Precipitation Processes in a β-Phase Cu-15 at%Sn Shape Memory Alloy

抄録

β-phase CuSn alloys, like many other β-phase alloys of the noble metals, exhibit various “extra” reflections in diffraction patterns. In the present work, the phases which give rise to extra reflections in a quenched Cu-15 at%Sn shape memory alloy were examined by means of electron microscopy, using a dark field imaging technique. The as-quenched alloy was found to consist of a DO₃ matrix containing scattered small particles (designated as the “s”-phase) and another phase which may be identified as a surface phase or a so-called “surface martensite.” The structure of the s-phase resembles that of an omega phase. In the same alloy annealed at 373 K after quenching, coarse precipitates (designated as the “L”-phase) formed. The L-phase forms as four variants in the β-phase each of which takes the shape of a hexagonal platelet, the basal plane of which is parallel to {111}_β. After considering the structure of the L-phase it is concluded that the unit cell has the lattice parameters a=b≈\sqrt6a_β, c≈\sqrt3a_β/2, and γ=120°, where a_β is the lattice dimension of the fundamental β-phase parent unit cell.