Atomic scale understanding the periodic modulation in ferroelastic alloy Ni-Mn-Ti

Ferroelastic alloys with modulated martensites have shown promising functionalities, such as giant elastocaloric and magnetoelastic effects, but understanding of the periodic modulation has prompted much controversy between atomic shuffling (or periodic distortion) and adaptive phase (or nano-twinning, assembly of blocks with fixed lattice parameters) concepts. The main difficulty lies in the capturing of atomic re-arrangement process during the fast-kinetics martensitic transformation. In this work, in-situ cooling TEM (transmission electron microscopy) and HR-TEM investigations were carried out on a Ni₅₀Mn₃₂Ti₁₈ Heusler alloy to visualize the atomic re-arrangement process from B2 austenite to 6O (6-layer modulated orthorhombic) and further to 4O (4-layer modulated orthorhombic) martensites. The results showed that the whole transformation involves modulation period change, lattice parameter adjustment, and gradual formation and dissociation of {110}<11¯0>_B2-type stacking faults (SFs). In particular, the 6O and 4O phases have distinct lattice parameters in the tetragonal blocks if following the adaptive phase concept. The successive B2 to 6O and 4O transformation can be explained by the changes in both distance and period of {110}<11¯0>_B2 shuffling. The experimental observations support that atomic shuffling may account for the formation of modulated martensites, at least, in the present Ni-Mn-Ti system. Consequently, this study, by providing atomic-scale evidence, may deepen the understanding of modulated phase transformation mechanism in an important class of functional alloys.