Comparative early stage high temperature oxidation of equimolar NiTi and high entropy Ti16Hf17Zr17Ni16Cu17Co17 shape memory alloy

Abstract

Isothermal oxidation experiments were conducted on equimolar NiTi and high entropy Ti16Hf17Zr17Ni16Cu17Co17 shape memory alloy from 1 to 276 min at 1000 °C in air. After oxidation, the thicknesses and structure features of the oxide scale were examined by scanning electron microscopy with a backscattered electron detector. X-ray diffraction was used to characterize the phase compositions of the oxide scale. For the oxide scale of NiTi, a three-layer structure was confirmed, whereas, a four or five-layer structure was observed in Ti16Hf17Zr17Ni16Cu17Co17 under different oxidation times. Furthermore, an outermost Cu2O layer was found for Ti16Hf17Zr17Ni16Cu17Co17 excessing 36 min of oxidation. Besides, Ti, Ni, Co, and Cu tended to diffuse outward, while Hf and Zr tended to diffuse inward, which was caused by the different diffusion coefficients of elements. Meanwhile, the oxidation kinetics of the two alloys were investigated, which revealed that the oxide layer thicknesses of both alloys showed a good parabolic relationship with the oxidation time, and the parabolic rate constant of Ti16Hf17Zr17Ni16Cu17Co17 (kP =2.653 ×10−7 cm2/s) was smaller than that of NiTi (kP =8.884 ×10−7 cm2/s), which indicated a better high temperature oxidation resistance of the high entropy shape memory alloy.