Characterization of fatigue life of ultrafine grained NiTi superelastic wires under uniaxial loading.

S. JAUREGUIZAHAR; H. SOUL; M.D. CHAPETTI; A. YAWNY

Santa Fe

Congreso; Congreso Internacional de Metalurgia y Materiales 14º SAM-CONAMET; 2014

In this work the pull-pull cycling of NiTi superelastic wire, with composition 50,9 at. % Ni and ultrafine grain (40-50 nm), was studied. Strain controlled experiments were performed at different material states: fully in austenite, fully in stress induced martensite and partially in stress induced martensite in a stress range determined by the forward and reverse transformation plateus (i.e., without involving the B19´ martensite stress induced transformation). The results were compared with fatigue lives obtained in pseudoelastic cycling experiments in order to analyze the effect of martensitic transformation. In addition, the applicability to NiTi shape memory materials of an approach used in the literature to rationalize fatigue data of conventional materials was analyzed. Finally, the different stages involved in the fracture process were evaluated by observing the fracture surface in the SEM. Based on these observations, the relative importance of crack initiation and propagation stages has been established.