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Significant prior-β grain refinement of as-cast Ti–6Al–4V alloy was achieved through additions of yttrium. Microstructural analysis revealed three mechanisms by which yttrium refines the prior-β grains during solidification: solute yttrium provides constitutional supercooling; yttrium scavenges oxygen and forms Y₂O₃ particles, which pin grain boundaries suppressing grain growth; Y₂O₃ particles also act as active nucleation sites. Even finer equiaxed grains were achieved through laser surface remelting suggesting that yttrium additions could be an efficient β-Ti grain refiner for the Ti–6Al–4V alloy processed by additive manufacturing.