Microstructural influences on crack initiation and growth in an equiatomic NiTi PE alloy

In this work the stress-induced microstructural transitions and the crack initiation and growth mechanisms in a near equiatomic NiTi shape memory alloy have been analyzed, by XRD and 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, at fixed values of the applied deformation. Direct and reverse stress-induced phase transition mechanisms, between the parent austenitc phase and the product martensitic one, have been captured by X-Ray diffraction tests while the crack initiation and propagation have been observed by scanning electron microscopy. These analyses revealed that stress-induced transformations, from austenite to martensite, occurs near the crack tip, as a consequence of the highly localized stress, which significaly affects the crack propagation mechanisms with respect to common metals. 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.