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

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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.