Change in Morphology of γ' of Prior-Crept Single Crystal Nickel-Based Superalloy, CMSX-4, with Simple Aging
It has been elucidated that the small creep strain must be essential to form the rafted γ/γ' structure using a single crystal nickel-based superalloy, CMSX-4. To confirm the amount of the strain required to make rafting, the single crystals interrupted the creep tests up to a strain of 0.01 at 1273K-250MPa was aged at 1273K without stress. To compare the difference in the morphology of the γ/γ' structure of the as-heat treated and the creep-interrupted single crystal with the simple aging, the as-heat treated single crystal was also aged. Microstructural observations by SEM were conducted for the specimens sectioned parallel to (100). The cuboidal γ' precipitates of the as-heat treated single crystal connected each other with three <100> directions after the simple aging. By employing the aging without stress on the specimen interrupted the creep test at a strain of 0.0074, the drastic morphological change in the γ/γ' structure was confirmed, that is, the γ/γ' structure changed to rafted one perpendicular to the creep-interrupted stress axis. The aspect ratios of the creep-interrupted specimens increased with increasing simple aging time, and attained to the maximum value at the simple aging time of 3.60x106s. The maximum value of the aspect ratios increase with increasing creep-interrupted strain and attained 4 at a strain of 0.0074. The dislocation density at the γ/γ' interface increases with increasing creep strain. Consequently, the formation of the rafted γ/γ' structure requires the dislocation substructure induced by the creep strain.
T. Chandra, K. Tsuzaki, M. Militzer , C. Ravindran
N. Miura et al., "Change in Morphology of γ' of Prior-Crept Single Crystal Nickel-Based Superalloy, CMSX-4, with Simple Aging", Materials Science Forum, Vols. 539-543, pp. 3024-3029, 2007