Successful hardening of 60NiTi under open atmosphere conditions

What is it about?

Hardened 60NiTi shows outstanding chemical and mechanical properties. To harden, 60NiTi parts are, in the first stage, solutionized above 1050 C where 60NiTi exists as a single B2 austenitic phase and in the second stage quenched under a fast cooling rate. 60NiTi shows high sensitivity to the heat treatment environment. Conducting solutionizing heat treatment in air results in depletion of Ti causing precipitation of soft Ni3Ti2 and Ni3Ti phases in the matrix of outer surface regions. These phases are not desirable and will deteriorate the mechanical and chemical properties of the hardened 60NiTi. Vacuum furnaces or furnaces backfilled with inert argon gas are commonly employed to conduct hardening treatment on 60NiTi alloy. However, vacuum furnaces with the possibility of drop-quenching the solutionized parts are not easily available, and backfilling the furnaces with inert argon gas is costly. This research investigates the effect of 1 at.% Hf on the microstructure and resulting hardness of 60NiTi parts hardened under an open atmosphere condition.

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Why is it important?

It was found that the addition of small amounts of Hf to 60NiTi would prevent the precipitation of these soft Ni-rich phases in samples solutionized under the same open atmosphere conditions. These Hf-added samples exhibited a high hardness of about * 60 HRC with minor hardness differences seen in outer regions as compared to inner areas. These results imply the suitability of cost-effective open atmosphere heat treatment for hardening of ternary NiTi–Hf alloys.