Comparative Oxidation Kinetics of a NiPtTi High Temperature Shape Memory Alloy

Abstract

A high temperature shape memory alloy, Ni–30Pt–50Ti (at.%), with an M _s near 600 °C, was isothermally oxidized in air for 100 h over the temperature range of 500–900 °C. Nearly parabolic kinetics were observed in log–log and parabolic plots, with no indication of initial fast transient oxidation. On average the rates were about a factor of 4 lower than values measured here for a binary Ni–49Ti commercial SMA. The overall behavior could be best described by the Arrhenius relationships:

$${\text{Ni}}{\text{Pt}}{\text{Ti}}{:}\,k_{\text{p}} = 1.54 \times 10^{12} \exp \left[(- 250\,{\text{kJ}}/{\text{mol}}) {RT} \right]{\text{mg}}^{2}/{\text{cm}}^{4} {\text{h}} $$

$${\text{Ni}}{\text{Ti}}{:}\,k_{\text{p}} = 6.39 \times 10^{12} \exp \left[(- 249\,{\text{kJ}}/{\text{mol}}) {RT} \right]{\text{mg}}^{2}/{\text{cm}}^{4} {\text{h}} $$

The activation energy was consistent with literature values for TiO₂ scale growth measured for elemental Ti and some NiTi alloys, at ~210–260 kJ/mol. However, a number of other studies produced activation energies in the range of 135–150 kJ/mol. This divergence may be related to various complex scale layers and depletion zones, however, no specific correlation can be identified at present.