On the formation of faceted Al₃Zr (β') precipitates in Al-Li-Cu-Mg-Zr alloys

Abstract

Trace additions of Zr to Al alloys inhibit recrystallization through the formation of spherical and coherent Al₃Zr (β') precipitates. Recently, observations have been made of faceted β' precipitates in several hot deformed Al alloys, although no systematic experimental study of either the causes of the formation of such precipitates or their orientation relationships with the Al matrix has so far been reported. A detailed examination of the orientation relationships shows that the cube-on-cube orientation relationship existing between spherical, coherent β' precipitates and the Al matrix does not hold good for the faceted β' particles and that the faceted β' particles are twin-related with the matrix. It is shown that the twin-related β' particles are not incoherent, but bound by large facets fully coherent with the matrix, and that such particles are associated with fairly significant coherency strains. A probable shape of the faceted β' is also described.