Microstructure and mechanical properties of a superelastic NiTi1Hf (at.%) bearing alloy

Superelastic NiTi alloys with composition in the range of 54-55Ni (at. %) have been shown to have very high hardness (~ 500-600 VHN) in addition to large recoverable strain, and intrinsic corrosion resistance. These properties make them very attractive for bearing applications for advanced aerospace applications. However, quenching cracks and residual stresses that have been observed while processing these binary alloys pose a concern. In this work, a ternary alloy with composition 54Ni-44Ti-1Hf (at. %) has been produced and studied at various steps of processing and heat-treatment. Single and multiple cycle stress-strain curves were generated in compression and tension after extrusion, after solution-treatment, and after aging. Detailed microstructural analysis was conducted after each step and various phases were analyzed. A high density of Ni4Ti3 phase that imparts precipitate strengthening was observed after extrusion and solution treatment. After aging, a second precipitate phase, so called the “H” phase that is rich in Hf, was observed in the B2 matrix. The undesirable Ni3Ti phase was observed under all conditions, which suggests further optimization of the solution treatment step. Extended results will be presented and a comparison will be made with the binary 55Ni45Ti (at. %) bearing alloy.