Study on Microstructure Refinement Mechanism in Deeply Supercooled Solidification of Ni-25at.%Cu Alloys

Maximum undercooling degrees were obtained by glass purification and superheating. Rapid solidification in an undercooled alloy was systematically studied. As the undercooling increases, the solidification front appears randomly. The microstructure undergoes a transformation from coarse dendrite grains to granular grains to oriented dendrite to equiaxed crystal. The results show two grain refinement under high and low undercooling. No high strength textures are found under high undercooling conditions, and many grain boundaries of high angle nature are observed, which confirms recrystallization. In the TEM characterization of high undercooling, high density dislocation strain indicates that accumulated stress in the rapid solidification process induces microstructure plastic strain and promotes recrystallization. The results show that the grain boundary of refined grain is coarser at low undercooling, but narrower at high undercooling. Twins and subgrains observed at large undercooling indicate that the grain refinement mechanisms are very different.