Modeling of critical grain size for shifting plasticity enhancement to decrease by refining grain size

The effect of grain size on plasticity was investigated based on the theories of dislocation pile-up and Frank-Read (FR) source activation in ultrafine grained materials. The possible reason of the experimental phenomena of decrease in plasticity as grain size refined in ultrafine grained materials was theoretically analyzed and the critical grain size for plasticity decrease was proposed. The results predicted that there was a critical grain size of about 4-5 μm with the present parameters of X80 microalloyed steel where the plasticity would shift from increase to decrease as grain refined, which well agreed with the experimental data described in the references in ultrafine grained materials. The mechanism of decreasing in plasticity as grain size refined was dominantly due to the decrease of probability of activation of FR source. The modeling for predicting the critical grain size has been built up which would be very helpful to optimize microstructure for obtaining excellent combination of strength and plasticity.