Experimental study of the dissolution and reprecipitation behaviors of ωo phase in high Nb containing TiAl alloy

The dissolution and reprecipitation behaviors of ωo phase during isothermal annealing and ensuing re-aging at 700-800 °C are investigated on an as-cast high Nb containing TiAl alloy with nominal composition of Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y (at.%). The microstructural characterization revealed that numerous micron-sized ωo particles exist within the ordered βo phase in the as-cast alloy. These ωo particles have high thermal stability when annealing at 800 °C and 850 °C for 100 h. Increasing the temperature to 900 °C, the ωo particles began to decompose into βo phase with a low transformation rate. Further increasing temperature causes accelerated dissolution kinetics of ωo phase. Ensuing re-aging at 700 °C and subsequent quenching lead to the reprecipitation of nano-sized ωo particles accompanied by extremely fine γ plates. When the aging temperature increases to 800 °C, the micron-sized ωo particles precipitate from βo phase accompanied by coarsened γ plates. Possible mechanism of the reprecipitation behavior of ωo phase is discussed.