A discrete dislocation plasticity study of the micro-cantilever size effect
A discrete dislocation plasticity study of the micro-cantilever size effect
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DOI：
10.1016/j.actamat.2015.01.030
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发表时间：
2015-04-15
期刊：
影响因子：
9.4
通讯作者：
Wilkinson, A. J.
中科院分区：
文献类型：
--
作者：
Tarleton, E.;Balint, D. S.;Wilkinson, A. J.
微悬臂梁越来越多地被用来通过使用纳米压头控制弯曲来提取弹性和塑性材料属性。聚焦离子束研磨可以用来制造截面尺寸在微米量级的小型单晶悬臂梁，而电子背散射衍射(EBSD)可以从具有所需晶体取向的颗粒中研磨出悬臂梁。微悬臂梁弯曲实验表明，足够小的悬臂梁更强，这通常被认为与中性轴对位错结构演化的影响有关。用离散位错塑性平面模型模拟端部加载悬臂梁来解释实验中观察到的行为。该模型允许初始位错源密度和产生的滑移带间距与实验载荷位移曲线相关联。该模型与早期的纯弯曲离散位错塑性模型的预测结果有相似之处。然而，也有显著的差异，包括强烈的源密度依赖的尺寸效应，不能用几何必要的位错(GND)参数来解释，以及悬臂应力分布对软堆积位置的影响。平面模型被用来确定零分辨剪应力等值线，而不是中性轴，作为控制软堆积位置的，以及震源间距作为在观察到的尺寸效应中限制滑移带间距的；在震源受限区域强化要大得多。研究了样品尺寸和位错源密度的影响，并与钛和锆的小尺寸力学试验进行了比较。计算预测，如果尺寸由平均源间距归一化，并且包括表示与尺寸无关的流动应力的项，则应力与尺寸的关系的标度指数n近似为1，而通常用于拟合实验数据的简单幂定律形式显著低估了n。(C)2015年，由Elsevier Ltd.出版的Acta Materialia Inc.
Micro-cantilevers are increasingly used to extract elastic and plastic material properties through controlled bending using a nanoindenter. Focused Ion Beam milling can be used to produce small scale single crystal cantilevers with cross-sectional dimensions on the order of microns, and electron backscatter diffraction (EBSD) allows cantilevers to be milled from a grain with a desired crystal orientation. Micro-cantilever bending experiments suggest that sufficiently smaller cantilevers are stronger, which is generally believed to be related to the effect of the neutral axis on the evolution of the dislocation structure. A planar model of discrete dislocation plasticity was used to simulate end-loaded cantilevers to interpret the behaviour observed in experiments. The model allowed correlation of the initial dislocation source density and resulting slip band spacing to the experimental load displacement curve. There are similarities between the predictions of this model and those of earlier discrete dislocation plasticity models of pure bending. However, there are notable differences, including a strong source density dependence of the size effect that cannot be explained by geometrically necessary dislocation (GND) arguments, and the effect of the cantilever stress distribution on the locations of soft pile-ups. The planar model was used to identify zero resolved shear stress isolines, rather than the neutral axis, as controlling the soft pile-up location, and source spacing as limiting the slip band spacing in the observed size effect; strengthening was much greater in the source-limited regime. The effect of sample dimensions and dislocation source density were investigated and compared to small scale mechanical tests conducted on titanium and zirconium. The calculations predict a scaling exponent n approximate to 1 for the dependence of stress on size if size is normalised by the average source spacing and a term representing the size-independent flow stress is included, whereas the simple power-law form ordinarily used to fit experimental data significantly underestimates n. (c) 2015 Acta Materialia Inc. Published by Elsevier Ltd.