The influence of strain rate on the microstructure transition of 304 stainless steel

In the top-down approach to tailor the microstructures of materials via plastic deformation, the strain rate plays a significant role. This paper systematically investigates the deformation mechanisms of 304 stainless steel subjected to surface impacts over a wide range of strain rates (10-10(5) s(-1)). Based on comprehensive analysis of X-ray diffraction and electron microscopy observations, we found that the strain rate between 10 and 103 s(-1) only activated dislocation motions and alpha'-martensite transformations, resulting in nanocrystallines and ultra-fine grains. However, higher strain rates (10(4)-10(5) s(-1)) produced a high density of twin bundles with nanoscale thickness in the bulk material. The transition from dislocation-mediated mechanism to twinning-mediated mechanism was interpreted in terms of the magnitude of the applied stress, which was calculated from the explicit finite-element simulation with the use of the Johnson-Cook model. A critical twinning stress, determined from the infinite separation of Shockley partials, renders the transition point. Deformation twinning occurs when the applied stress exceeds this critical twinning stress. Larger stress leads to thinner and denser twin lamellae. Conversely, the stress below the transition point can only induce dislocation motions and alpha'-martensite transformations. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.