Phase transformations in a Cu-14.2Al-7.8Ni alloy

Abstract

In the as-quenched condition, the microstructure of the Cu-14.2 wt pct Al-7.8 wt pet Ni alloy was D0(3) phase containing extremely fine L-J precipitates with two variants. The L-J precipitate has an orthorhombic structure with lattice parameters a = 0.413 nm, b = 0.254 nm and c = 0.728 nm, which was a new phase found firstly by the present workers. The orientation relationship between the L-J phase and the matrix is (100)(L-J)//(011)(m), (010)(L-J)//(111) and (001)(L-J)//(211)(m). The rotation axis and rotation angle between two variants of the L-J phase are [021] and 90 deg. When the alloy was aged at 500 degreesC for short times and then quenched, gamma(2) particles started to precipitate within the D0(3) matrix at the aging temperature and the remaining D0(3) matrix would transform to gamma(1)' martensite during quenching. This is similar to that observed by other workers in a Cu-14Al-4Ni alloy. However, when the alloy was aged at 500 degreesC for longer times, the remaining D0(3) matrix would transform to a mixture of (B2 + alpha) phases. This feature has never been observed by other workers in the Cu-Al-Ni alloys.