Nano-structural refinement of metallic surfaces by using low-compression cyclical loading

Abstract

Recent studies on very high cycle fatigue (VHCF) have observed a unique fracture surface with a fine granular area around the crack origin that arises from long-term repeated contacts of fracture surfaces (over 10⁷ cycles). A fine microstructure including nanocrystalline is known to form in the matrix just beneath the fracture surface. Focusing on these phenomena, this study devised a new surface modification technique, called Cyclic Press (CP), that creates a nanocrystalline layer using cyclic compressive loading. Repeated low-compression loadings were applied to the surfaces of SNCM439 high-strength steel and ADC12 die-cast aluminum alloy with an indenter for 5 × 10⁷ cycles. Then, the surfaces and cross sections were analyzed by using a laser microscope, a scanning electron microscope, a scanning ion microscope, and energy dispersive X-ray spectrometry (EDS). As a result, a nanocrystalline structure very similar to that of the cross section beneath the fracture surface in the VHCF regime clearly formed in the surface layer of both materials. Additionally, an oxygen-rich layer formed at the surface of the CP-treated ADC12. This result was considered to be caused by diffusion of elemental oxygen in the atmosphere by a huge number of repeated contacts between the indenter and specimen. Overall, the results indicate that CP is a useful technique for forming nanocrystalline layers in metal surfaces and can also be expected as a new method for diffusing surrounding gaseous elements into surfaces at room temperature.