Precipitation processes in a Cu-0.9mass% be single crystal

Abstract

The precipitation processes in a Cu-0.9 mass%Be alloy single crystal containing only G.P. zones parallel to the matrix (001)α plane have been studied by high-resolution transmission electron microscopy. The precipitation sequence found is: G.P. zones → γ″ → γ′1 → γ1 + γ′ → γ. The γ″ phase has a two-layer structure of Be atoms separated by a matrix layer parallel to (001)α, and a body-centered tetragonal (bet) lattice. The γ′1 phase consisting of five to ten Be-layers is body-centered monoclinic. The γ1 phase continuously transforms from the G.P. zones via γ″ and γ′1. The heterogeneous formation of γ′ occurs on the γ1 phase. The γ1 or γ′ phase is bct with a = b = 0.254nm and c = 0.352 nm or a = 0.254 nm and b = c = 0.268 nm. The γ″, γ′1 or γ1 phase aligns with the matrix according to the Bain orientation relationship. The plate-shaped γ′ precipitates have the (11̄2̄)α habit plane and the orientation relationship, (11̄2̄)α // (11̄2̄) γ′; [110]α // [100] γ′-(OR I), or the (11̄3)α habit plane and the orientation relationship, (11̄3)α // (11̄3)γ′; [110]α // [100] γ′ (OR II). The structure of γ′ successively changes into that of the γ phase with a = 0.280 nm for OR I or 0.268 nm for OR II. The γ precipitates of OR I and OR II are elongated along approximately [11̄1]α and [33̄2̄] α, which are in good agreement with the invariant-line directions. © 2006 The Japan Institute of Metals.