Cyclic microstructural transitions and fracture micromechanisms in a near equiatomic NiTi alloy

• In-situ XRD observation of stress-induced phase transformation mechanisms during cyclic mechanical loading.
• SEM observation of crack initiation and growth mechanisms.
• Decrease of the critical cell parameter during cyclic loading as a consequence of the increased fraction of microstrucural defects.
• Fracture surfaces show different morphologies in the inner and outer region of the specimen.

In this work stress-induced microstructural transitions and crack initiation and growth mechanisms in a near equiatomic NiTi shape memory alloy have been analyzed, by means of both X-ray diffraction (XRD) and scanning electron microscopy (SEM) investigations. In particular, miniaturized dog-bone shaped specimens and a special testing machine have been used, which allow in situ XRD and SEM investigations during mechanical loading. Direct and reverse stress-induced phase transition mechanisms, between the parent austenitic phase and the product martensitic one, have been captured by XRD while crack initiation and propagation mechanisms have been observed by means of SEM investigations. These analyses revealed that stress-induced transformations occurs near the crack tip, as a consequence of the highly localized stress, which significantly affect the crack propagation mechanisms with respect to common metallic alloys. In fact, blunting does not occurs during mechanical loading and, in addition, complete crack closure is observed during unloading, as a consequence of the reverse transformation from product to parent phase.