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

超细和纳米晶材料因其具有比传统材料优越的上级性能而引起了材料科学与工程领域研究人员的极大兴趣。然而，这些材料中的微结构通常是结晶不稳定的，具有转变成具有较粗晶粒尺寸的正常多晶体的强烈倾向，导致其改进的或新颖的性质的损失。因此，UF/纳米材料在热和机械活化后的微观结构响应和演变出于实践和科学原因而引起极大兴趣，如在许多出版物中有充分记载的。近年来，许多新开发的UF/纳米材料加工技术在这些材料中引入了大量的UF/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，它们可以诱导许多特殊的力学现象，如强烈的强化效应。然而，发现这些孪晶具有低稳定性，即它们可能在退火和/或塑性变形时后退、消失甚至转化为位错结构。到目前为止，还没有系统的调查报告，详细说明这些现象以及相关的机制。事实上，这些成长双胞胎的许多方面仍然是模糊的。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。将特别注意位错-孪晶转换机制。

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.