Effect of Withdrawal Rate on Microstructure and Creep Performance of Directionally Solidified Mar-M247LC Superalloy

The effect of withdrawal rate on microstructure and creep performance of directionally solidified (DS) Mar-M247LC superalloy was investigated. Results show that an increase in withdrawal rate of DS specimens leads to a reduction in primary dendrite arm spacing (from 479 μm to 322 μm), and the average size of γ' precipitate decreases from 460 nm to 345 nm in interdendritic region and from 298 nm to 203 nm in dendritic core. In addition, the carbide morphology changes from blocky to script-like. The heat treatment leads to the formation of distinct cuboidal γ' precipitates. And the volume fraction of γ' precipitates in heat-treated microstructure has a significant increase compared to that in DS microstructure. The DS superalloy under the withdrawal rate of 40 μm/s exhibits elongated raft-like γ' structure with narrowed matrix channels and regular dislocation networks, synergistically prolonging creep rupture life to 96.6 h. Fractographic analysis confirms that the superalloy exhibits a transgranular ductile fracture mode, with cracks initiating at decomposed MC carbides.