Unexpected ordering behaviour of Pt3Al intermetallic precipitates

Alloys with a microstructure analogous to the γ/γ′ system of Ni-based superalloys are found in the Pt–Al system. A two-phase microstructure consisting of Pt3Al precipitates in an fcc (Pt) solid solution was developed to mimic the basic microstructure of Ni-based superalloys. The Pt3Al precipitates were found to consist of plates that are twin related and have a specific orientation relationship with respect to the matrix. Each plate contains a high density of thin platelets lying perpendicular to the c-direction. The precipitates were expected to crystallise in the D0′c unit cell. However, the experimental results showed that a different ordered structure, based on two modifications of the D0′c unit cell, occurred instead. The observed matrix/precipitate orientation relationship could be justified by misfit considerations.