The influence of interaction geometry on the obstacle strength of voids and copper precipitates in iron

Interaction between a 1/2 < 1 1 1 > {1 1 0} edge dislocation and voids or coherent bcc Cu precipitates (diameter D = 2 or 4 nm) in Fe with their centre displaced by +/-Delta z from the dislocation glide plane is investigated by computer simulation for temperature T = 0 to 450 K. Voids provide the highest critical stress, tau(c), when Delta z = 0. The dislocation climbs up on release when Delta z >= 0, but down when Delta z < 0. Void-surface facets influence the sense of climb. There is no correspondence between tau(c) and the sense or magnitude of climb. 2 nm precipitates give highest tau(c) when Delta z < 0 and lowest when Delta z > 0, due to a combination of the modulus difference and size misfit between bcc Cu and Fe. 4 nm precipitates are partially transformed to fcc structure by the dislocation when T <= 300K and Delta z >= 0. Surprisingly, the transformed fraction of Cu at low T is highest for Delta z = D/2, due to the compressive field of the dislocation. The geometries that produce large transformed fractions result in the lowest tau(c), in contrast to expectation from previous studies.