| 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 zinc to the system produces a new ternary system, CuZnAl, which is of commercial importance. These alloys have a useful transformation temperature that can be modified to lie between –100 and 100°C. While CuZnAl alloys exhibit shape memory capabilities, they are less common than the CuAlNi alloys. Advantages of CuZnAl SMA’s CuZnAl SMA’s have the advantage that they are made from relatively cheap metals using conventional metallurgical processes. These reasons make them amongst the cheapest of the commercial SMA’s available, especially when compared to NiTi SMA’s. Disadvantages of CuZnAl SMA’s The major drawback with these alloys is that the martensitic phase is stabilised by long term ageing at room temperatures. This causes an increase in transformation temperature over time and a decomposition of structure when exposed to temperatures in excess of 100°C. Compared to other SMA’s, CuZnAl SMA’s have only modest shape memory properties, with a maximum recoverable strain of approximately 5%. Without the addition of grain growth control additives, the grain size of these alloys can be quite large leading to brittleness. In most instances, these disadvantages outweigh the cost advantages. Composition and Transformation Temperature CuZnAl SMA’s usually contain 15-30% zinc and 3-7% aluminium, with the balance being copper. The addition of small quantities (usually less than 1%) of boron, cerium, cobalt, iron, titanium, vanadium and zirconium are commonly added to control grain size. Use of grain growth control additives keeps grain size down and overcomes brittleness issues. However, additions should be made carefully as they can upset the stability of the structure, thus affecting the shape memory chastacteristics. Production
CuZnAl alloys can be produced using conventional processes such as induction melting. Nitrogen or other inert gases must be used for shielding purposes over the melt and during pouring to prevent zinc evaporation. Powder metallurgy processes can also be used to produce fine grained structures without the need to grain size control additives. Alloys with lower aluminium contents can be cold worked with interpass annealing. Cold working becomes increasingly more difficult with increasing aluminium content. 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. Prolonged solution heat treatment should be avoided as the zinc tends to evaporate and the alloy excessive grain growth can occur. 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. If quenched too rapidly into the martensite phase, CuZnAl alloys are susceptible to martensite stabilisation. This effect inhibits and may totally negate the shape memory effect. In practice it increases the reverse transformation shift temperature. Slower quenching or step quenching may be necessary for alloys with Ms above room temperature. Thermal Stability Thermal stability of copper-based SMA’s is limited by decomposition kinetics and hence, prolonged exposure to temperatures above 150°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. Key Properties • Density 7.60-7.65 g/cm3 • Values for Young’s modulus and yield strength are higher for the high temperature phase of the alloy. • Recoverable strain is approximately 4-5%. |