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

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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 and lowers the activation energy required to form a critical nucleus. A planar precipitate interface with the orientation (001)M23C6||(221)γ, [110]M23C6||[110]γ can form when the precipitate is in a ‘twinned’ orientation relationship with the matrix. The formation of this interface can be explained in terms of the good fit which exists between precipitate and matrix atoms on these planes. It is suggested that, in general, precipitation at first and second-order twin boundaries will be strongly dependent on the precipitating phase.