Ab initio description of segregation and cohesion of grain boundaries in W-25Â at.% Re alloys

We investigate grain boundaries (GBs) in W-25 at.% Re alloys with special focus on the segregation of Re to the GBs and the changes in their cohesive properties that arise therefrom. Our simulations rely on density functional theory and a mean-field approximation, the virtual crystal approximation, to model the highly alloyed system. Based on a gamma-surface approach, the geometry of a wide range of GBs, including tilt, twist and mixed GBs, is obtained. Segregation energies are found to vary strongly between different segregation sites, with the strongest segregation energy amounting to −0.75 eVs. We show that bond-order parameters are able to identify strong segregation sites based on purely geometric information only. With a thermodynamic model based on a Bragg-Williams approach, the concentration of Re atoms is calculated as a function of GB character at 1913 K and compared with atom probe experiments. The segregation levels are similar, with a trend towards higher segregation levels observed in the calculations for certain misorientation angles. Finally, Re alloying is found to lead to a general increase in the work of separation of the GBs, suggesting a reduction in the tendency for intergranular fracture.