2012 Volume 53 Issue 11 Pages 1910-1914
The microstructure evolution and microsegregation behavior of alloying elements in a Ni-based superalloy at a series of growth rates under super-high thermal gradient directional solidification were quantitatively characterized. Both microstructure and dendritic segregation are functions of growth rate. The high growth rate solidification leads to refinement in primary (λ1) and secondary (λ2) dendrite arm, decreased size of γ/γ′ eutectics and γ′ phase, as well as reduced microsegregation degree of alloying elements. Also with the growth rate increased, the morphology of γ′ precipitates changes from irregular shape to a cuboidal one. The electron probe microanalysis (EPMA) reveals that Cr, Mo and γ′ forming elements (Al, Ta and Nb) all partition to interdendritic region, while W and Re both preferably segregate to dendrite core. Thus at fast growth rates (or high cooling rates), finer dendrites, uniformly coherently cuboidal γ′ precipitates and decreased segregation favorable to the performance of superalloys are obtained. These quantitative results provide a basis for the establishment of cast and heat treatment process.
