Effect of morphology of γ′ phase on creep resistance of a single crystal nickel-based superalloy, CMSX-4

The influence of the formation of a lamellar gamma-gamma' structure on creep resistance was investigated using the single crystal nickel-base superalloy, CMSX-4. The evolution of lamellae, or rafts was controlled by the duration of the prior-creep test conducted at 1273 K - 160MPa up to 3.24 x 10(6)s. Under these creep condition, the cuboidal gamma' phase in the as-heat treated specimens turned to the rafted one through a transient creep stage. The stress enhanced creep tests were conducted at 1273 K - 250MPa to evaluate creep resistance of the prior-creep tested specimens, and the minimum creep rates decided by the stress enhanced creep test were compared with that of the as-heat treated specimen. The minimum creep rates of the prior-creep tested specimens were always larger than that of the as-heat treated one. The dislocation substructure was not formed in the cuboidal gamma' and rafted gamma', and was observed at the gamma/gamma' interface and gamma channel. The thickness of the gamma channel increased with increasing the prior-creep testing time. The correlation between the thickness of the y channel and the minimum creep rate in the stress enhanced creep test shows linear and is independent of the shape of gamma'. The TEM observation of specimens interrupted the stress enhanced creep test at the minimum creep rare showed that the radius of dislocation curvature was directly proportional to the thickness of the y channel. Consequently, the loss of creep resistance through rafting of gamma' is caused by the increase in the radius of dislocation curvature, not by the shape and the size of gamma'.