Dislocation instabilities and microstructure in anisotropic -Fe

Abstract

We compare theoretical predictions of the shapes of dislocation shear loops in highly anisotropic crystals with TEM of -Fe irradiated at high temperatures. The sharp-cornered microstructure we observe is explained via instability of certain dislocation orientations, emerging as the temperature, and hence anisotropy, increases above a critical value. The microstructure features shear dislocations with -type Burgers vectors, which casts doubt on the traditional view that plasticity is mediated exclusively by -type dislocations in bcc crystals, at least in Fe and ferritic steels. These observations provide experimental evidence for a new mechanism for high-temperature plastic deformation, which may contribute to the severe loss of strength observed above 500C.