A molecular dynamics study of grain boundary free energies, migration mechanisms and mobilities in a bcc Fe-20Cr alloy

Curvature driven migration of a series of 〈1 1 0〉 tilt grain boundaries in a bcc Fe-20Cr alloy is simulated using molecular dynamics to investigate the relationship between the atomic migration mechanism and mobility at medium to high temperatures. The boundaries studied include low angle boundaries (LAGB), high angle boundaries (HAGB) and singular boundaries, such as coherent twins. The steady-state boundary shape and curvature are compared with a simple analytical model which incorporates the dependence of absolute mobility and free energy on boundary inclination. The comparison indicates that the 109.5° (11¯2) Σ3 coherent twin boundary will have relatively low energy but high mobility. This result is attributed to a particularly effective repeated shuffle mechanism which occurs on the twinning plane. Two other migration mechanisms are observed, one involving the motion of 〈1 1 1〉 glissile dislocations in LAGB and the other involving uncorrelated atomic shuffles in HAGB, sometimes associated with interfacial steps.