Growth velocity-undercooling relationship and structure refinement mechanism of undercooled Ni-Cu alloys

The growth velocity-undercooling relationship as a function of bulk undercoolings was systematically measured using high speed thermal imaging of highly undercooled Ni-Cu alloy samples. It was found that the growth velocity of Ni80Cu20 alloy was almost coincident with that of Ni95Cu5 alloy and the growth velocity of Ni85Cu15 alloy was almost coincident with that of Ni90Cu10 alloy, which was a new result that was unreported previously for concentrated solid solution alloys. It was also found that the solidification velocity increased smoothly with undercooling up to the maximum value. Growth velocity can be adequately represented by the current dendritic growth model. A clear discontinuity in the growth velocity-undercooling curve was observed at a critical undercooling. This discontinuity in the velocity-undercooling curve can be attributed to the finite speed of solute diffusion in the bulk liquid ahead of the growing dendrite. Microstructural evidences in these samples indicative the occurrence of dendritic fragmentation. Microstructural analysis using light microscopy and EBSD analysis reveals that recrystallization accompanies the grain refinement at high undercoolings. Furthermore, at high undercoolings, a high density of annealing twins in the as solidified samples indicates the occurrence of recrystallization in the solidification microstructure and a high density of subgrains in the quenched samples are observed within the microstructure, which indicates the occurrence of extensive recovery, a result previously unreported in samples solidified from highly undercooled melts.