Microstructure evolution of a Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy during massive transformation and subsequent annealing

It has been widely reported that the microstructure refinement of TiAl alloys can be achieved by massive transformation and subsequent annealing in α₂ + γ two phase field. To achieve this goal, several heat treatment parameters must be adjusted, including the heat treatment temperature around single α phase field, the annealing temperature, and the annealing time for the precipitation of α₂ phase. Thus, a systematic study is needed for each alloy with different compositions. In this study, the heat treatment parameters for grain refinement via massive transformation of a high Nb-containing TiAl are investigated. Precipitation of α₂ phase during annealing is observed by transmission electron microscopy. It is found that 30 min at single α phase field is appropriate for the massive transformation; a full, massively transformed microstructure cannot be obtained by oil or water quenching. A short annealing time can result in a refined microstructure, whereas the sizes of the precipitated α₂ phase increases with the increase of annealing time. The α₂ phase can form at the interface of twin boundaries of the γ phase, following the Blackburn orientation relationship with both sides. The Vickers hardness is measured for the annealed samples, which remains relatively stable for different annealing times.