Twinning effects on strength and plasticity of metallic materials
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Introduction Twinning in a crystalline material results in the formation of a domain crystal inside a parent crystal, where the two share some of the same crystal lattice points in a symmetrical manner and are separated by twin boundaries (TBs).1 While twinning may or may not contribute to plastic deformation depending on the specific twinning mechanism, it does result in substantial evolution of microstructures and in the formation of TBs.1,2 Twins can be classified into growth twins (formed during crystal growth), annealing twins (induced by heat treatment, recrystallization), and deformation twins (generated by mechanical loading), and deformation twinning contributes to plastic shear deformation. TBs that separate a twin domain from the parent crystal can effectively strengthen materials by impeding dislocation motion due to the slip discontinuity caused by the mirror symmetry, and increase the ductility and work-hardening capability of twinned metallic materials. For metals that plastically deform via both slip and twinning, three types of twins can be generated during material synthesis, heat treatment, and mechanical deformation. However, for other crystalline materials, such as ceramics and semiconductors, deformation twinning rarely occurs. Twins in these materials are mainly formed during the growth process, such as crystal growth from solution or recrystallization, leading to growth twins or annealing twins.3
For example, fine twin domains with an average thickness of ∼3.8 nm have been observed in cubic boron nitride (cBN), and the resulting nanotwinned (NT) cBN bulk samples have an extremely high Vickers hardness; in the case of NT cBN, the hardness exceeds 100 GPa—the optimal hardness of synthetic diamond.4 Because of the mirror symmetry associated with twinning, the formation of twin boundaries has been used to tailor the morphologies of crystalline nanowires and nanorods, such as zigzag SiC nanorods and nanowires,5 Y-shape branched nanorods,6 and kinked Ge-Si semiconductor nanowires.7 This issue of MRS Bulletin includes six articles that highlight current active research on twins and twinning in metals, including the synthesis and mechanical behavior of NT metallic materials, plasticity induced by mechanical twinning in hexagonal-close-packed (hcp) metals, and twinninginduced plasticity in steels. The articles identify outstanding scientific issues and provide suggestions for future research directions.
Formation of twins in nanotwinned metals Growth twins can be introduced into crystalline materials via several techniques. The article by Bufford et al. in this issue highlights several examples in which nanotwins are introduced into monolithic metals via pulsed electrodeposition (PED) or magnetron sputtering techniques, the latter being a
Jian Wang, Mechanical and Materials Engineering, University of Nebraska–Lincoln, USA; [email protected] Xinghang Zhang, Departments of Mechanical Engineering and Materials Science and Engineering, Texas A&M University, USA; [email protected] DOI: 10.1557/mrs.2016.67
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