Alpha-case kinetics and high temperature plasticity of Ti-6Al-4V alloy oxidized in different phase regions.

The mechanical properties are impacted by the surface condition of metals. Titanium alloy is oxidized and diffused oxygen into substrate, leading to phase transition in an atmosphere with oxygen. The alpha-case kinetics and tensile behavior of Ti-6Al-4V alloy by high temperature oxidized in different phase regions were investigated by static oxidation test over the temperature range of 850-1100 °C and tensile tests over the temperature range of 830-950 °C. The relationship between high temperature plasticity, oxidation and microstructural transformation of titanium alloy was analyzed by metallographic method, hardness measurements and scanning electron microscopy. The results showed that the inward diffusion of oxygen was the reason of alpha-case generation, which modified the different effects at the surfaces of titanium in different phase regions. The cracks were observed at the surface of the specimens whose depths were greater at higher temperatures. The thickness of this alpha-case was increasing with elevated temperature. Under the high strain rate, the lower stress value of the thermally oxidized Ti-6Al-4V alloy in the vicinity of yielding point was observed compared with that unoxidized. The fractographic appearances of specimens oxidized were brittle and sharp type, while the fracture mode of specimens was ductile fracture without alpha-case. The higher amount of oxygen in α phase both stabilized α phase forming a microstructure and strengthened the α phase via solid solution strengthening. The existence of alpha-case led to easier surface crack initiation and propagation under the tension stress state. [ABSTRACT FROM AUTHOR]