Nanocrystalline fcc metals: bridging experiments with simulations
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Nanocrystalline fcc metals: bridging experiments with simulations H. Van Swygenhoven, P. M. Derlet; A. G. Frøseth, S. Van Petegem; Z. Budrovic; A. Hasnaoui Paul Scherrer Insitute, ASQ/NUM – Computer Modelling and Experiments, PSI-Villigen, Switzerland. ABSTRACT Atomistic simulations have provided unprecedented insight into the structural and mechanical properties of nanocrystalline materials, highlighting the role of the non-equilibrium grain boundary structure in both inter- and intra-grains deformation processes. One of the most important results is the capability of the nanosized grain boundary to act as a source and sink for dislocations. However the extrapolation of this knowledge to the experimental regime requires a clear understanding of the temporal and spatial scales of the modelling technique and a detailed structural characterisation of the simulated samples. In this contribution some of the synergies that can be developed between atomistic simulations and experiments for this research field are briefly discussed by means of some typical examples. INTRODUCTION The understanding of the mechanical properties of nanocrystalline (nc) materials (with grain sizes less than 100 nm) poses a fundamental challenge to materials science research [1, 2]. With decreasing grain sizes, it is generally believed that there exists a transition from dislocation mediated plasticity within the grains to a plasticity that is primarily accommodated by the GB structure. Many controversial and conflicting results have been reported and despite the extensive efforts over the past decade no complete and consistent picture exists concerning the relation between the geometrical grain boundary (GB) network, including details of GB structure, and overall nc mechanical behaviour. Tensile deformation studies of nc materials demonstrate a significant increase in strength and decrease in ductility when compared to their coarse grained counterparts, in a way that strongly depends on the synthesis method and the different obtained microstructures [1, 3, 4]. Other typical features characterizing the deformation mechanism of nc-metals is an increased strain rate sensitivity (up to 10 times higher than the value of the coarse grained material but still low compared to what is observed during superplastic deformation) [5], a relatively low activation volume measured by strain rate jump tests, and a fast decrease in the work hardening leading to limited uniform deformation and the onset of instabilities, resulting in shear bands at higher strain rates [3,4]. THE SYNERGY BETWEEN SIMULATION AND EXPERIMENT Atomistic simulation of nc systems provide a unique and complementary approach to the ongoing experimental investigation of the mechanical and structural properties of nc systems, demonstrating that nc materials cannot be characterized by grain size alone, and that many other structural parameters play an important role in their mechanical behaviour [6, 7]. Such modelling can therefore be very helpful in providing better insight int
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