Influence of microstructural purity on the bending fatigue behavior of VAR-melted superelastic Nitinol

The bending fatigue resistance of commercially-available Standard versus High Purity Nitinol was evaluated at 3% mean strain and a range of strain amplitudes with the simple wire Z-specimen geometry. The Standard grade Nitinol demonstrated a 10(7)-cycle fatigue strain limit of 0.50% alternating strain, comparable to results reported elsewhere in the literature. Conversely, the High Purity grade VAR Nitinol demonstrated a 5-fold improvement in fatigue resistance with an impressive 10(7)-cycle fatigue strain limit of 2.5% alternating strain. The High Purity Nitinol has an oxygen+nitrogen content of 60wppm, maximum wrought-material inclusion length of 17µm, and inclusion volume fraction of 0.28%, all substantially less than industry standards. With all processing variables held constant except for inclusion content, it is clear that this marked fatigue superiority is due exclusively to the reduction in both size and area fraction of inclusions.