Initial stages of γ2 precipitation in an aged Cu-Al-Ni shape memory alloy

Effect of ageing on martensitic transformation in a low-temperature Cu-Al-Ni shape memory alloy was investigated by calorimetry and transmission electron microscopy. Shifts of the transformation temperatures were followed and the microstructural observations with special attention to the early stages of precipitation of the γ2 phase were done on samples annealed for 30 min between 175 and 325 °C. The most remarkable structural features in the as-quenched β1 phase are prismatic dislocation loops with Burgers vectors in 〈100〉 directions, which were formed by coalescence of excess vacancies. Most probably, some off-stoichiometric Al atoms segregated to the plane of the loops during quenching, thus predetermining the places where the first coherent precipitates of the equilibrium γ2 phase form upon ageing. Stresses and concentration changes in the close vicinity of the precipitates can facilitate the nucleation of martensite and, consequently, raise the Ms temperature. However, an unambiguous explanation of the observed increase of the As temperature has not been found. In the later stages of precipitation, rows of relatively large γ2 particles arranged along 〈100〉 directions may act as effective obstacles to the motion of the phase interfaces and impede the martensite transformation.