Precipitation behavior of (Mo) phase on dislocations in Mo ₅SiB₅ phase

The precipitation behavior of a Mo solid solution in a Mo ₅SiB₂(T₂) matrix has been characterized by transmission electron microscopy. The T₂ phase has a distinct range of homogeneity and a temperature dependent solubility range. In order to provide for the homogeneity range of the T₂ phase, a significant concentration of constitutional vacancies has been proposed as a defect mechanism. Upon annealing, the excess vacancies aggregate and collapse into dislocations, which then act as the heterogeneous nucleation sites for the subsequent Mo precipitation. The formation of the preferential variants of precipitates is discussed in terms of the interaction between the transformation strain upon precipitation and the stress fields around dislocations, as well as the crystallographic mismatch between the Mo precipitates and the T₂ matrix.