Modeling fatigue crack growth resistance of nanocrystalline alloys

The description of fatigue crack growth in metals has remained an empirical field. To address the physical processes contributing to crack advance a model for fatigue crack growth (FCG) has been developed utilizing a combined atomistic-continuum approach. In particular, the model addresses the important topic of the role of nanoscale coherent twin boundaries (CTB) on FCG. We make the central observation that FCG is governed by the dislocation glide resistance and the irreversibility of crack tip displacement, both influenced by the presence of CTBs. The energy barriers for dislocation slip under cyclical conditions are calculated as the glide dislocation approaches a twin boundary and reacts with the CTB. The atomistically calculated energy barriers provide input to a mechanics model for dislocations gliding in a forward and reverse manner. This approach allows the irreversibility of displacement at the crack tip, defined as the difference between forward and reverse flow, to be determined. The simulation results demonstrate that for both refinement of twin thickness and a decrease in crack tip to twin spacing FCG resistance improves, in agreement with recent experimental findings reported in the literature.