Effect of Precipitation on Martensitic Transformation and Shape Memory Behaviour in Rapidly Solidified Ni

The effects of precipitation of Ni5Al3 in rapidly solidified Ni66Al34 were studied using DSC, X-ray diffraction and TEM. Precipitation in the martensite during heating at 10°C/min commenced at about 250°C and subsequently degraded the shape memory behaviour by inhibiting reversibility of transformation. On the other hand, precipitation in the β phase during aging at 550°C after rapid heating, had no influence on reversible martensitic transformation during subsequent cycling. Clearly, precipitation in the martensite interfered with reverse transformation, whereas precipitation in the parent phase had no such effect. Therefore, it is concluded that, to sustain reversibility of the martensitic transformation and consequential shape memory behaviour, precipitation of Ni5Al3 in the martensite must be avoided

Microestrutural evolution in a CuZnAl shape memory alloy: kinetics and morphological aspects

The microstructural evolution of the CuZnAl shape memory alloys was studied by indirect techniques relating to the atomic migration rate of grain boundaries. Addition elements were used in a Cu-15,5Zn-8,0Al alloy to provide a comparison with the same alloy without microelement additions. The alloys were melted in an induction furnace of 24 kVA. After casting, the bulk samples of the alloys were homogenized. Then they were solution treated and hot-rolled followed by water-quenching to initiate the recrystallization. Finally, annealing produced at different temperature ranges was made in different samples in order to establish a law for the grain growth. Following the heat treatments, all annealed samples were examined by statistical metallography and the grain sizes were measured. After measurements, the same empirical law of grain growth was found for the different alloys and the ln [D-Do] x 1/T diagrams were plotted in order to establish the kinetic behavior. Based on the estimated values of the activation energy, important conclusions were obtained concerning the addition elements