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 pct Ni alloy was D0₃ 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//(0$\bar 1$1) _m , (010)_L-J//(1$\overline {11}$) _m 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 °C for short times and then quenched, γ ₂ particles started to precipitate within the D0₃ matrix at the aging temperature and the remaining D0₃ matrix would transform to γ 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 °C for longer times, the remaining D0₃ matrix would transform to a mixture of (B2+α) phases. This feature has never been observed by other workers in the Cu-Al-Ni alloys.