Martensitic transition and structural modulations in Ni₅₁Fe₂₄Ga₂₅ ferromagnetic shape-memory alloy

The structure of near-stoichiometric Ni₅₁Fe₂₄Ga₂₅ Heusler alloy synthesized by a melt-spinning technique has been studied using transmission electron microscopy. The main phase possessed a local, well-defined L2₁ high-atomic order structure, and some fcc structural γ phase and lamellar twin structures were also present. At room temperature, a rich variety of micro-modulated domains in the parent phase was observed. The domain structures were aligned along the 〈110〉, 〈100〉, 〈211〉, and ~14° off 〈110〉 directions, resulting in a complex tweed contrast. These are possibly the precursors of martensitic transformation (MT). Below the MT temperature, Ms, the cubic parent phase transformed into modulated martensitic variants, which were composed of lamellar structures with predominantly 10 and 14 M modulated structures along the 〈110〉 directions.