Martensitic Transformation and the Ageing Effect in Cu-Al-Ni Alloys

The effects of ordering and precipitation on martensitic transformation behaviour in Cu72−xNixAl28 (x=0∼4) alloys have been studied by measuring changes in electrical resistance, 63Cu nuclear magnetic resonance intensity, X-ray diffraction and hardness on isochronal or isothermal ageing of the quenched alloys, with particular reference to the role of Ni in these phenomena. The main results are summarized as follows:(1) In alloys with low concentrations of Ni (x≤2), the ordering of the β1 phase (DO3 structure) is almost completed during quenching. On ageing after the quench, the equilibrium γ2 phase precipitates coherently with the matrix, impeding the nucleation and growth of the martensite phase. As a result, the Ms temperature is lowered and the hysteresis loop becomes broad and smooth, while in the as-quenched state the loop is narrow and sharply sloping. Further precipitation of γ2, however, raises the Ms temperature owing to the decrease in the Al concentration of the matrix. (2) In alloys with higher concentrations of Ni (x≥3), the ordering of the DO3 structure is not completed during the quench but continues on the subsequent ageing. In addition, the change of structure of β1 from the DO3-type to the Heusler-type ordered structure, accompanied by a rise in Ms temperature, was observed prior to the precipitation of γ2. (3) The ordering of β1 and the precipitation of γ2, during quenching and on subsequent ageing, are retarded by the addition of Ni because the diffusion of Al and Cu is suppressed.