Semiconductors and Devices

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

V. Pelosin^a1, M. Gerland^a1 and A. Rivière^a1

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

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 (B2 L2₁), and second, the precipitation of the (Cu₉Al₄) γ ₂ 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.

(Received November 27 2000)

(Revised March 27 2001)

(Accepted March 30 2001)

(Online publication July 15 2001)

PACS:

• 81.30.Hd – Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder;
• 81.30.Kf – Martensitic transformations;
• 61.72.Ff – Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, X-ray topography, etc.)

pelosin@lmpm.ensma.fr