The Influence of loading directions on the failure of the bicrystal in a-ti with grain-boundary penetrating dislocations

Some fatigue fracture of a and near a titanium alloys is believed to originate from the slip transfer from soft to hard grains under cyclic loading. Based on experimental observations, a model bicrystal is constructed with: l)a soft-and a hard-oriented grain, connected by a prismatic-prismatic grain boundary; 2) dislocations gliding on the prismatic planes in the soft grain and penetrating into the hard. Molecular dynamics simulations are then carried out to investigate the interaction between the dislocations and the grain boundary, as well as the plastic deformation processes afterwards,in various loading directions. It is found that the loading direction has a pivotal influence on the resulted dislocations after the initial dislocations are absorbed by the grain boundary, since it determines the resolved shear stress on the slip plane of the transferred dislocations in the hard grain. As the direction changes, the hard grain exhibits various types of plastic deformation, including dislocation nucleation and cracking. Failures due to crack initiation in the hard grains and on the grain boundary are observed.