Unraveling the mechanics of directional coarsening in single crystal Ni-based superalloys via laser peening: Insights from experimental characterization and microstructural analysis

Surface treatment techniques, like laser peening, enhance the longevity of Ni-based superalloy components by reinforcing their microstructure against surface-initiated damage. Recent findings suggest that these methods may also influence precipitate coarsening behavior at high temperatures, leading to rafting phenomena. This study extensively examined γ’ rafting in single crystal Ni-based superalloy CMSX-4 post laser peening and heat treatment. X-ray diffraction revealed surface compressive stresses (-400 MPa to -800 MPa), transitioning to tensile stresses at greater depths before returning to an unstressed state. Correlation with electron microscopy indicated horizontal coarsening in compressive regions and vertical coarsening in tensile regions due to misfit and residual stresses aiding diffusion. Plastic strain near the LPed surface was measurably increased with lattice misorientation values around 5° before returning to an unstrained state after heat treatment due to rafting-aided recovery and defect reorganization. Secondary γ’ precipitates with a radius of approximately 10 nm occupied γ channels between rafted precipitates, indicating solute element diffusion and supersaturation. Energy dispersive x-ray spectroscopy showed a significant depletion of γ’-forming elements around rafted primary precipitates, highlighting preferential solute diffusion.