Effect of grain size on strength and strain rate sensitivity in metals

The effect of the grain size on the mechanical properties of metallic materials has been a topic of significant interest for researchers and industry. For many decades, a relationship defining the mechanical strength proportional to the inverse of the square root of the grain size has been widely accepted despite some reports of deviations from this behavior. Nevertheless, the initial explanations for this relationship, based mainly on the activation of slip systems by dislocation pileups at grain boundaries, have provided essentially no predictive capability. Here, we show that a physically based model for grain boundary sliding predicts, in excellent agreement with experimental data, the flow stress for plastic deformation for a broad range of materials using the fundamental properties of each material over a wide range of grain sizes and testing conditions. This mechanism also successfully predicts the reported enhanced strain rate sensitivity in ultrafine and nanocrystalline materials at different temperatures.