MICROSTRUCTURAL EVOLUTION OF DIRECTIONALLY SOLIDIFIED Ni-BASED SUPERALLOY DZ125 UNDER HIGH TEMPERATURE GRADIENT

Abstract

The Ni-based superalloy DZ125 was prepared by liquid metal cooling(LMC) directional solidification and quenching technology with withdrawal rate(V) range of 2—400μm/s and temperature gradient up to 250 K/cm.The morphologies of solid/liquid(S/L) interface, cellular/dendritic arm spacings and the morphologies of MC carbide were studied systematically. The shallow cellular interface arised at V=2μm/s.With increasing the withdrawal rate,the S/L interface turns into deep cellular(V=3μm/s) and dendritic(V≥5μm/s) interfaces successively. The cellular spacing is increased with increasing the withdrawal rate.However,the primary dendritic arm spacing is decreased with increasing the withdrawal rate.The maximum value of cellular/dendritic spacings appears at transition from cellular to dendritic interfaces(V=5μm/s). Meanwhile,the morphology of MC carbide changes from octahedron to frame-like,Chinese-script and finally to fine dendrite with increasing the withdrawal rate.Compared with the theoretical models of primary dendrite spacing,the results are good in agreement with Trivedi's and Ma's models. Furthermore,they are also in agreement with Hunt-Lu model only at lower withdrawal rates(V≤50μm/s).The relationships of primary and secondary dendritic arm spacings with withdrawal rates can be described asλ_1=314.6V~(-0.24±0.02) andλ_2=97.76V~(-0.33±0.01),respectively.MC carbide precipitated from the melt during solidification,and its morphology is dependent both on the withdrawal rate and the morphology of S/L interface.