Distinctive performance evolution of surface layer in TC4 alloy oxidized at high temperature: Softening or hardening?

The characteristics of metal surface layer at high-temperature significantly affect the service performance of TC4 titanium alloy components, so that the evolution mechanism of mechanical properties of metal surface layer under high-temperature oxidation conditions is an important scientific issue. In the present work, the mechanical behavior and the relevant mechanism of metal surface oxidized at the temperatures of 500 ℃, 700 ℃, and 900 ℃ for different times have been systematically investigated. The experimental results show that with the increase of temperature, the oxidation kinetics curve of the alloy changes from parabolic law to linear law, and the oxidation reaction changes from diffusion control to interfacial reaction control. Furthermore, as the temperature increases from 500 °C to 900 °C, the feature of oxide scale changes from the single-layer structure to the multi-layer alternating structure, and the thickness of oxide scales and depth of alpha case increased with increasing temperature. The surface oxide scale is mainly composed of low valent oxides such as Ti3O and Ti6O during low-temperature oxidation as well as short-term high temperature oxidation. After long-term high temperature oxidation, the oxide scale is mainly composed of high valent oxides such as TiO2, Al2O3, and TiVO4. Simultaneously, significant changes occur in the composition, microstructure and properties of metal surface layer. In general, the microstructure and mechanical properties of the surface-layer metal are strongly dependent upon the competitive influence of the outward diffusion of Al and the inward diffusion of O. In the temperature range of 500–900ºC, when the temperature is relatively low, the softening effect caused by the outward diffusion of Al is stronger than the strengthening effect caused by the inward diffusion of O, and a surface softening effect thus occurs. On the contrary, when the temperature is high, the strengthening effect caused by the inward diffusion of O is stronger than the softening effect caused by the outward diffusion of Al, and a surface strengthening effect accordingly happens.