Supported by the National Natural Science Foundation of China (Grant Nos. 92463302, 92163202), an international research team led by Prof. Lei Lu from the Institute of Metal Research, Chinese Academy of Sciences, has made significant progress in the study of cyclic creep behavior of metallic structural materials. This research, titled “Superior resistance to cyclic creep in a gradient structured steel”, was published in Science on April 3, 2025 (https://www.science.org/doi/10.1126/science.adt6666).

Fatigue is one of the primary failure modes of metallic materials, particularly in engineering structures subjected to cyclic or alternating loads. Fatigue failure typically occurs at a stress level far below the yield strength of the materials. It is characterized by sudden and unnoticeable nature, posing a significant threat to the safety of engineering structures. Cyclic creep (ratchetting effect) is a severe fatigue deformation mechanism, manifested as the unidirectional accumulation of cyclic plastic strain under asymmetric stress cycles and non-zero mean stress, eventually leading to irreversible structural failure. Conventional high-strength materials often suffer from cyclic softening and strain localization, both of which exacerbate ratchetting, accelerating premature fatigue failure. Therefore, enhancing the resistance of high-strength metallic materials against cyclic creep damage has become a significant technical challenge in materials engineering.

The research team led by Prof. Lei Lu, together with international collaborators, proposed a fatigue-resistance strategy based on gradient dislocation structures in high-strength metallic materials. By introducing spatially graded dislocation cell structures into the classical 304 austenitic stainless steel, they successfully achieved a synergistic optimization of high strength and excellent cyclic creep resistance. As a result of this strategy, the yield strength was increased by 2.6 times. Compared to conventional stainless steels and other alloys with similar strength, the average ratchetting strain rate was reduced by 2 to 4 orders of magnitude (Figure), breaking through the long-standing bottleneck in improving the ratchetting resistance of structural materials.

This achievement marks a further step, following their previous groundbreaking progress in gradient dislocation structure alloys — including the gradient cell-structured high-entropy alloy with exceptional strength and ductility (Science, 2021) and the gradient cell–structured high-entropy alloy with exceptional strength and strain hardening at low temperature (Science, 2023). The gradient hierarchical dislocation structure represents a versatile strengthening-toughening strategy with broad application prospects in various engineering alloys, providing an important technical foundation for the long service life and high reliability of critical components under extreme service conditions such as in aerospace applications.

Figure. Cyclic creep behavior of gradient dislocation structured 304 austenitic stainless steel. (A) Ratchetting strain vs. number of cycles under maximum stress (σ_max). (B) Average ratchetting strain rate vs. normalized σ_max.