The r/+/-l a tt ice 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-temperature creep deformation. During creep deformation beyond the minimum creep rate (total strain w 0.5 %), the lattice mismatches, measured in and perpendicular to the [OOl] t s ress axis, respectively, undergo changes in opposite directions. This reflects the build-up of a complex

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... induced triaxial state of internal stress in the phases y and 7'. The overall resolved shear stresses which act in y' and y due to the combined action of the external and internal stresses are estimated and the conditions under which cutting of the y/-phase by dislocations should occur are discussed.