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

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 supersaturatedα′phase (face-centred cubic) phase decomposes not into the equilibriumαandεphases as reported to date, but into theαandζ(space groupP63,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 ofζlamellae are often parallel to the {}ζplane. 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)αand (0001)ζplanes, which match across the broad faces of theζlamellae, are nearly the same, while, in a perpendicular direction, mismatch between (111)αandspacings amounts to 0.5%. The facing (111)αandplanes 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 depletedαphase is often correlated with that of the adjoiningα′grain. This correlation indicated that the OR hold successively uponα′ → ζ → αtransformation. Possible orientations of theαphase have been enumerated, as supervariants to the adjoiningα′grains mediated by theζphase. [ABSTRACT FROM AUTHOR]