Microfracture Mechanism and Fracture Properties of Ni-Mn-Ga-Fe Ferromagnetic Shape Memory Alloys

Abstract

The fracture toughness improvement of Ni-Mn-Ga-Fe ferromagnetic shape memory alloys containing ductile particles was explained by direct observation of microfracture processes using an in situ loading stage installed inside a scanning electron microscope (SEW chamber. The Ni-Mn-Ga-Fe alloys contained a considerable amount of ductile particles in the grains after the homogenization treatment at 800 similar to 1100 degrees C. gamma particles were coarsened and distributed homogeneously along P grain boundaries as well as inside beta grains as the homogenization temperature increased. The in situ microfracture observation results indicated that gamma particles effectively acted as blocking sites of crack propagation, and provided stable crack growth that could be confirmed by the R-curve analysis. This increase in fracture resistance with increasing crack length improved overall fracture properties of the alloys containing gamma particles.