Mechanical/thermal responses and microstructure evolution of ultrafine - (UF) and nano-grained materials with high density UF/nano-sized growth twins

超细材料和纳米材料由于其优越的性能而引起了材料科学与工程领域的广泛关注。然而，这些材料的微观结构通常是热力学不稳定的，有很强的转变为晶粒尺寸更粗的正常多晶的趋势，导致其改进或新性能的丧失。因此，UF/纳米材料在热活化和机械活化下的微观结构响应和进化在实践和科学上都是非常有趣的，并且在许多出版物中都有记载。近年来，许多新开发的超滤/纳米材料加工技术在这些材料中引入了大量的超滤/纳米生长孪晶，如Cu、Ni和奥氏体钢。这些孪晶通常非常小，宽度为亚微米和纳米级，它们会产生许多特殊的力学现象，如强强化效应。然而，这些孪晶具有低稳定性，即在退火和/或塑性变形后，它们可能后退、消失甚至转变为位错结构。到目前为止，还没有系统的研究报告详细说明这些现象及其相关机制。事实上，这些生长双胞胎的许多方面仍然是模棱两可的。目前的工作将解决这些问题，目的是了解上述现象的基本方面。本文将特别关注位错-孪晶转化机理。

Ultrafine-(UF) and nano-grained materials have attracted much interest of researchers in the field of materials science and engineering due to their superior properties over the traditional counterparts. However, the microstructures in these materials are generally thermodynamically unstable, with a strong tendency to transform into normal polycrystals with coarser grain size, leading to the loss of their improved or novel properties. Therefore, microstructural responses and evolution in UF/Nano-materials upon thermal and mechanical activations are of great interest for both practical and scientific reasons, as well documented in numerous publications. In recent years, many newly developed techniques for processing UF/Nano-materials introduce a large amount of UF/Nano growth-twins in these materials, such as Cu, Ni and austenite steels. These twins are usually very small with submicro- and nano-meter width, and they induce many special mechanical phenomena such as strong strengthening effect. However, these twins are found with low stability, i.e. they may recede, disappear and even convert to dislocation structures upon annealing and/or plastic deformation. Up to the present, no systematic investigations have been reported that detail these phenomena as well as related mechanisms. In fact, many aspects of these growth twins have still remained ambiguous. The present work will tackle these issues with the objective to understand the fundamental aspects of the above phenomena. Special attention will be paid to the dislocation - twin conversion mechanism.