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

The dissolution and reprecipitation behaviors of ω_o 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 ω_o particles exist within the ordered β_o phase in the as-cast alloy. These ω_o particles have high thermal stability when annealing at 800 °C and 850 °C for 100 h. Increasing the temperature to 900 °C, the ω_o particles began to decompose into β_o phase with a low transformation rate. Further increasing temperature causes accelerated dissolution kinetics of ω_o phase. Ensuing re-aging at 700 °C and subsequent quenching lead to the reprecipitation of nano-sized ω_o particles accompanied by extremely fine γ plates. When the aging temperature increases to 800 °C, the micron-sized ω_o particles precipitate from β_o phase accompanied by coarsened γ plates. Possible mechanism of the reprecipitation behavior of ω_o phase is discussed.