From Withdrawal Rate to Creep Life: Microstructure-Mediated Performance in Directionally Solidified Mar-M247LC

Abstract:

Directionally solidified (DS) nickel-based superalloys exhibit excellent creep resistance due to the elimination of transverse grain boundaries perpendicular to the stress axis. Given the strong dependence of creep performance on the initial solidification microstructure, this study reveals how withdrawal rate governs the as-cast microstructure of DS Mar-M247LC and dictates its post-heat-treatment creep life at 980 °C/220 MPa. It was found that an increase in the withdrawal rate of the directionally solidified specimens led to a reduction in the values of primary dendrite arm spacing (from 479 μm to 322 μm) and the average size of γ" precipitates (from 460 nm to 345 nm in interdendritic regions and from 298 nm to 203 nm in dendritic core). In addition, the carbide morphology changes from blocky to script-like. The heat treatment led to the formation of distinct cuboidal γ" precipitates and was accompanied by a significant increase in the γ" volume fraction compared to the directionally solidified microstructure. The alloy solidified at 40 μm/s exhibits elongated γ" rafts with narrowed matrix channels and regular dislocation networks, synergistically extending creep rupture life to 96.6 h. Fractographic analysis confirmed transgranular ductile, with fracture initiation occurring at decomposed MC carbides.