Influence of lattice misfit on the internal stress and strain states before and after creep investigated in nickel-base superalloys containing rhenium and ruthenium

The microstructure of nickel-base superalloys changes significantly during creep. The strengthening γ′-precipitates coarsen directionally and dislocation networks develop at the interfaces between γ′-precipitates and γ-matrix. Coherency stresses due to the lattice misfit between γ and γ′ phase act as the driving force for these microstructural changes. The measurement of the lattice constant of the γ-matrix and γ′-phase by X-ray diffraction before and after creep experiments allows the changes of internal stress and strain states during creep to be revealed. In this work, three experimental alloys with systematically varied amounts of lattice misfit were studied. It was found that the separation of X-ray peaks of γ and γ′ after creep is directly dependent on the amount of lattice misfit. Also the distortion of the lattice, visible in the broadening of the respective rocking curves, was measured and found to be dependent on the lattice misfit as long as the γ/γ′-interfaces stay coherent. The lattice misfit is directly influenced by the alloy composition. Rhenium and ruthenium decrease the lattice misfit and modify the coherency stresses between γ- and γ′-phase accordingly.