Discontinuous precipitation with metastable ζ phase in a Cu–8.6% Sn alloy

Abstract

Discontinuous precipitation (DP) in Cu–8.6 at% Sn alloy at 573 K has been studied using transmission electron microscopy and electron back scattered diffraction techniques. DP reactions occur only sporadically at grain boundaries after prolonged aging, giving rise to a cellular morphology. The supersaturated α′ phase (face-centred cubic) phase decomposes not into the equilibrium α and ε phases as reported to date, but into the α and ζ (space group P63, a = 0.733, c = 0.786 nm) phases, the latter being stable above 835 K. These two phases possess the following orientation relationship (OR): . Well-defined habit planes do not exist. However, broad faces of ζ lamellae are often parallel to the {}ζ plane. The observed OR and cellular morphology have been interpreted by matching atomic rows across the phase boundaries in two perpendicular directions. Namely, the spacings of (110)α and (0001)ζ planes, which match across the broad faces of the ζ lamellae, are nearly the same, while, in a perpendicular direction, mismatch between (111)α and spacings amounts to 0.5%. The facing (111)α and planes form a coincidence site lattice, and atomic configurations in these planes both contain similar triangular atomic networks. It was also found that the orientation of the depleted α phase is often correlated with that of the adjoining α′ grain. This correlation indicated that the OR hold successively upon α′ → ζ → α transformation. Possible orientations of the α phase have been enumerated, as supervariants to the adjoining α′ grains mediated by the ζ phase.