Solidification structures of high niobium containing TiAl alloys

To understand the effect of alloy stoichiometry on the microstructural development and mechanical behavior of γ-TlAl-based materials, it is necessary to have a determination of the phase relationships for the TiAl alloy system near the γ phase field. Cast structures and phases of Ti-(43-47)Al-8Nb-(1-2)Mn (at%) alloys have been studied by using scanning electron microscope and X-ray diffraction. Their solidification path and microstructure development during the solidification were analyzed. The experimental results show that the alloys with different Al contents form different macrostructure and microstructural morphologies. This indicates that the solidification paths are different with different Al contents. The alloy with 43Al forms equiaxed grain structure, and the solidification path is as follows: L → L+β → β → α+β → α+β cores → α₂+γ+B2 cores. Whereas the alloy with 47Al forms columnar grain structure, and the solidification path is as follows: L → L+β → α+β+L → α+γ+β cores → α₂+γ+B2 cores. The β phase is their primary solid phase and can be retained to ambient temperature. The alloy with 43Al solidifies completely into β phase. The peritectic reactions L+β → α and L+α → γ appear when the Al content increases to 47Al.