| Single crystal specimens were solidified by modified Bridgman method. The surface recrystallized single crystal specimens were prepared by shot peening followed by heat treatment. Recrystallization of single crystal superalloys was supposed to decrease its creep rupture lives, however, creep tests showed that the creep life of the recrystallized specimen was similar(or slightly longer) to that of the single crystal specimen at 982℃ 240MPa and 1050℃ 140MPa in air. The surface oxide layer of the recrystallized specimens was thicker than that of the single crystal specimens after creep rupture at 982℃, 240MPa and 1050℃, 140MPa in air. Surface oxidation was enhanced by the existence of recrystallized grain boundaries. The creep ruptured microstructures showed that the cracks were initiated at the recrystallized grain boundaries, which were perpendicular to the stress direction. The initiated cracks propagated along the recrystallized grain boundaries but the crack growth was obstructed by the surface oxide layer. In vacuum, the single crystal specimens showed longer creep rupture lives than the surface recrystallized specimens at 982℃, 240MPa. Creep at 871℃ 500MPa in air, where oxidation is less severe, was similar to the result at 982℃, 240MPa in vacuum. |
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