Interfacial structure and transformation mechanism of the μ phase in Al-3.9 Cu-0.5 Mg-0.5 Ag alloy

Abstract

Conventional transmission electron microscope (TEM) contrast analyses were performed to identify interfacial defects on the {111} α/(001) μ interface of μ precipitates. These defects were found to be ledges, which exhibit a 6 <112> α and a 2 <110> α dislocation contrast. High-resolution transmission electron microscopy (HRTEM) was also employed to observe the faces and edges of one unit-cell thick μ precipitates at various aging temperatures. These observations showed the μ precipitates to be completely coherent with the matrix at early stages of growth. At the edges of relatively thick precipitates and for multiple unit-cell ledges, HRTEM indicated an extra (001) μ plane spaced approximately 2.6 nm apart and this is necessary to accomodate an approximately 9% misfit normal to the precipitate plate. A possible transformation mechanism from the α matrix to μ phase that provides an atomic correspondence between the two structures is proposed. Growth of the μ phase is explained in terms of four building blocks which relate the two structures crystallographically. At each growth step, the selection of a building block is dictated by the minimization of misfit strain energy. The transformation and growth models are in good qualitative agreement with the conventional and high-resolution TEM observations.