TEM study of the austenitic phase of high Ms CuAlNi alloys

Abstract
CuAlNi shape memory alloys, with Ms close to 130 °C (in the as-quenched state) have been studied on single crystalline and polycrystalline samples. The specimens have been quenched after heat treatment at 850 °C. The structural study discussed in the present paper has been performed using electrical resistivity and TEM (Transmission Electron Microscopy) analysis. The electron diffraction patterns show that the austenite of the two as-quenched samples is a disordered and body-centred cubic structure. Then, after one annealing at 165 °C performed in the TEM, the polycrystalline austenite reorders in a facecentred cubic structure (L2₁-type), while the single crystalline austenite (annealed at 172 °C) stays in the disordered state. Nevertheless, during the heating of the single crystal sample above A_F, ordered precipitates appear in the parent phase along the former martensite- martensite interface traces. In addition, precipitates of the Cu₉Al₄ (γ₂) phase are evidenced in the two samples after annealings below 172 °C. These observations, the structural difference between the single- and the polycrystalline austenite and the low temperature precipitation of the γ₂ phase, are quite original and have to be discussed by taking into consideration previous investigations performed on similar alloys.