Atomistic scale behaviors of crack propagation in nanocrystalline bcc iron

Nanocrystalline materials have an extensive application in engineering due to their excellent performance on strength and toughness. This work carries out molecular dynamics simulation to identify the atomistic scale behaviors of crack propagation in nanocrystalline bcc iron, where the effect of grain size less than 15 nm is examined. The crack, instead of the grain boundary, is found to play a major role in the engineering stress-strain behavior. The maximum stress of the cracked model is no longer consistent with the inverse Hall-Petch relationship. Based on three-dimensional investigation, the diverse propagation behaviors on different cross sections along crack front are revealed, and the mechanism of intergranular decohesion is clarified. Through intergranular decohesion, the fast brittle cleavage on one cross section can accelerate the ductile propagation on the other cross sections. With a decrease in grain size, the intergranular decohesion effect weakens, and the crack propagation becomes more ductile. The threshold for crack ductile growth also increases with grain size decrease, which is very different from the inverse Hall-Petch relationship. The nanocrystalline iron with a small grain size has lower tensile strength but higher crack ductile growth threshold. Besides, the convictive evidence of asymmetrical propagation in nanocrystalline bcc iron is found. The asymmetrical propagation behavior is determined by the different abilities of leading partial dislocation nucleation on the asymmetrically distributed atomic close-packed planes. Since the grain orientation is random, the atoms are usually distributed asymmetrically along the crack plane. The asymmetrical behavior dominates the crack propagation inside grain for nanocrystalline bcc iron.