In-Situ Fracture Observation and Fracture Toughness Analysis of Ni-Mn-Ga-Fe Ferromagnetic Shape Memory Alloys

Abstract

The fracture property improvement of Ni-Mn-Ga-Fe ferromagnetic shape memory alloys containing ductile gamma particles was explained by direct observation of microfracture processes using an in-situ loading stage installed inside a scanning electron microscope (SEM) chamber. The Ni-Mn-Ga-Fe alloys contained a considerable amount of gamma particles in beta grains after the homogenization treatment at 1073 K to 1373 K (800 A degrees C to 1100 A degrees C). With increasing homogenization temperature, gamma particles were coarsened and distributed homogeneously along beta grain boundaries as well as inside beta grains. According to the in-situ microfracture observation, gamma particles effectively acted as blocking sites of crack propagation and provided the stable crack growth, which could be confirmed by the R-curve analysis. The increase in fracture resistance with increasing crack length improved overall fracture properties of the Ni-Mn-Ga-Fe alloys. This improvement could be explained by mechanisms of blocking of crack propagation and crack blunting and bridging.