| The Copper/aluminium binary alloy displays shape memory characteristics but has a transformation temperature that is generally considered too high for practical use. The addition of nickel to this system has resulted in another family of shape memory alloys (SMA’s), the CuAlNi alloys with transformation temperatures in the range 80 to 200°C. CuZnAl alloys also exhibit shape memory capabilities, but are less common than the CuAlNi alloys. Advantages of CuAlNi SMA’s CuAlNi SMA’s are popular due to their wide range of useful transformation temperatures and small hysteresis. They are also the only SMA’s that can be used at temperatures over 100°C. Compared to Ni-Ti SMA’s, the CuAlNi alloys are much cheaper to make as they use cheaper raw materials and do not require sophisticated processing as do the NiTi alloys. Composition and Transformation Temperature CuAlNi SMA’s usually contain 11-14.5% aluminium and 3-5% nickel, with the balance being copper. The aluminium content strongly influences the alloys’ transformation temperature.
Reducing the aluminium content below 12% can also improve the alloys’ mechanical properties. Adding manganese (approximately 2%) can reduce the transformation temperature, while the addition of small quantities (approximately 1%) of boron, cerium, cobalt, iron, titanium, vanadium and zirconium are also commonly added to control grain size. However, additions should be made carefully as they can upset the stability of the structure. The alloy Cu13Al14Ni is a commonly used commercial grade from this family of alloys. Production The most common method of producing these alloys is induction melting. Powder metallurgy processes can also be used to produce fine grained structures without the need to grain size control additives. Hot working is the only satisfactory fabrication method. CuAlNi alloys can be hot worked in air. Following hot working, they are subject to a suitable solution heat treatment involving controlled cooling (often water quenching), which will help to dictate properties such as transformation temperature. Postquench ageing is often required to establish the transformation temperature, as the as-quenched transformation temperature is usually unstable. This process is normally carried out above the Af. Thermal Stability Thermal stability of copper-based SMA’s is limited by decomposition kinetics and hence, prolonged exposure to temperatures above 200°C should be avoided. Similarly, ageing at lower temperatures can also have an effect on the transformation temperature, e.g. ageing in the martensitic state will tend to stabilise this phase. |