First stage of the structural evolution of austenite in Cu-Al-Ni shape memory alloys

Laboratoire de Mécanique et de Physique des Matériaux ENSMA, BP 40109, 86961 Futuroscope-Chasseneuil, France

Corresponding author: pelosin@lmpm.ensma.fr

Received: 27 November 2000
Revised: 27 March 2001
Accepted: 30 March 2001
Published online: 15 July 2001

Abstract

Two shape memory Cu-Al-Ni alloys, a polycrystal and a single crystal, exhibiting a martensitic transformation close to 130 °C (in the as-quenched state) have been studied. Specimens have been quenched after heat treatment at 850 °C. The structural evolutions of the high temperature phase (austenite) have been studied for thermal treatments performed below 200 °C. Investigations have been carried out using electrical resistivity measurements, TEM (Transmission Electron Microscopy) observations and X-ray diffraction analysis. The main structural modifications are observed in the polycrystalline alloy and concern first, the reordering process of the austenite structure (B2L2₁), and second, the precipitation of the (Cu₉Al₄) γ₂ phase. In the single crystal alloy, the evolutions are very slight and localized on the structural defects. Particular attention is paid to the role of the quenched-in vacancy elimination on the observed mechanisms. In addition, the incidence of the structural evolution on the transformation temperatures is also discussed.