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Degradation caused by self-multiplication of damage induced by an interplay between hydrogen and the martensite transformation in a Ni–Ti superelastic alloy

ABSTRACT The role of damage in the degradation of tensile properties related to a stress-induced martensite transformation in a hydrogen-charged Ni–Ti superelastic alloy has been investigated. By damage, we mean vacancy clusters and dislocation structures induced in advance by a dynamic interplay between hydrogen and the martensite transformation. To homogenise the hydrogen concentration, the specimen was aged at room temperature after charging with a small amount of hydrogen (approximately 30 mass ppm). In this case, no fracture occurs within 2000 cycles during a cyclic tensile deformation test in the stress plateau region generated by stress-induced martensite and reverse transformations. With this hydrogen concentration, cyclic interplay between hydrogen and the martensite transformation scarcely leads to the degradation of tensile properties. Nevertheless, following cyclic martensite transformations before aging that induces pre-damage, fracture occurs after around 1000 cycles under a cyclic tensile deformation test despite the hydrogen concentration being the same. The present results clearly indicate that pre-damage induced by hydrogen affects the subsequent transformations, thereby causing the self-multiplication of damage, which degrades the tensile properties.