过冷DD3单晶高温合金凝固组织演化

MICROSTRUCTURE EVOLUTION OF UNDERCOOLED DD3 SINGLE CRYSTAL SUPERALLOY

用复合熔盐净化与循环过热相结合的方法，研究了DD3单晶高温合金过冷熔体凝固组织的演化规律，获得了最大过冷度210K．在所获得的过冷度范围内，其凝固组织的形态发生三次突变：第一次是在过冷度为30K时,因校晶熟化、重熔，高度发达的树枝晶转变为第一类粒状晶；第二次发生在过冷度为78K时，因技晶熟化被抑制，组织转变为细枝晶；当过冷度大于等于180K时。

The structure evolution of DD3 single crystal superalloy was systematically investigated using the method of molten salt denucleating combined with thermal cycle, by which a substantial degree of undercooling up to 210 K was produced. Within the achieved range of undercooling 0-210 K, two grain refinements occur. At the lower range of undercooling 0-30 K, the dendrite growth is dominantly controlled by solute diffusion, and the solidified dendritic morphologies are similar to those of the conventional as- cast structure. The first grain refinement occurs in a certain range of undercooling 30-78 K, because of the dendrite break-up or ripening owing to remelting. At the higherrange of undercooling 78-150 K, however, solute trapping that results from high dendrite growth velocity weakens the effect of solute diffusion on the dendrite growth. In this case, highly developed fine delldrite is formed as a result of the restrained ripening process. The decrease of the grain size above the critical undercooling (△T*=180 K) is attributed to the stress that originates from the extremelyrapid solidification process, which results in the dendrite distortion, disintegration and recrystallizationfinally.