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 gamma-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 similar to 600 mu m (compared to the powder layer thickness of 70 mu m). 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 alpha-phase field (i.e. L+beta ->alpha and alpha ->alpha(2)+gamma phase transformation processes). The epitaxial growth of prior beta grains and the anchoring effect of residual B2-phase are very likely to be responsible for the formation of CLC microstructure. (C) 2019 Elsevier B.V. All rights reserved.