Two-step martensitic transformation in Ni-Mn-Ga alloys

The Ni-Mn-Ga L2(1) ordered alloys undergo a martensitic transformation (MT) which characteristics have been extensively analysed in relation to the electron to atom ratio (e/a). In that sense, alloys with e/a<7.60 are known to exhibit soft mode condensation into a premartensitic phase prior to the NIT itself, manifested as a weakly first order transformation, while alloys with e/a>7.70 do not undergo any premartensitic transformation but they show instead a first order intermartensitic transformation during cooling. Alloys with intermediate e/a do not show such defined behaviours, but they are highly sensitive to the external applied fields. In the present work, three single crystalline Ni-Mn-Ga alloys with e/a between 7.60 and 7.70 are studied. The obtained results indicate that, even though the transformation temperatures are quite similar and the martensitic structures are in all cases modulated crystal lattices of 5 or 7 layer period, the transformation behaviour is different for each alloy, as revealed by the DSC and, especially, by the internal friction (IF) and modulus (E) evolution during cooling and heating these alloys. Particularly, one of them show a two step MT which is mainly attributed to inhomogeneous storage of elastic energy during the transformation.