Interrogation of the microstructure and residual stress of a nickel-base alloy subjected to surface severe plastic deformation

A low stacking-fault energy nickel-base, single-phase, face-centered-cubic (fcc) alloy has been subjected to surface severe plastic deformation (S2PD) to introduce nano-grains and grain size gradients to the surface region of the alloy. The simultaneous microstructural and stress state changes induced by S2PD have been investigated via the X-ray diffraction (XRD) analysis that includes evaluation of annealing and deformation twins, deformation faults, in-plane lattice parameters and elastic strains of the crystal lattice, macroscopic residual in-plane stresses, crystallite sizes, internal strains, dislocation densities, and crystallographic texture as a function of the depth measured from the processed surface. Microstructural changes have also been characterized using optical and electron microscopy in order to corroborate the findings from the XRD analysis. The results from the XRD analysis are in excellent agreement with those derived from the microscopy analysis. This is the first systematic and comprehensive study using XRD to quantify depth-profile changes in a wide range of microstructural features and stress states in a fcc material resulting from the S2PD process.