Pei Li

Ultrastrong steels are widely used to manufacture load‐bearing components in the automotive and aerospace industries, for light‐weighting, enhanced fuel efficiency, and reduced costs. A martensitic steel with fine grains, containing 0.34 wt% carbon and 2.85 wt% alloying elements, was produced through the combined technique of thermomechanical controlled processing, austenitizing, water quenching, and tempering (TMCP‐Q&T). A lath martensitic matrix with high‐density dislocations, accompanied by the twinned martensite and retained austenite, was obtained using this technique. The dispersed nanoscale ε‐carbide and (Ti,Mo)C phases were distributed at both boundaries and interiors of martensite laths. The mean size of prior‐austenite grains was refined to 7.7 μm, and the average width of martensite laths was 190 nm. Notably, the novel steel exhibited an excellent combination of ultimate tensile strength (2.2 GPa) and ductility (9.4%), which were superior to those of 300 M steel (2 GPa, 8%). Moreover, a significant 40% reduction in the alloying element content was achieved relative to 300 M steel, and the raw material cost was decreased by 17% correspondingly. The enhanced tensile strength of this martensitic steel is mainly attributed to dislocation strengthening and precipitation strengthening, which contribute 53% and 21% to the yield strength, respectively. This discovery provides a new strategy for fabricating a 2.2‐GPa grade ultrastrong steel with good ductility at a low cost, making it highly desirable for industrial applications.

In this paper, the low-cycle fatigue deformation behavior of polycrystalline γ-TiAl alloys at different temperatures was investigated by molecular dynamics simulations. The results showed that the fatigue process comprises an initial cyclic softening stage followed by saturation, and the stress–strain response of the material shows significant asymmetry. With an increase in temperature, the asymmetry between tensile and compressive stresses gradually decreases, and the amplitude of saturated stress decreases significantly. The decrease in dislocation density leads to the cyclic softening of the alloy, and the evolution of dislocation density is temperature-dependent. The dislocation density first decreases and then tends to be stable, while at 900 °C and 1000 °C, it shows an abnormal trend of decreasing first and then increasing. In addition, microscopic mechanism analysis shows that grain coarsening, dislocation annihilation, and phase instability lead to the cyclic softening of the alloys. The fatigue plastic accumulation at low temperatures is mainly dominated by dislocation slip, while at high temperatures, grain boundary slip gradually replaces dislocation slip and becomes the main deformation mechanism. This work reveals new insights into the mechanical behavior of polycrystalline γ-TiAl alloys under cyclic plasticity and temperature-dependent deformation mechanisms.