Confined martensitic phase transformation kinetics and lattice dynamics in Ni-Co-Fe-Ga shape memory alloys

Abstract

Insights into the phase transformation kinetics and lattice dynamics associated with the newly discovered confined martensitic transformation are of great significance to the in-depth understanding of the phase transformation behavior responsible for the rich new physical phenomena in shape memory alloys and could shed light on the design of novel multifunctional properties through tuning the confined martensitic transformation. Here, we employ in-situ elastic and inelastic neutron scattering techniques to reveal the characteristics of transformation kinetics and lattice dynamics in two Ni55-xCoxFe18Ga27 magnetic shape memory alloys with x = 5.5 and 6.5. With a remarkable difference in transformation temperature, these two alloys show similar vibrational properties. Phonon softening of the TA(2)[xi xi 0] phonon branch, manifested as a dip at xi similar to 033 on the phonon dispersion curves, was disclosed by inelastic neutron scattering experiments; but this phonon softening is much less pronounced than that observed in stoichiometric Ni2MnGa or near-stoichiometric Ni-Fe-Ga Heusler alloys. The dispersion curves for the TA(2)[xi xi 0] phonon branch are only weakly dependent on temperature. The less pronounced phonon softening and very weak temperature dependence of the phonon dispersion curves could be attributed to the martensitic transformation which is confined by local inhomogeneities. This confined martensitic transformation shows a sluggish nature and occurs in a broad temperature range. The sluggish transformation was evidenced by the existence of residual austenite at low temperatures far below the martensitic transformation start temperature. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.