MICROSTRUCTURAL EVOLUTION OF DIRECTIONALLY SOLIDIFIED Ni--BASED SUPERALLOY DZ125 UNDER PLANAR GROWTH

By using liquid metal cooling method, the Ni-based superalloy DZ125 was directionally solidified under planar interface growth condition of drawing rate of 1.5 μm/s. The microstructures at different solidified fractions were examined by OM and SEM. The results showed that the solid/liquid interface is planar and the microstructure evolution undergoes three stages: the γ phase was formed as solidified fraction (fs) was below 0.26, and the fine HfC phase sparsely distributed in γ matrix; the coupled γ/MC growth appeared as fs ranged from 0.26 to 0.86, and the morphology of M Cw as fibrous or plate-like shapes; the γ/γ � eutectic was obtained as fs was more than and equaled 0.86 0.86, in which the octahedral M C was precipitated simultaneously. EPMA was used to determine the solute distribution along the longitudinal direction of the sample. The contents of Al, Ti, Ta and Mo increased with fs increasing, while the contents of W, Cr and Co decreased. The analysis showed that the microstructure transformation is attributed to macro-scale non-uniform solute distribution which resulted from solute redistribution during directional solidification.