Fatigue and fracture of shape memory alloys in the nanoscale: An in-situ TEM study

This study is aimed at demonstrating the fatigue and fracture mechanisms in nanoscale NiTi shape memory alloy thin films. During functional fatigue, permanent strains are attributed to residual martensite pinned by Ni4Ti3 precipitates and interfacial dislocations parallel to type II internal twins of the martensite. These residual martensite-austenite interfaces acted as heterogeneous nucleation sites for stress-induced martensite in subsequent cycles thereby explaining the reduction in transformation stress. Finally, structural failure was observed to occur along the dislocation-rich austenite-martensite interface, making it the weakest link in the microstructure thus establishing a direct link between mechanisms of functional fatigue that result in the accumulation of permenant strain and mechanisms of structural fatigue that result in fracture. This work provides a comprehensive understanding of deformation mechanisms in nanoscale shape memory alloys and points to fundamental mechanisms that are applicable to macro scales.