A Molecular Dynamics Simulation of High Strain-rate Deformation in Nanocrystalline Silicon Carbide
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1021-HH04-02
A Molecular Dynamics Simulation of High Strain-rate Deformation in Nanocrystalline Silicon Carbide Yifei Mo, and Izabela Szlufarska Department of Materials Science and Engineering, University of Wisconsin, Madison, WI, 53706 ABSTRACT Multi-million atom molecular dynamics simulations of tensile testing have been performed on nc-SiC. Reduction of grain size promotes simultaneous enhancement of ductility, toughness, and strength. Simulations show that the nc-SiC fails by intergranular fracture preceded by atomic level necking. Atomic diffusion can prevent premature cavitation and failure, and therefore it sets an upper limit on high strain-rate deformations of ceramics. We report a non-diffusional mechanism for suppressing premature cavitation, which is based on unconstrained plastic flow at grain boundaries. In addition, based on the compositeís rule of mixture, we estimate Youngís modulus of random high-angle grain boundaries in nc-SiC to be about 130 GPa. The effect of temperature and strain rate on mechanical properties is studied. INTRODUCTION One of the main limitations in the applications of ceramics is their low fracture toughness[1,2]. In general, both toughness and strength of ceramics are determined by the largest preexisting flaw. Preexisting cracks open up under tensile stress, leading to failure by fast brittle fracture [3-5]. However, even ceramics without preexisting flaws are inherently brittle, and one of the mechanisms of crack nucleation in these materials is related to cavitation at grain boundaries (GBs). Cavities develop when stress becomes localized and reaches a critical value, for example at the GB triple junctions or along GBs perpendicular to the direction of the applied stress [6]. Traditionally, in so-called superplastic ceramics cavitation is suppressed by atomic diffusion, which relaxes the accumulated local stress [7-9]. Significant enhancements of ductility and toughness can be accomplished only when the time-scales associated with the deformation strain rate are large in comparison to the diffusion time-scales. For this reason there is an upper limit on deformation strain rates at which increased ductility of ceramics can be obtained. Recently, a synthesis of high strain-rate (up to 1 s-1) superplastic ceramic composites has been reported by Kim et al., [2] where an additional stress relaxation was accomplished by dislocation-induced plasticity in the grains (of average diameter of 210≈). The ability to overcome brittleness of ceramics at even higher strain rates is of large technological interest for shape forming of ceramics and it could have a large impact on the manufacturing processes. Computer simulations are perfectly suited to explore the limits of high strain rate deformation and to correlate mechanical response of a material with its atomistic structure [10]. We have performed molecular dynamics (MD) simulations of tensile deformation of nanocrystalline (nc) silicon carbide (SiC)
samples with varying grain size and at a strain rate of 108 s-1. We discover a n
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