Effect of grain size and misorientation angle on fatigue crack growth of nanocrystalline materials

It is known that the unique microstructure and deformation mechanism of nanocrystalline (nc) materials make the crack initiation and propagation different from the conventional coarse grained materials. The research of fatigue crack propagation (FCP) in nc metals has remained an empirical field. A theoretical model for I type crack growth was established to address the physical processes. The model describes the crack initiation and propagation and discusses the important topic of the role of the grain size and misorientation angle on the fatigue crack growth on the interface of the nc materials. We make the major research that fatigue crack growth is governed by the irreversibility of displacement at the crack tip and the dislocation glide resistance. The characteristics are observed that the dislocation glide suffer larger resistance from ultrafine grain size and large misorientation angle. It can be found that the critical stress of dislocation penetrating grain boundary and the misorientation angle of the grain boundary are positively correlated. The resent experiment found that grain refinement serves to reduce the extent of crack path tortuosity and cause an increase in fcp. The model results demonstrate the crack growth rate increases with the decrease of the grain size and the misorientation angle increases with the crack growth rate decreasing, in agreement with experimental discovery.