No abstract

A gradient structure (GS) design is a prominent strategy for strength-ductility balance in metallic materials, including Cu alloys. However, producing a thick GS surface layer without surface damage is still a challenging task limited by the available processing technology. In this work, a gradient structure (GS) surface layer with a thickness at the millimeter scale is produced in the Cu-38 wt.% Zn alloy using ultrasonic severe surface rolling technology at room temperature. The GS surface layer is as thick as 1.1 mm and involves the gradient distribution of grain size and dislocation density. The grain size is refined to 153.5 nm in the topmost surface layer and gradually increases with increasing depth. Tensile tests indicate that the single-sided USSR processed alloy exhibits balanced strength (467.5 MPa in yield strength) and ductility (10.7% in uniform elongation). Tailoring the volume fraction of the GS surface layer can tune the combination of strength and ductility in a certain range. The high strength of GS surface layer mainly stems from the high density of grain boundaries, dislocations and dislocation structures, deformation twins, and GS-induced synergistic strengthening effect. Our study elucidates the effect of the thick GS surface layer on strength and ductility, and provides a novel pathway for optimizing the strength-ductility combination of Cu alloys.

Designing a gradient nanostructure is regarded as an effective strategy for strengthening commercial pure Ti without seriously sacrificing ductility. However, the corrosion behavior of the gradient nanostructured (GNS) pure Ti is far from clear, especially in reducing acid in which pure Ti shows poor corrosion resistance. The present paper aims at investigating the corrosion behavior of GNS pure Ti in hydrochloric acid by electrochemical method. The GNS surface layer is produced by a recently developed method called ultrasonic severe surface rolling. The GNS pure Ti exhibits spontaneous passivation behavior as well as the coarse-grained one in 1 M HCl. Due to the GNS surface layer, the corrosion current density and passive current density decrease by 70% and 54%, respectively, giving rise to significantly enhanced corrosion resistance and passivation ability. The better corrosion resistance is believed to be ascribed to the high-density grain boundaries and dislocations induced by the surface nano-grained structure as well as the smooth surface with few surface defects. The USSR processing also enlarges the static water contact angle of the pure Ti to 61.0 ± 0.3°.