014-10-06
期刊：
影响因子：
6.4
通讯作者：
Bassim, Nabil
中科院分区：
文献类型：
--
作者：
Yiadom, Solomon Boakye;Khan, Abdul Khaliq;Bassim, Nabil
虽然在塑性变形过程中可能发生均匀失稳，但绝热剪切带(ASB)的形成并不遵循冲击过程中的均匀失稳。几何应力集中部位和/或显微组织的不均匀导致了剪切应变局部化的形核和启动。在这项研究中，发现初始的微观结构不均匀性产生了ASBS的成核位置。观察到在析出碳化物周围形成了双错配界面和原子柱随机排列的边界层，它们增加了样品中位错源的体积分数。经最低温度回火的AISI4340钢试样，析出碳化物较小，长径比大，在基体中密集分布，极易形成ASB。随着回火温度的升高，碳化物的相对尺寸增大，其长径比和在基体中的分布密度相应减小，从而更能抵抗ASB的形成。研究表明，在冲击过程中，被激活的位错源与试件内的最大剪切方向(应力集中区)相交是相交点作为ASB可能形核场所的必要条件，这取决于位错产生的速率、局部应变和应变率。在碳化物体积分数不变的情况下，回火试件具有较高的ASB萌生敏感性，这与冲击时激活位错源与最大剪切方向交点的体积分数有关。此外，较小的碳化物具有较高的长径比和分布密度，加剧了应变梯度的影响和与回火样品相关的组织不均匀。(C)2014爱思唯尔B.V.保留所有权利。
While instability may occur homogenously during plastic deformation, the formation of adiabatic shear band (ASBs) does not follow a homogenous instability during impact. Geometrical stress concentration sites and/or microstructural inhomogeneities result in the nucleation and initiation of shear strain localization. In this study, initial microstructural inhomogeneity was found to produce nucleation sites for the initiation of ASBs. It was observed that double misfit interfaces and boundary layers with random arrangement of atomic columns are formed around precipitated carbides and they increase the volume fraction of dislocation sources within the specimens. The AISI 4340 steel specimens which were tempered at the lowest temperature had smaller precipitated carbides with high aspect ratios densely distributed within the matrix and were easily susceptible to the formation of ASBs. As the tempering temperature increased, the relative sizes of the carbides increased with a corresponding reduction in their aspect ratios and their distribution density within the matrix and thus were more resistant to the formation of ASBs. In this study, it is demonstrated that the intersection of an activated dislocation source with the direction of maximum shear (regions of stress concentrations) within the specimens during impact, is a necessary condition for the point of intersection to act as a possible site for the nucleation of ASBs, depending on the rate of dislocation generation, local strain and strain rate. At a constant carbide volume fraction, the higher susceptibility of the tempered specimens to the initiation of ASBs is attributed to the volume fraction of the points of intersection between activated dislocation sources and direction of maximum shear during impact. Additionally, the smaller carbides, with their higher aspect ratios and distribution densities, accentuate the effect of strain gradients and the microstructural inhomogeneities associated with the tempered specimens. (C) 2014 Elsevier B.V. All rights reserved.