THERMOMECHANICAL BEHAVIOR OF MONOCRYSTALLINE CU-AL-BE SHAPE-MEMORY ALLOYS AND DETERMINATION OF THE METASTABLE PHASE-DIAGRAM

Stress-strain behaviour of monocrystalline Cu-Al-Be alloy was investig ated by uniaxial tensile tests in a temperature range between -70 degr ees C and +160 degrees C. Four kinds of typical behaviours were examin ed in this study. Main interest was the determination of the sigma-T d iagram obtained from the sigma-epsilon curves, This metastable phase d iagram is comparable to the Cu-AI-Ni one. Isothermal mechanical cyclin g was made at 6% strain up to 1500 cycles. As a result, a deterioratio n of the superelastic behaviour was found. A thermal flash allows to p artially recover this behaviour. Rupture tests performed at a temperat ure above 120 degrees C have shown the critical stress above which eit her brittle fracture of alpha' martensite occurs or plastic deformatio n of beta phase takes place. Finally, comparative tests have been made between a single crystal, a polycrystal with small grains and a polyc rystal with columnar structure (bamboo-type).