Mechanical Behavior of a Ni-based Crystalline and a Zr-based Amorphous Materials Subjected to Surface Severe Plastic Deformation

A surface-treatment process, surface-severe-plastic deformation (S²PD), is developed and applied on both crystalline and amorphous materials to introduce the 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 near-surface structures of the processed specimens were characterized by means of the optical microscopy, scanning-electron microscopy (SEM), X-ray diffraction (XRD), and transmission-electron microscopy (TEM). Mechanical properties, such as the microhardness, yield strength, and four-point-bend fatigue, 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 grainsize gradient with a nc surface and a coarse-grained interior for the polycrystalline superalloy. Improved fatigue properties were found after the process. However, excessive treatments deteriorate the fatigue properties, and the possible reasons are discussed.

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 face. DSC shows that the free volumes of the deformed BMG have increased, and possible crystallization may occur during the process. XRD and high-energy 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.