A Gradient Structural Steel with Ultra-High Ratchetting (Cyclic Creep) Resistance
Ratchetting, also termed cyclic creep, denotes the progressive accumulation of plastic deformation in metals subjected to asymmetric stress-controlled cyclic loading. This phenomenon induces dimensional intolerance and premature fatigue failure in critical engineering structures such as steel rails, nuclear power pipelines, and aircraft engines. Existing strategies to enhance ratchetting resistance—including pre-strain treatment of coarse-grained metals and nanostructuring—often compromise plastic hardening capacity and promote strain localization, thereby degrading long-term cyclic performance. Recent work by Lu's group proposed three prerequisites for high ratchetting resistance: high plastic strain hardening capacity, low dynamic recovery, and suppression of microstructural coarsening during cycling. Based on this framework, a gradient dislocation structured (GDS) 304 austenitic stainless steel (Fe-18%Cr-8%Ni, wt.%) was fabricated via pre-torsion cyclic deformation. While grain size remains uniform at 37 μm, the initial dislocation structure exhibits a radial gradient. Transmission electron microscopy reveals dislocation cell structures with cell size and thickness of 290 nm and 50 nm, respectively, in the surface region, accompanied by abundant low-angle boundaries. This gradient architecture effectively balances strength and ratchetting resistance, offering a viable route for designing structural metals with ultra-high cyclic creep resistance.