Formation of columnar lamellar colony grain structure in a high Nb-TiAl alloy by electron beam melting.

The grain morphology and texture control in electron beam melted (EBM) Ti-47Al-8Nb γ-TiAl alloy is considered. The EBM process window to obtain a columnar lamellar colony (CLC) grain structure was defined following a critical assessment of thermal gradient and liquid-solid interface velocity by using numerical simulation. Experimentally, an epitaxial grain growth during solidification of Ti-47Al-8Nb has been realised by using the optimum EBM parameter sets. The length of the CLC grain structure reached up to ∼600 μm (compared to the powder layer thickness of 70 μm). The texture analysis and phase identification performed using electron backscatter diffraction (EBSD) provided important insights in understanding the solidification and phase transformation processes during the EBM fabrication. It was found that the solidification path for EBM high Nb-TiAl alloy involves the high-temperature α-phase field (i.e. L+β→α and α→α 2 +γ phase transformation processes). The epitaxial growth of prior β grains and the anchoring effect of residual B2-phase are very likely to be responsible for the formation of CLC microstructure. Image 1 • We provide a new method to overcome the difficulty of lamellar orientation control caused by the uncertain orientations of α phase generated from β→α solid phase transformation between adjacent layers during EBM. • With the tight control of both thermal gradient and velocity of solidification front, EBM of high Nb-TiAl alloy produced a highly columnar prior β grain structure within which the presence of γ/B2 lamellae and extended lamellar colonies along the Z-direction was found. • The length of the columnar lamellar colony structure reached up to ∼600 μm (compared to the powder layer thickness of 70 μm). [ABSTRACT FROM AUTHOR]