Deformation Mechanisms of Nanocrystalline Hexagonal Close-Packed Metals
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0924-Z03-19
Deformation Mechanisms of Nanocrystalline Hexagonal Close-Packed Metals GUANGPING ZHENG Mechanical Engineering, University of Hong Kong, Hong Kong, 10000, Hong Kong
ABSTRACT Using molecular dynamics simulation, a reverse martensitic transformation from hexagonal close-packed (hcp) to face-centered cubic (fcc) structure is observed in nanocrystalline (nc) cobalt and zirconium undergoing plastic deformation. In nc-cobalt hcp-to-fcc transformation is prevalent and deformation twinning is rarely observed. The transformation mechanism involves the motion of Shockley partial dislocation 1/3 in every other (0001)hcp /(111)fcc plane. From the simulation results, it is suggested that the interaction among partials should be considered to understand the deformation mechanisms of hcp nc metals. INTRODUCTION Nanocrystalline (nc) metals have been found to show some exceptional mechanical properties such as high yield strength and super-plasticity and are promising for engineering applications. Among the nc metals, face-centered cubic (fcc) metals have been extensively investigated. Although the hexagonal close-packed (hcp) nc metals are important structural materials and the nanocrystalline hcp metals such as Co, Ti, Mg and Zn have attracted considerable attentions [1-3], the deformation mechanisms in these nc metals have not been well understood. This study is dedicated to investigate the deformation mechanisms of nanocrystalline hcp metals. Experimental and simulation investigations have revealed some underlying deformation mechanisms in fcc nanocrystals. It has been shown by high resolution transmission electron microscopy (HRTEM) [4,5] and molecular dynamics (MD) simulations [6,7] that partial dislocations emitted from the grain boundaries (GBs) play an active role during the plastic deformation process. One of the remarkable features of plastic deformations in nc fcc metals is the deformation twinning [4,5], which is not commonly observed in coarse-grained fcc metals with high stacking-fault energy, e.g. Al and Pd. Since coarse-grained hcp metals are susceptible to deformation twinning, it is interesting to investigate the roles of deformation twinning in their nanocrystalline counterparts. MOLECULAR DYNAMICS SIMULATIONS Simulation is performed in nc-cobalt and nc-zirconium using molecular dynamics. The nc-cobalt and nc-zirconium samples have exactly the same nanocrystalline structures which are constructed using a kinetic model for grain growth [8]. The grain boundaries in these nc structures are high-angle ones. Detailed methods of construction of nanostructures and nc samples can be found in Ref. [8]. Before deformation, samples are relaxed at T=300K to
equilibrate the GB structures. For the interaction among cobalt atoms, we use a semi-empirical tight binding potential developed by Cleri and Rosato [9]. The interaction among zirconium atoms is described by a Finnis-Sinclair potential developed by Ackland [10]. The bulk singlecrystal cobalt and zirconium samples described by these potentials do not show a
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