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

超细(UF)和纳米晶材料由于具有传统材料无法比拟的优异性能，引起了材料科学和工程领域研究人员的极大兴趣。然而，这些材料中的微观结构通常在热力学上是不稳定的，有很大的倾向于转变为具有更粗晶粒度的正常多晶，导致其改进或新颖性能的丧失。因此，UF/Nano材料在热激活和机械激活时的微观结构响应和演化引起了人们极大的兴趣，这在实际和科学上都是有意义的，许多出版物都有记载。近年来，许多新发展的UF/纳米材料加工技术在铜、镍和奥氏体钢等材料中引入了大量的UF/纳米生长孪晶。这些孪晶通常非常小，宽度为亚微米和纳米级，并产生许多特殊的力学现象，如强烈的强化效应。然而，这些孪晶的稳定性很低，即它们在退火和/或塑性变形时可能后退、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以及相关的机制。事实上，这对双胞胎的许多方面仍然是模棱两可的。目前的工作将处理这些问题，目的是了解上述现象的基本方面。将特别关注位错-孪晶转化机制。

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.