Precipitation Processes in a Cu–0.9 mass% Be Single Crystal

Abstract

The precipitation processes in a Cu-0.9.9mass%Be alloy single crystal containing only G.P. zones parallel to the matrix (001)a plane have been studied by high-resolution transmission electron microscopy. The precipitation sequence found is: G.P. zones -» ->y'i -> Y\+ / → y. The phase has a two-layer structure of Be atoms separated by a matrix layer parallel to (001 )a, and a body-centered tetragonal (bct) lattice. The y'i phase consisting of five to ten Be-layers is body-centered monoclinic. The Yi phase continuously transforms from the G.P. zones via and y'i. The heterogeneous formation of y' occurs on the yi phase. The yi or y' phase is bct with a = b = 0.254nm and c = 0.352nm or a = 0.254nm and b = c = 0.9.268nm. The y, y'i or y phase aligns with the matrix according to the Bain orientation relationship. The plate-shaped y' precipitates have the (112)a habit plane and the orientation relationship, (112)a // (012) Y .; [110]a // [100]y. (OR I), or the (113)a habit plane and the orientation relationship, (113)a //(013)y; [110]a // [100]y. (OR 11). The structure of y' successively changes into that of the phase with a = 0.280 nm for OR I or 0.268 nm for OR II. The precipitates of OR I and OR II are elongated along approximately [111]a and [332]a, which are in good agreement with the invariant-line directions.