Crack Propagation Mechanisms of an Aged Nickel-Titanium-Hafnium Shape Memory Alloy

Mechanisms of crack propagation in a NiTiHf sample were studied from post-mortem microstructure characterizations about cracks using transmission electron microcope (TEM). Results showed that cracks propagate mainly along the <011> type II twin planes, zig-zagging between different twin planes to avoid H-phase precipitates. The crack tip terminated outside of the martensite band, within surrounding austenite. Stress-induced amorphizition of the crack edge was also observed in both the austenite and martensite phases. A larger amorphous region ahead of crack tip compared with the amorphous region along the crack edge, suggests that amorphization occurs before the crack propagates. Ni enrichment by 25% around the crack tip confirms the amorphization mechanism is accompanied by a Ni doping effect, consistent with the motivating ideas of strain glass theory. This work demonstrates that engineering H-phase precipitate morphologies is one viable path toward engineering the fracture toughness of NiTiHf alloys.