Investigation of metastable γ′ precipitate using HRTEM in aged Cu–Be alloy

Plate-like precipitates in Cu–1.9Be alloy have been studied using high-resolution transmission electron microscopy (HRTEM). The metastable γ′ phase and its matrix orientation were determined in samples which were solutionized, after aging at 320 °C for 2, 6 and 72 h. The relationships were determined as follows: [0 1 1] Cu-matrix (α)//[0 0 1] precipitate (γ′) and (1 0 0)α//(1 0 0)γ′. The habit planes were determined to be the (3 1 1¯) and (2¯ 1 1¯) planes of α for the γ′ phase. The angle between [1 0 0]γ′ and [2 0 0]α was found to be 6° and 6.5° for aging times of 6 h and 72 h, respectively. As the aging time increased, the length and width of the Cu–Be precipitates increased from 10–20 nm to 50–100 nm and 2–5 nm to 10 nm, respectively, along the trace of the habit planes.

Research highlights
► γ′ is an intermediate metastable phase in the precipitation sequence of CuBe alloy.
► The γ′ phase has a b.c.t. lattice with a = 0.254 nm, b = c = 0.268 nm.
► The relationship was found as follows: [0 1 1]α//[0 01 ]γ′ and (1 0 0)α//(1 0 0)γ′.
► The precipitates grew on the trace of (3 1 1¯) and (2¯ 1 1¯) habit planes.
► Further aging times increase the misfit angle between the γ′ and the α.