Precipitation and martensitic transformation in polycrystalline CuAlNi shape memory alloy – Effect of short heat treatment

In this work, the influence of different heat treatments on the martensitic transformation and precipitation in polycrystalline Cu13%Al4%Ni shape memory alloys have been investigated. A CuAlNi strip were subjected to different heat treatments in order to obtain a variety of microstructures. In this way, the samples were aged at a temperature between 200 and 450 °C. Some specimens were heat-treated at 900 °C followed by ageing processes. The transformation temperatures were measured using Differential Scanning Calorimetry (DSC) method. To determine the mechanical properties, the samples were subjected to the tensile and hardness tests. Microstructural studies were performed using optical and scanning electron microscopies. According to the experimental results, increasing the ageing temperature up to 370 °C results in strength and transformation stress to increase. As the temperature rises to 350 °C, some precipitates appear in the microstructure. Once the temperature reaches 450 °C, there is no martensite in the microstructure. Accordingly, the hardness increases and the elongation decreases drastically in such a way that the alloy breaks in the elastic region. The solution treatment at 900 °C for 20 min yields the martensite to reappear in the microstructure resulting the hardness and strength to decrease and the elongation to increase. Ageing of the samples after solution treatment, increases the mechanical properties again but no precipitation takes place.