A gradient nanostructured (GNS) surface layer was generated on an ultrahigh strength bearing steel (AISI 52100) by surface mechanical rolling treatment (SMRT). The initial martensite plates with cementite and retained austenite were transformed into nanograined martensite with a mean size of ∼24 nm at the topmost surface. Axial tension–compression fatigue tests showed that an enhanced fatigue property was achieved on the GNS sample due to the suppressed fatigue crack initiation at the surface. Effects from hardness, microstructure, and residual stress in the GNS surface layer were discussed in relation with the fatigue mechanism of the SMRT sample. GRAPHICAL ABSTRACT IMPACT STATEMENT A gradient nanostructured surface layer was generated on an ultrahigh strength bearing steel, and significantly improved the fatigue property of the steel by suppressing surface crack initiation at lower stresses.

Gradient nanostructures (GNSs) have been optimized in recent years for desired performance. The diffusion behavior in GNS metals is crucial for understanding the diffusion mechanism and relative characteristics of different interfaces that provide fundamental understanding for advancing the traditional surface alloying processes. In this paper, atomic diffusion, reactive diffusion, and surface alloying processes are reviewed for various metals with a preformed GNS surface layer. We emphasize the promoted atomic diffusion and reactive diffusion in the GNS surface layer that are related to a higher interfacial energy state with respect to those in relaxed coarse-grained samples. Accordingly, different surface alloying processes, such as nitriding and chromizing, have been modified significantly, and some diffusion-related properties have been enhanced. Finally, the perspectives on current research in this field are discussed.