Research on grain refinement mechanism of undercooled solidified DZ125 superalloy

The grain refinement mechanisms of undercooled DZ125 superalloy melt were systematically investigated by the method of molten salt purification combined with cycled superheating. Within the achieved range of undercooling 0-180 K, there were two processes of grain refinements in the alloy. One took place in the undercooling range of 48-85 K, and the other beyond the critical undercooling 160 K. Based on dendrite growth theory, the tendency of the dendrite remelting can be evaluated with the dimensionless superheating. With the increase of undercooling, the dimensionless superheating increases first and then decreases. Both theoretical calculations and experimental observations indicate that the grain refinement at low undercoolings is caused by the serious dendrite remelting produced by recalescence superheating, and that at high undercoolings by stress originating from the rapid solidification process, which leads to the dendrite disintegration and recrystallization.