2016 Volume 102 Issue 5 Pages 237-243
To obtain the fundamental information on the microstructure evolution during cooling from the β region in a Fe containing α+β type titanium alloy “Ti-5Al-1Fe”, microstructures of the specimens continuously cooled from the β region at various cooling rates were closely examined and compared with those in conventional titanium alloys Ti-6Al-4V and Ti-5Al-2.5Sn.
The kinetics of the α phase formation during cooling in Ti-5Al-1Fe, in which fast diffusive Fe is used as an alloying element, was considerably different from that in Ti-6Al-4V containing slow diffusive V although the two alloys have the same strength level and the similar phase constitution. In the specimens cooled at cooling rate of 300°C/s or higher, the Fe-enriched β phase retained between the lath α plates, showing that diffusional phase transformation occurred even at relatively high cooling rate of 300°C/s. In the specimens cooled at cooling rates ranging from 1 to 300°C/s, cooling rate dependence on Vickers hardness at room temperature was considerably small compared to Ti-6Al-4V. It is similar to that in Ti-5Al-2.5Sn containing around 0.2% Fe, indicating that the diffusional component of phase transformation dominated the phase transformation during cooling and shear component was suppressed at cooling rates lower than 300°C/s. In the specimen cooled at 0.1°C/s, the ω phase formed in the retained β phase probably as a result of an aging effect due to the low cooling rate, while equilibrium FeTi was not detected. Based on the microstructure observations, the CCT diagram of Ti-5Al-1Fe was formulated.