High temperature oxidation behavior of Ti–Ni–Hf shape memory alloy

• Hf-rich layer and their formation mechanism are first reported in TiNiHf alloy.
• The effect of Hf on the overall oxidation rate was studied for the first time.
• Growth rate of Hf-rich layer was compared with other oxide layers.

The effect of Hf addition on the high temperature oxidation behavior and oxide structure of Ti–49Ni–12Hf high temperature shape memory alloy was investigated. Thermogravimetric analyses (TGA) were conducted at temperatures ranging from 800 to 1000 °C for 100 h in dry air. The kinetic curves of the oxidation were measured by TGA, and then the microstructure and chemical elements distribution in the different regions of the specimens were analyzed by SEM/EDS and XRD after oxidation. The experimental results showed that the oxidation rate in the primary stage was high, and then the mass-gaining-rate decreased gradually with the oxidation time. The oxidation behavior of Ti–50.5Ni alloy obeys parabolic rate law for whole oxidation period while the Ti–49Ni–12Hf alloy showed initially parabolic rate law followed by linear rate law. The Hf-rich oxide layer that formed beneath the outer oxide layer can impede the diffusion of Ti ions, resulting in a notable improvement of oxidation resistance of Ti–49Ni–12Hf alloy at high temperature.

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