Ordering, Martensitic and Ferromagnetic Transformations in Ni–Al–Mn Heusler Shape Memory Alloys

Abstract

Order-disorder, martensitic and magnetic transformations in the Ni₂AlMn alloys have been investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. The critical temperatures corresponding to the above transformations have been determined. It is shown that, on aging at low temperature, the parent phase structure changes from that of B2 to L2₁ by an ordering reaction causing the Ms temperature to decrease. The Curie temperature which has the highest value of about T_c=92°C at the Ni₂AlMn composition decreases steeply on deviating from the stoichiometric composition. Two kinds of new martensite phases have also been detected in the as-aged Ni–21Al–25Mn and Ni–22Al–25Mn alloys.