Experimental and theoretical study of the effect of Si on the oxidative behavior of Ti-6Al-4V alloys

This paper reports the first study of the oxidative behavior of Si-containing Ti-6Al-4V alloys in air. The alloys were studied in the temperature range 910–1210 K for a maximum period of 518.4 ks, and their tensile properties at room temperature are reported. The addition of Si decreased the total mass gain during oxidation and enlarged the region in which the mass gain obeyed the parabolic rate law. The oxide scale mainly consisted of rutile and α-Al₂O₃ phases. The results of the XPS analysis suggest that in rutile, Si exists as both Si⁴⁺ ions and SiO₂. First-principles calculations indicated that Si⁴⁺ ions stably dissolved into the interstitial sites in rutile, which suppresses oxygen migration through rutile by decreasing the number of oxygen vacancies, thereby inhibiting oxide scale formation. The reaction layer consisting of Ti₃Al and α″ phases on Ti-1mass% Si alloy suppressed the dissolution of oxygen into the substrate, i.e., α-case formation. The ratio of mass gain in α-case formation to total mass gain during oxidation at 1010 K and 1110 K ranged from 0.077 to 0.42. The addition of Si to the Ti-6Al-4V alloy was effective in improving both its oxidative resistance and tensile strength.