γ Precipitates in Cu-Zn-Al Alloys Studied by High Resolution Electron Microscopy

A HREM study of small y-phase precipitates in P CEL-211-A1 single crystals is presented. The images taken at atomic resolution show the perfect coherency of the precipitates when they are embedded in the P austenitic phase. In this case, the precipitates are located at the antiphase boundaries of the matrix. In turn, several microdomains separated by internal antiphase-like boundaries are present inside the precipitates. Nevertheless, the study of the boundaries at atomic level shows that the shift associated to the boundary (one atomic plane along <1 lo>) is not a translations vector of the lattice. When the precipitates are embedded in a martensitic (18R) matrix, the precipitates loose the coherency. A mechanism of martensitelprecipitate accommodation based on the formation of a small nanoplate at the interface between the main martensite plate and the precipitate, together with the formation of dislocations, has been observed.