Effect of Mn on microstructure of as-cast Ti47Al8Nb (1 ∼ 2) Mn alloys

The microstructures of Ti47 Al8 NbxMn (mole fraction, %) alloy with x = 1 or 2 were studied on as-east, as-HIPped and on heat-treated samples. The results showed that Mn element is beta phase stabilizer and promotes the formation of beta(B-2) phase. B-2 phase exists at the dendrite cores of the as-cast microstructures. This phase is metastable and can completely decompose into alpha and( or) gamma during 1 200 degreesC, 200 MPa, 4 h HIP process. In the as-HIPed alloy with 1% ( mole fraction) Mn addition, the alpha(2) + gamma lamellae structure is interrupted and decomposed resulting in grain refined near gamma microstructure. The alpha(2) + gamma lamellae structure near interdendritic regions decomposes almost completely into gamma grains, but near dendrite cores incompletely into gamma grains with a small amount of alpha(2) particles or needles around or in it. However, compared with the alloy with 1% (mole fraction) Mn, more alpha(2) phase is retained in the alloy with 2% (mole fraction) Mn. alpha(2) 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 alpha(2) phase decreases following (1250 degreesC, 7 h-1 150 degreesC, 15 h) for 3 cycles heat treatment.