The Effect of Additions During Nitinol Melting to Improve Fatigue Performance

The source of many failures in nitinol can be linked to the inclusions present in the product. As such, the industry is always on the lookout for cleaner material with fewer or smaller inclusions. In this work, elements were added to nitinol during the melting process in an attempt to reduce the oxide inclusions present. Elemental additions studied in this work were calcium and yttrium. The effectiveness of these changes was measured by inclusion rating per ASTM F2063, oxygen testing at ingot, rotary beam fatigue testing, and scanning electron microscope analysis of fractures from rotary beam fatigue testing. Findings to be presented include process issues with excess yttrium and manufacturing challenges of using calcium. Improved fatigue has been observed, up to 10,000,000 cycles to failure with rotary beam fatigue testing and future work will continue to improve results. Improvement in fatigue life will allow designers to push the current limitations of nitinol materials currently available in today’s marketplace.