Superior resistance to cyclic creep in a gradient structured steel

Cyclic creep, or ratcheting, is a severe form of fatigue deformation caused by cumulative unidirectional plastic strain under asymmetrical stress cycling with a nonzero mean stress. It often causes premature failure of structural materials, and enhancing ratcheting resistance is a challenge in materials engineering. We demonstrate superior ratcheting resistance in high-strength austenitic stainless steel with a gradient hierarchy of dislocation cells. The ratcheting rate is two to four orders of magnitude lower than for coarse-grained counterparts. Its resistance results from sustained microstructural refinement through deformation-induced coherent martensitic transformations to hexagonal close-packed nanolayers within stable dislocation cells. The progressively refined microstructure mitigates cyclic softening and suppresses strain localization during stress cycling, thus reducing ratcheting strain. The gradient dislocation architecture represents a promising design for high-strength, ratcheting-resistant materials.