Experimental study of grain refinement mechanism In undercooled Ni-15at.%Cu alloy

Applying glass fluxing and cyclic superheating, rapid solidification of undercooled Ni-15at.%Cu alloy was performed by rapidly quenching the sample after recalescence. The evolution of micro structure and microtexture has been analyzed. At both low and high undercoolings, well-developed dendrites, within and around which are distributed by the fine cquiaxcd grains, arc observed. At low undercooling, the completely grain-refined microstructure shows a highly oriented texture without annealing twins, whereas at high undercooling a fully random texture as well as a number of annealing twins is observed. On this basis, all the possible mechanisms for grain refinement, as well as their effects on the microstructure formation, were discussed. The grain refinement at both low and high undercoolings is concluded to originate from dendrite fragmentation. Particularly, at high undercooling, recrystallization, as a consequence of dendrite deformation (by fluid flow) and dendrite fragmentation (which provides grain boundary sites for recrystallization nucleation and for the "appearing"recrystailized grains), occurs and plays a role in the grain refinement and the formation of fully random texture.