An X-ray study of creep-deformation induced changes of the lattice mismatch in the γ′-hardened monocrystalline nickel-base superalloy SRR 99

Abstract

X-ray line profile measurements were performed on monocrystalline specimens of the γ′-hardened nickel-base superalloy SSR 99 with axes close to the [001] direction. The aim was to measure the local lattice parameters in the γ-matrix and in the γ′-particles and to obtain information on the lattice mismatch and internal stresses. For this purpose, a special high-resolution double crystal diffractomoter with negligible instrumental line broadening was used. Measurements were performed on specimens in the initial state with cuboidal γ′-particles and on creep-deformed specimens containing the so-called γ/γ′-raft structure. In several cases the line profiles were measured as a function of the rocking angle, and the intensity distributions were mapped in reciprocal space around the (002) and (020) Bragg reflections. In the undeformed state these intensity distributionsindicate that the local lattice parameter varies spatially in the γ-phase. The line profiles of specimens in the initial state were asymmetric. A remarkable result obtained on creep-deformed specimes was that, whereas the asymmetry of the (020) line profiles was enhanced to the extent that a hump or a second peak appeared, the asymmetry of the (002) line profiles was reversed in sign. A quantitative evaluation yielded mean values of the constrained lattice mismatch which, in the case of creep-deformed specimens, differ significantly for the (001) and (010) lattice plane spacings. It is concluded that the orientation-dependent lattice spacings represent a triaxial state of residual stress which has its origin in the superposition of the originally present coherency stresses and the deformation-induced internal stresses. All observed features could be explained in detail in terms of a composite model of plastic deformation, which takes into account the dislocation networks deposited at the γ/γ′-interfaces during deformation.