Cyclic heat treatment is an effective approach for enhancing the mechanical properties of 18Ni(C250) maraging steel, and the selection of cyclic heat treatment temperature is a key factor. In this study, a cyclic heat treatment process with a two-step solution treatment is employed to investigate the influence of cyclic heat treatment temperature, specifically the first solution treatment temperature (920 °C, 950 °C, and 980 °C), on the microstructure and mechanical properties of 18Ni(C250) maraging steel. The results indicate that with an increase in the cyclic heat treatment temperature, the average grain size of the 18Ni(C250) maraging steel decreases initially and then increases. When the cyclic heat treatment temperature reaches 950 °C, the grain size is at its minimum, exhibiting optimal grain uniformity. Additionally, the increase in cyclic heat treatment temperature results in a reduction in the size of martensitic lath with the same orientation inside the grains, along with an increase in the relative quantity of low-angle grain boundaries. Furthermore, the volume fraction and size of retained austenite show a monotonous increase with the rise in the temperature of the cyclic heat treatment, and the rate of increase becomes notably larger when the temperature is raised from 950 °C to 980 °C. Based on the observed microstructural changes, the variation in the mechanical properties of the 18Ni(C250) maraging steel was analyzed. Specifically, as the cyclic heat treatment temperature increases, the tensile strength of the 18Ni(C250) maraging steel initially increases and then stabilizes, while the elongation and fracture toughness exhibit a monotonic increase.

The evolution of microstructures and mechanical properties with tempering temperature of a novel 2.5 GPa grade ultra-high strength steel with synergistic precipitation strengthening was investigated. With increasing tempering temperature, the experimental steel initially progressed from ε-carbides to M₃C and then to M₂C, followed by further coarsening of the M₂C carbides and β-NiAl. Concurrently, the martensite matrix gradually decomposed and austenitized. The ultimate tensile strength and yield strength initially increased and subsequently decreased with rising tempering temperature, reaching peak value at 460 and 470 °C, respectively. Conversely, the ductility and toughness initially decreased and then increased with rising tempering temperature, reaching a minimum at 440 °C. The increase in strength was attributed to the secondary hardening effects resulting from carbide evolution and the precipitation of β-NiAl. The subsequent decrease in strength was due to the recovery of martensite and coarsening of precipitates. The decrease in ductility and toughness was linked to the precipitation of M₃C, while their subsequent increase was primarily attributed to the dissolution of M₃C and an increase in the volume fraction of reverted austenite. The high dislocation density of martensite, the film of reverted austenite, nanoscale M₂C carbides, and ultrafine β-NiAl obtained during tempering at 480 °C resulted in the optimal mechanical properties of the experimental steel. The strength contributions from M₂C carbides and β-NiAl were 1081 and 597 MPa, respectively.

As a high-performance lightweight structural material with superior strength, Ti55531 titanium alloy has been widely adopted in critical load-bearing components such as landing gears and airframe frames in the aerospace sector to achieve significant weight reduction. However, when the tensile strength of Ti55531 exceeds 1250 MPa, the fracture toughness typically falls below 50 MPa·m^1/2. In this study, we addressed this challenge by precisely controlling the cooling rate during β annealing heat treatment. Through careful regulation of the cooling rate from the high-temperature β phase region to the aging temperature region, the Widmanstätten structure was successfully introduced into the Ti55531 titanium alloy. The experimental results demonstrate that this microstructure achieves a high tensile strength of 1252 MPa at a cooling rate of 2.5 °C/min, while simultaneously improving the elongation and fracture toughness to 9% and 84 MPa·m^1/2, respectively. Microstructural analysis reveals that the basket-weave structure plays a crucial role in maintaining high strength. Meanwhile, the Widmanstätten structure effectively increases the energy required for crack extension by resisting crack propagation and altering the crack propagation path, thus significantly enhancing fracture toughness. These findings offer a promising pathway for overcoming the traditional trade-off between strength and toughness in high-performance titanium alloys.

This study investigated the high-temperature tensile durability of G115 steel after aging at 650 °C for different periods of time using a high-temperature tensile testing system. The results show that with the increase in aging time, the microstructure of G115 steel changes to a certain extent, which shows that as the martensite lath width and HAGB increases, the dislocation density decreases and the second precipitate shows obvious growth. As the tensile test temperature increases, the tensile strength decreases from ~750 MPa to ~350 MPa. The effect of aging time on the high-temperature tensile durability of steel after a 3000 h aging process is not significant, which should be related to the relatively high microstructure thermal stability of G115 steel during the aging process.