CREEP-INDUCED LOCAL LATTICE-PARAMETER CHANGES IN A MONOCRYSTALLINE NICKEL-BASE SUPERALLOY

The gamma-gamma' lattice mismatch of specimens of the monocrystalline nickel-base superalloy SRR 99 has been measured by a high-resolution X -ray diffraction technique for the undeformed state and after high-tem perature creep deformation. During creep deformation beyond the minimu m creep rate (total strain almost-equal-to 0.5 %), the lattice mismatc hes, measured in and perpendicular to the [001] stress axis, respectiv ely, undergo changes in opposite directions. This reflects the buildup of a complex deformation-induced triaxial state of internal stress in the phases gamma and gamma'. The overall resolved shear stresses that act in gamma' and gamma due to the combined action of the external an d internal stresses are estimated, and the conditions under which cutt ing of the gamma' phase by dislocations should occur are discussed.