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Microstructure evolution and segregation of ti-45al-8nb alloy during directional solidification

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Document pages: 9 pages

Abstract: High Nb TiAl alloy has good oxidation resistance and mechanical properties, but segregation seriously affects the microstructure and properties of the alloy. In order to quantitatively study the segregation behavior of aluminum during solidification, the microstructure of directionally solidified (DS) ti-45al-8nb (atomic percentage) alloy was prepared at 30 extraction rate μ M s and 200   μ M s and 4200 temperature gradients   K M was observed by optical microscope and electron probe microanalyzer. The micro segregation was characterized by wave dispersion spectroscopy. The results show that the direct drawn ingot includes non melting zone, columnar directional solidification zone, paste zone and quenching liquid zone. The arm spacing of primary dendrites is 353   μ M and 144   μ m,respectively, for the two ingots. But the solidified microstructures of the ingots are large lamellar colonies, which contain a few B2 patches and γ bands induced by microsegregation. From dendritic zone to columnar zone, the volume fractions of B2 patches and γ bands decrease. The segregation extents of Al and Nb decrease with the increase of solidification rate. There exists an obvious back diffusion process of Al during solidification and cooling after solidification. According to evolution of Al concentration profiles from mushy zone to columnar grain zone, interdiffusion coefficient for Al in β-Ti at near solidus temperature is semiquantitatively calculated, and the value is (6 – 11) × 10−11 m2 s.

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