Ageing Phenornena in a Cu-Al-Ni Alloy

Abstract
Effect of ageing on martensitic transformation has been investigated in a Cu-Al-Ni alloy. Single crystalline specimens were annealed for 30 min at temperatures T_an = 175 to 350°C. Changes in the martensitic transformation were followed by calorimetry (DSC) and optical microscopy. Detailed observations of structural changes due to annealing were made by TEM. The most remarkable structural feature in the as-quenched β₁ phase are prismatic dislocation loops which have been formed by coalescence of excess quenched-in vacancies. Most probably, some off-stoichiometric Al atoms segregated to the plane of the loops during quenching. The first coherent precipitates of the equilibrium γ₂ phase are observed in the vicinity of the dislocation loops after ageing at 200°C, and their size and density raise with increasing T_an. A linear increase of the transformation temperatures is observed for T_an = 200 - 300°C. This shift can be most consequently explained by depletion of the matrix of Al atoms and annealing out of excess quenched-in vacancies. Moreover, stresses and concentration changes in the vicinity of the γ₂ precipitates could facilitate the nucleation of martensite. The DSC data as well as optical observations indicate that γ₁' martensite is induced in samples annealed at T_an up to 300°C, whereas a different type of martensite suddenly appears in the samples aged above this temperature.