Effect of Oxidation on the Transformation Temperatures and Microstructure of Ti-Ta-Al High Temperature Shape Memory Alloys

In this study, the effect of oxidation on the transformation temperatures and microstructure of Ti-Ta20-Al5 high temperature shape memory alloys (Ti-Ta20-Al5 HTSMAs) is studied. The alloy was studied under two conditions, as cast and after cold rolling and recrystallizing (CR +RE). The microstructure, phase composition, and phase transformation temperature of Ti-Ta20-Al5 HTSMAs before oxidation were studied. Thermogravimetric analysis (TGA) combination with microstructural investigations were performed after oxidation at temperatures range from 25°C up to 1000°C. For both conditions, the alloy consisted mostly of β – BCC and martensite ʺα – orthorhombic phase before oxidation. Ti-Ta20-Al5HTSMAs as (CR +RE) shows martensitic transformation temperature higher than 400°C. The oxidation behaviour was linear until 650°C, while parabolic at elevated temperatures. For both conditions, the alloy consisted of β – BCC phase (Ta – rich) and α-hexagonal phase (Ti – rich) with not strong precipitation of ω – phase after oxidation. The results show that the formation of multi – layer oxide, which consists of Al2O3, TiO2, and Ta2O5. For both conditions, oxidation can suppress martensitic phase transformation by precipitation of α-hexagonal phase (Ti – rich), Al2O3, TiO2, and Ta2O5, and ω phase, would result in degradation of the martensitic phase transformation.