Directional coarsening of γ′ phase in single crystal nickel based superalloys during tensile creep

The gamma' precipitate rafting kinetics and morphological evolution for two model single crystal superalloys have been studied. The microstructure of the alloys after different stages of tensile creep at 1040 degrees C and under a range of stresses are examined using TEM and SEM, The chemical compositions of both gamma and gamma' phases are analysed by energy dispersive spectrometry. Results show that a meshlike gamma' raft structure is formed along the direction normal to the stress axis during primary creep. The applied stress causes a decrease in the coherent strain energy at the gamma'/gamma interfaces in the (001) crystal plane. The released energy is the driving force for the diffusion of elements, leading to the formation of the gamma' rafts. A longer time is required for the formation of gamma' rafts in alloy 2 owing to its higher content of the refractory element W which obstructs the migration of the other elements in the diffusion field of the gamma' rafts.