Plasticity in Nanocrystalline and Amorphous Metals: Similarities at the Atomic Scale
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Plasticity in Nanocrystalline and Amorphous Metals: Similarities at the Atomic Scale Alan C. Lund and Christopher A. Schuh Department of Materials Science and Engineering, Massachusetts Institute of Technology 77 Massachusetts Avenue, Cambridge, Massachusetts, USA 02139 ABSTRACT For metallic alloys, the amorphous state is often regarded as the limiting structure as grain size is reduced towards zero. One interesting consequence of this limit is that the properties of the finest nanocrystalline metals must begin to resemble those of metallic glasses. In this work we focus upon the nature of the plastic yield mechanisms in these material classes, and seek to identify commonalities and disparities in the nature of plastic yield in glasses and nanocrystals. The discussion is presented with reference to static atomistic simulations of (i) an amorphous binary alloy, and (ii) a nanocrystalline Ni specimen with grain size of 3 nm. We show that both these materials deform by the operation of fine atomic shearing events, and both exhibit asymmetric yielding as a consequence.
INTRODUCTION Much scientific and practical interest is currently focused on both nanocrystalline and amorphous metals, which exhibit uniquely favorable properties as a result of their non-equilibrium microstructures. In nanostructured materials, the experimentally [1-3] and computationally [4, 5] observed transition in deformation mechanisms between micro- and nano-scales has garnered much interest. The commonalities between amorphous and nanostructured metals have received relatively less attention, despite the amorphous state having long been recognized as a limiting length scale for nanocrystal grain sizes [6]. The relationship between these states, after the view of Nieh and Wadsworth [6], is depicted schematically in Fig. 1. One interesting analogy between nanocrystalline and amorphous metals is in the atomic-scale mechanisms of plastic deformation. In the very finest nanoscale range (grain size, d < 10 nm), plastic yield occurs primarily by shear shuffling of atoms located at intercrystalline boundaries [4, 7], ultimately leading to cooperative, large-scale sliding of grain boundaries [8]. This type of behavior is reminiscent of that seen in metallic glasses, where local ‘shear transformation zones’ (STZs), comprised of a small number of neighboring atoms, undergo shear distortion and selfassemble into large planar ‘shear bands’. The operation of STZs in both nanocrystalline and amorphous metals presents a clear contrast to conventional engineering metals, and classical behavior such as work hardening, symmetric von Mises yielding, etc., cannot be taken for granted. Furthermore, the operation of STZs in both types of materials suggests that they should have many features in common. In this contribution, we discuss static simulations that consider the nature of the plastic yield criterion in both an amorphous binary alloy, and a pure nanocrystalline Ni specimen, and discuss the commonalities and differences between plasticity in these mater
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