高熵合金CoCrFeMnNi纳米压痕中位移突变现象和空位活动的关联性验证

Experimental investigation of the correlation between pop-in and vacancy activities in a high-entropy CoCrFeMnNi alloy

摘要:

随着纳米压痕技术的进步，越来越多的数据表明纳米压痕中发生的位移突变现象不是完美晶格均质形核过程而是借助于空位的异质位错形核过程。但是这个结论主要基于通过纳米压痕力学统计模型提取的激活能和激活体积这两个参数确立的，而并未有实验直接验证位移突变或位错形核与空位之间的关联。根据先前对高熵合金的纳米压痕研究，本文通过巧妙设计的纳米压痕实验来验证这种关联性。实验发现，当施加的载荷保持在亚临界水平一段时间后，依然可以观察到位移突变；而且，位移突变的大小（位移突变结束和开始时刻的位移差）与纳米压痕的压头尺寸成正比。位移突变对时间和压头尺寸的依赖性可以通过空位的特性得到合理的解释：第一，空位在任何载荷水平下都会发生迁移，而且需要一定时间形成空位堆作为位错的形核中心；第二，空位数量与压头作用区域的体积成正比，而这个体积又与压头尺寸成正比，另一方面，空位数量又决定了位错形核的数量进而解释了位移突变尺寸与压头尺寸成正比的现象。这也就从实验上直接印证了空位在位移突变或位错形核的过程扮演着重要的角色。

Abstract:

With the advancement of nanoindentation technology, more and more data are supporting that the pop-in event is not a homogeneous but a heterogeneous dislocation nucleation process mediated by vacancy-like defects. However, the conclusion is based on the two model-extracted parameters, activation energy and activation volume. There hasn’t been any experiment directly correlating the pop in event with vacancy activities. This work, following our previous nanoindentation study on a FCC high entropy alloy CoCrFeMnNi, aims to verify this correlation with specially-designed nanoindentation experiments. It was found that when the indenter was hold at a subcritical load (in reference to the critical load that triggers instantaneous pop-in) for a period of time, delayed pop-in was observed. Furthermore, the displacement bust size increased with the tip radius up to a few hundred nanometers. These two observations suggested the vacancy was at play because the vacancy can migrate under any applied stress and its number within the indented volume depends on the tip radius.