Microstructural specificities and martensitic transformations in hot-rolled NiFeGa(B) alloys

In the present study the effects of hot rolling and boron microalloying on the features of microstructure and martensitic transformation in ferromagnetic NiFeGa(B) shape memory alloys were studied. The possibility of hot rolling of Ni54Fe19Ga27 and (Ni54Fe19Ga27)99.7B0.3 alloys at temperatures above 873 K with plastic strain of up to 50 % without destruction and cracking has been demonstrated. The hot rolling of NiFeGa(B) alloys leads to the precipitation of dispersed particles along grain boundaries and in the grain volume. Microalloying with boron prevents grain boundary migration during hot rolling and results in the reduction of average grain / subgrain size from 50 to 20 μm in (Ni54Fe19Ga27)99.7B0.3 alloy. Hot-rolled (Ni54Fe19Ga27)99.7B0.3 alloys had a specific microstructure: the γ-phase contained ordered γ'-phase particles 3 –10 nm in size. In the Ni54Fe19Ga27 alloy, a similar microstructure of γ / γ'-phase appeared only after hot rolling followed by annealing at 1273 K for 0.5 h. The specific microstructure of the γ / γ'-phase particles and a sharp texture of austenite (100)<001> parallel to rolling direction found in hot-rolled and post-deformation annealed NiFeGa(B) alloys with average grain / subgrain size of 60 μm are expected to increase the strength properties of austenite and martensite and promote cyclically stable functional properties (superelasticity and the elastocaloric effect).