THE EFFECT OF TWINNING ON THE SHAPES OF CUBE-CUBE-RELATED GE PRECIPITATES IN AL

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