Effect of Co addition on solidification velocity, hardness and refined structure transformation mechanisms of undercooled Ni-Cu-Co alloys

In this work, the effect of Co addition on the solidification velocity, hardness and refined structure trans-formation of the undercooled Ni-Cu-Co alloys have been systematically investigated. The results show that the grain refinement of Ni-Cu-Co ternary alloys at low undercooling is caused by the intense dendrite remelting. However, the grain refinement at high undercooling is attributed to recrystallization, and the driving force of recrystallization comes from the stresses and plastic strains accumulated by the interaction between liquid flow induced by rapid solidification and primary dendrites. The Co addition can significantly increase the solidification velocity and recalescence effect of Ni-Cu-Co alloys by increasing the number of solidified crystal nucleus and solute diffusion velocity. Under the same undercooling level, the increase of solidification velocity and recalescence effect have strengthened the remelting effect and stress accumu-lation, which can enlarge the critical range for grain refinement gradually. Meanwhile, due to the significant improvement of solidification velocity and stress-induced deformation effect by Co addition, the low angle GBs in the refined structure of Ni-Cu-Co alloys increase and the grain size decreases significantly, which have significantly improved the average hardness of the refined structure. When Co addition reaches 6 %, the refined structures can appear in almost the whole undercooling range and the hardness are improved by more than 100 % compared with the as-cast structure.(c) 2022 Elsevier B.V. All rights reserved.