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"" = 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 Pi 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 y 2 phase are observed in the vicinity of the dislocation loops after ageing at 200°C, and their size and density raise with increasing T m . A linear increase of the transformation temperatures is observed for T^ = 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 y 2 precipitates could facilitate the nucleation of martensite. The DSC data as well as optical observations indicate that yi' martensite is induced in samples annealed at T m up to 300°Cj whereas a different type of martensite suddenly appears in the samples aged above this temperature.

Ageing effects in a Cu-Al-Ni shape memory alloy

“…2) is similar to that described in the literature [1,5,6,8,10]. While it seems to be commonly accepted that an improvement in the short-range order may occur in stage 1, the increase of the transformation temperatures in stage 2 has not been unequivocally explained yet.…”

“…Ageing of Cu-AI-Ni alloys showing the thermoelastic martensitic transformation has been studied by many authors [1][2][3][4][5][6][7][8][9][10][11][12][13]. A shift of the transformation temperatures and changes in the transformation hysteresis due to annealing have been well documented.…”

“…In the early stages of ageing, changes in the degree and type of order of the parent Pi phase are thought to be a significant process [3,8,10]. Precipitation of stable phases (y 2 , a) has been documented to occur in the later stages, leading to the decomposition of the metastable parent phase and in turn to the lost of the shape memory [2][3][4][5][6].…”

TEM study of the austenitic phase of high Ms CuAlNi alloys

“…Many discussions have already been focused on the origin of the transformation temperature increase [2][3][4][5][6] and the present results clearly show that the main modifications are not induced by the 72 precipitation. On the other hand, the sharp rise of As and Ms, similarly observed in the two samples, could be ascribed to the recovery of the second neighbouring order in the austenite phase [6][7].…”

“…If the quenching treatment, that usually allows to obtain the martensitic phase, can hardly suppress the occurrence of the first ordering transition, the B2 order could be retained at low temperature [1], Nevertheless, by annealing in the austenite phase, it is possible to modify the order degree. Ageing phenomena observed in CuAINi austenite are commonly described in three more or less independent stages associated with reordering process and precipitation of stable phases [2]. In the first one, typically for an annealing below 200 °C, some authors have observed short range ordering, but the transformation temperatures are not significantly changed [3][4].…”