Thermo-Mechanical Behavior of Cu-Al-Be Shape Memory Alloys

Cu-based shape memory alloys (SMA) are being developed because they can exhibit higher transformation temperatures than the binary Ti-Ni conventional alloys, as well as lower hysteresis. However some Cu-based SMA could also exhibit very low transformation temperatures being potentially useful for developing cryogenic applications as sensors and actuators. This is the case of Cu-Al-Be SMA in which beryllium strongly decreases the transformation temperatures while maintaining a good thermal stability against overheating, as well as a low thermal hysteresis.

One of the main technical drawbacks of Cu-based SMA, and in particular of Cu-Al-Be, is the stabilization of the martensite during the required thermal treatments or even during further cycling.

The aim of the present work is to develop the experimental methodology to avoid the above commented problems and then we have initially studied the influence of several thermal treatments in order to find the optimal conditions for a stable and reproducible martensitic transformation in this Cu-Al-Be family, avoiding both the stabilization of the martensite and the precipitation of the stable phases. The study of the thermal transformation has been carried out by differential scanning calorimetry and by internal friction measurements.

Once the thermal transformation has been properly controlled, superelastic tests and thermal cycles under load have been carried out on different single crystals of Cu-Al-Be, with the aim of establishing the Clausius-Clapeyron line and characterising the one-way shape memory effect. In addition the superelastic behaviour has been studied as a function of the maximum strain and the loading rate on several alloys of different composition.

Present results are very promising, allowing consider Cu-Al-Be SMA as good candidates for cryogenic applications.