Microstructure evolution of directionally solidified Mar-M247 LC superalloy and its dependence on solidification variables

Solidification process and microstructure, gamma' phase and carbide formati on in directionally solidified Mar-M247 LC alloy were studied and correlate d with solidification variables. The results indicate that gamma dendrite a rm/cell spacing changes with the solidification variables and exhibits a ma ximum value. Liquid volume fraction decreasing, though deviating from modif ied Scheil equation at the lower part of the mushy zone, can be fitted by t he modified Scheil equation at the upper part of the mushy zone. The larges t segregation and the largest amount of defects such as micropore, gamma/ga mma' eutectic and discontinuous gamma' phase occur in the sample with the c oarsest gamma dendrite structure. Discontinuous gamma' phase can nucleate e ither spontaneously at the gamma dendrite boundary or grow from the gamma/g amma' eutectic. The gamma/gamma' eutectic and the discontinuous gamma' phas e formations are suppressed at the near equilibrium solidification. Both sp lit cube and dendritic shape morphology of the gamma' precipitate can be ob tained directly by cooling after solidification. The carbide forming elemen t enrichment above solid-liquid interface at near equilibrium solidificatio n is not high enough for large carbide growth. However, if nitrogen content in the liquid is high, carbonitride nucleation and carbonitride growth can occur above the solid-liquid interface. The amount of solute segregation w here the carbide grows decides carbide nucleation and carbide growth. The c arbide volume fraction decreases at the near equilibrium solidification due to lack of secondary gamma dendrite arm formation. The melt composition wh ere the carbide grows and the carbide growth time control the carbide compo sition.