Martensitic transformation and effects of thermal cycling in Ni50Mn33Al17 magnetic shape memory alloy

The phase transformations of a Ni50Mn33Al17 alloy in the range from room temperature to 1000 K are investigated. Differential scanning calorimetry, electrical resistometry and thermomagnetic measurements are presented. The critical temperatures and entropy change for the direct and inverse martensitic transformation are determined. The effects of thermal cycling are studied by performing repetitive measurements on different temperature ranges after quenching. For cycling below 500 K martensitic transformation/retransformation do not show significant changes. However, if the cycling reaches higher temperatures, modifications appear in the values of electrical resistivity and magnetization at room temperature, and the volume fraction that transforms martensitically is reduced. These effects could be attributed to an excess of vacancies retained by quenching, which are only eliminated above 500 K, and to the growth of equilibrium phases with high-temperature magnetic properties.