深过冷DD3高温合金的两次细化机制

用复合熔盐净化与循环过热相结合的方法，获得了最大210K过冷度，研究了DD3高温合金过冷熔体凝固组织的演化规律，在所获得的过冷度范围内，凝固组织的形态发生两次晶粒细化，发生第一次细化的过冷度为30－70K，因枝晶熟化，重熔，高度发达的树枝晶转变为第一类粒状晶；发生第二次细化的过冷度超过153K，凝固组织因枝晶碎断和再结晶而志变为第二类粒状晶。

GRAIN REFINEMENTS IN UNDERCOOLED DD3 SINGLE CRYSTAL SUPERALLOY

The structure evolution of DD3 single crystal superalloy was systematically investigated employing the method of molten salt denucleating combined with thermal cycle. Within the achieved range of undercooling 0-210K, the solidification structure undergoes two grain refinements. Because of the dendrite break--up or ripening owing to remelting. the first grain refinement occurs in a certain range of undercooling 30-70K. The dendrite growth is dominantly controlled by solutal diffusion, and the remelting of primary dendrite evaluated by adopting the calculation of the dimensionless Superheating in recalescence is most seriously. With the increase of undercooling, however, solute trapping that results from high dendrite growth velocity weakens the effect of solute diffusion on the dendrite growth. Therefore, the decrease of the grain size above the critical undercooling (T*=180K) is attributed to the stress originating from the extremely rapid solidification process, which results in the dendrite distortion, disintegration and recrystallization finally.