Phase and microstructure of TC21 titanium alloy during slow cooling

The phase and microstructure of TC21 titanium alloy during cooling from β phase field at differently slow rates were studied by OM, XRD and SEM. The results show that the main phase transformation was β→α during cooling, and the precipation of Ti₂AlNb(O)and Ti₂AlNb(B₂)phases were discovered. When cooling rate was 1°C/min, α precipitates nucleated at the grain boundary, then growed towards transgranular, and finally forms fully lamellar microstructure. Whereas, nucleation of α precipitates during cooling with 5°C/min taked place at grain boundaries and was also accompanied by appearing at intragranular sites, which resulted in final microstructure was lamellar microstructure with a little basketweave microstructure. The effect of cooling rate on microstructure received at room-temperature was visible. Thickness of α laths, α colonies size and fraction of α laths all decreased with the cooling rates reducing.