Strain-induced dimensionality crossover and associated pseudoelasticity in the premartensitic phase of Ni₂ MnGa

The in situ high-energy x-ray diffraction was used for revealing an atomic mechanism on the two-step pseudoelastic behavior found in the premartensitic phase of Ni₂ MnGa magnetic shape memory alloy. The applied stress first suppresses the three-dimensional modulated structure of the premartensitic phase to a two-dimensional modulated one, which is accompanied by a change in the modulation wave vector and accommodates a large lattice strain reaching ∼1%. With further increasing stress, the two-dimensional modulated premartensite transforms to the five-layered modulated martensite. The observation of the stress-induced dimensionality crossover of atomic modulation has broad impacts in understanding not only the mechanical properties of advanced shape memory alloys but also the physical properties of condensed matter with heterogeneous structures.