Abstract:The microstructures of Ti47Al8NbxMn (mole fraction, %) alloy with x=1 or 2 were studied on as-cast, as-HIPped and on heat-treated samples. The results showed that Mn element is β phase stabilizer and promotes the formation of β(B₂) phase. B₂ phase exists at the dendrite cores of the as-cast microstructures. This phase is metastable and can completely decompose into α and(or) γ during 1200℃, 200MPa, 4h HIP process. In the as-HIPed alloy with 1% (mole fraction) Mn addition, the α₂+γ lamellae structure is interrupted and decomposed resulting in grain refined near gamma microstructure. The α₂+γ lamellae structure near interdendritic regions decomposes almost completely into γ grains, but near dendrite cores incompletely into γ grains with a small amount of α₂ particles or needles around or in it. However, compared with the alloy with 1% (mole fraction) Mn, more α₂ phase is retained in the alloy with 2% (mole fraction) Mn. α₂ phase contains more Mn and is more stable in the alloy with 2% (mole fraction) Mn than in that with 1%(mole fraction) Mn. This makes its grain refinement more difficult. The amount of α₂ phase decreases following (1250℃, 7h—1150℃, (15h)) for 3 cycles heat treatment. 

Key words:  cast high-niobium TiAl alloy; microstructure; effect of Mn element