Understanding the solute segregation and redistribution behavior in rapidly solidified binary Ti-X alloys fabricated through non-equilibrium laser processing

The solute segregation and redistribution during non-equilibrium rapid solidification using laser additive manufacturing (LAM) process directly influence the microstructure morphology and phase distribution, which in turn affects their mechanical properties. In this work, a laser micro-alloying strategy was utilized to preserve the original solidification microstructure in the considered Ti-9Mo, Ti-9Cr, Ti-9Fe, and Ti-9Ni (wt%) alloys. The addition of different β-stabilizing elements (Mo, Cr, Fe, and Ni) resulted in distinct microstructures: Ti-9Mo and Ti-9Cr alloys exhibited larger grains (∼502 μm and ∼733 μm) and cellular morphologies due to minimum constitutional undercooling at the solid-liquid interface. Because of the increased constitutional undercooling, the Ti-9Fe grains are significantly refined (∼398 μm), showing a dendritic morphology with elongated primary dendrite arms. Ti-9Ni exhibited the highest constitutional undercooling, forming equiaxed dendrites. However, due to the significant consumption of solute atoms by the interdendritic eutectic phase Ti₂Ni, the grains did not further refine (∼396 μm). Combined with the temperature field simulation, the solidification conditions of the alloys were determined. In addition, based on the solute partitioning coefficients (k), the different solute redistribution and diffusion behaviors at the solid-liquid interface during the laser micro-alloying process of Ti-9Mo with k > 1 and Ti-9Cr with k < 1 were elucidated, providing essential insights into the formation of typical cellular morphology and enhanced Mo enrichment phenomenon in the Ti-9Mo alloy.