Effect of atomic order on the martensitic and magnetic transformations in Ni–Mn–Gaferromagnetic shape memory alloys

Abstract

The influence of long-range L21 atomic order on the martensitic and magnetic transformations of Ni–Mn–Ga shape memoryalloys has been investigated. In order to correlate the structural and magnetic transformationtemperatures with the atomic order, calorimetric, magnetic and neutron diffractionmeasurements have been performed on polycrystalline and single-crystalline alloys subjected todifferent thermal treatments. It is found that both transformation temperatures increasewith increasing atomic order, showing exactly the same linear dependence on the degree ofL21 atomic order. A quantitative correlation between atomic order and transformationtemperatures has been established, from which the effect of atomic order on the relativestability between the structural phases has been quantified. On the other hand, the kineticsof the post-quench ordering process taking place in these alloys has been studied. It isshown that the activation energy of the ordering process agrees quite well with theactivation energy of the Mn self-diffusion process.