Abstract
Decomposition processes of the hyper-eutectic Cu–Be β phase containing 6.65wt%Be upon quenching and successive aging treatments were investigated by means of transmission electron microscopy and diffraction. Upon quenching, the β phase could not be retained at room temperature, and decomposed into the γ phase (a=2.71Å) with the B2 type of structure and a metastable phase (denoted with αt) with a fct lattice of c⁄a=0.87 (a=3.75Å). The latter αt phase transformed to the equilibrium fcc α phase (a=3.57Å) upon successive agings at temperatures higher than 300°C. Morphological examinations suggested that the structural changes occurred in three steps: a spinodal-like decomposition to β+γ, a quasi-eutectic reaction of the decomposed β to γ+αt, and finally a change of the αt to α. The orientation relationships between the γ and αt or α phases were found to change with the aging process. That is, the angle between the (001)αtorα and (001)γ planes was zero in the as-quenched and 150°C-aged states, 4° in the 300°C-aged state and 7° in the 500°C-aged state, although the parallelism between the [1\bar10]αtorα and [100]γ directions was retained throughout the agings. Besides the lattice rotation, the habit plane of the αt or α crystals also rotated from the initial (011)γ to (012)γ planes around the [1\bar10]αtorα or [100]γ axis. These rotations of the lattice and habit planes of the products were well consistent with an energetical consideration of the interface between the γ and αt or α phases.
