The effect of twinning on the shapes of cube-cube-related Ge precipitates in Al

Abstract

This work establishes a correlation between the shape and internal twin structure of cube-cube-related Ge precipitates in Al. Using conventional and high-resolution electron microscopy it is shown that most triangular plates have only one twin variant, parallel to the habit plane. Hexagonal plates have two or more twins of the same variant, parallel to the habit plane. Most tetrahedral precipitates have two non-coplanar twin variants. Small octahedral precipitates are twin free, while larger octahedra can have up to two twin variants. The different shapes are due to preferential growth at grooves formed by re-entrant twin junctions, and an increased mobility of the interfaces between the matrix and a twinned precipitate. It is shown that precipitates nucleate as twin-free octahedra. The effect of single, double and secondary twinning on precipitate growth is analysed systematically and compared with experimental observations. Symmetry considerations show that all three precipitate forms are consistent with Curie's principle if the desymmetrizing effect of twinning is taken into consideration.