Effects of minor boron addition on phase transformation and properties of Ti-47.5Al-2Cr-2Nb alloy

The effect of 0.2 at.% boron addition on phase transformation of Ti-47.5Al-2Cr-2Nb alloy during cooling from the α single phase field was investigated. The room and elevated-temperature tensile properties, creep rupture properties of the boron-free alloy and the boron-modified alloy were compared. The results indicate a very strong effect of boron addition on α to γ phase transformation during continuous cooling at various rates. During air cooling and oil quenching, a large number of Widmanstätten colonies and feathery colonies form in the boron-free alloy, while in the boron-modified alloy there forms a fully lamellar microstructure without Widmanstätten and feathery colonies. A water quenched microstructure in the boron-free alloy consists of about 50 vol% massive transformed region and 50 vol% featureless untransformed region, whereas a 100 vol% massive transformed microstructure is formed in the boron-modified alloy during water quenching process. Due to the effect of boron addition on promoting the formation of fully lamellar microstructure during air cooling and oil quenching, the 760 °C/275 MPa creep rupture life of the boron-modified alloy is two times longer than that of the boron-free alloy when they are cooled from the α single phase field at the same cooling rates. In addition, the boron addition raises the yield strength at room and elevated-temperatures and improves the ductility at room temperature.