Precipitation of M23C6at first and second-order twin boundaries in austenitic stainless steels. (precipitation at twin boundaries)

Abstract

Abstract Transmission electron microscopy has been used to study the precipitation of M23C6, at first and second-order annealing twin boundaries. During precipitation at non-coherent first-order twin boundaries thin twin bands emanate from the boundary and act as nuclei for further precipitation. The precipitates nucleate within the twin bands so that they are in a ‘twinned’ orientation relationship with the surrounding matrix. This accounts for the characteristic plate shape of these precipitates. Second-order twin boundaries are of two types, special planar boundaries and general boundaries (Vaughan 1970). The planar boundaries contained small triangular prisms of a common first-order twin and their precipitation behaviour can be explained in terms of the observations described above. Precipitates which form at the general second-order twin boundaries always have a ‘twinned’ orientation relationship with both boundary grains. This enables the precipitates to form low energy interfaces with both grains a...