A surface-treatment process, surface-severe-plastic deformation (S2PD), is developed and applied on both crystalline and amorphous materials to introduce plastic deformation in the near-surface layer. A S2PD-processed crystalline component is expected to have enhanced fatigue properties because the refined grains in the near-surface layer and the coarse grains in the interior have good resistance to the crack initiation and propagation, respectively.
The microstructures and mechanical properties of the processed specimens were systematically investigated. It is shown that the S2PD process has the capability of simultaneously creating (a) a work-hardened surface layer, (b) a nanocrystalline
(nc) surface layer, (c) a surface region with compressive-residual stresses, and (d) a grain-size gradient with a nc surface and a coarse-grained interior for the polycrystalline superalloy. Improved yield strength and fatigue strength were found after the process.
For the amorphous material, thermal properties of the processed near-surface layer were characterized by means of the differential-scanning calorimetry (DSC). Effects of the treatment on the microhardness were studied by the nanoindentation. After the treatment, the plastic-flow deformation in the unconstrained sample edge was observed. In the sub-surface layer, the impact-induced shear-band operations generate the extrusion and intrusion marks on the side surface. XRD and highenergy synchrotron diffraction techniques were used to inspect the possible crystalline phase. A nanoindentation test shows that on the side surface, the hardness increases and, then, decreases with the distance from the processed surface. Four-point-bending-fatigue behavior has been studied and related to the modified surface structure and the compressive-residual stress induced by the process.