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Interrupted aging and creep tests were performed in the condition of 650 °C/70 MPa for P91 steel. The mechanisms of hardness and microstructure evolution during aging and creep were studied through hardness test, SEM/EBSD and TEM/EDS. By comparing the difference between aging and creep, the coupling effect of temperature and stress in P91 steel was elucidated. Results show that creep stress could accelerate the entire degradation process in P91 steel. Firstly, the dislocation density increased rapidly at the beginning of creep, and then decreased also at a high rate after reaching its peak value. On the other hand, the dislocation density decreased slowly in aging condition. Additionally, with the migration of grain boundaries under creep stress, some intergranular M23C6 entered inside grains and afterwards dissolved into matrix. On the other hand, other M23C6 which remained intergranular continuously ripened and congregated with time, and finally turned into large ones at triple junctions of grain boundaries. Furthermore, stress had the greatest impact on dislocation density, whereas temperature affected volume fraction of M23C6 the most. In creep condition, the degradation of mechanical properties of P91 steel was mainly caused by the reduction of dislocation strengthening which was followed by grain boundary strengthening.

[Display omitted] • More serious degardation happened on P91 steel during 650 °C/70 MPa creep than the one during aging. • Evolution of dislocation during creep was quite different from the one during aging. • The mechanism of M 23 C 6 evolution under stress and high temperature (creep) was elucidated. Mechanism of M 23 C 6 evolution during creep was elucidated. • Contribution of each strengthening mechanism was calculated and analyzed in both creep and aging conditions. Interrupted aging and creep tests were performed in the condition of 650 °C/70 MPa for P91 steel. The mechanisms of hardness and microstructure evolution during aging and creep were studied through hardness test, SEM/EBSD and TEM/EDS. By comparing the difference between aging and creep, the coupling effect of temperature and stress in P91 steel was elucidated. Results show that creep stress could accelerate the entire degradation process in P91 steel. Firstly, the dislocation density increased rapidly at the beginning of creep, and then decreased also at a high rate after reaching its peak value. On the other hand, the dislocation density decreased slowly in aging condition. Additionally, with the migration of grain boundaries under creep stress, some intergranular M 23 C 6 entered inside grains and afterwards dissolved into matrix. On the other hand, other M 23 C 6 which remained intergranular continuously ripened and congregated with time, and finally turned into large ones at triple junctions of grain boundaries. Furthermore, stress had the greatest impact on dislocation density, whereas temperature affected volume fraction of M 23 C 6 the most. In creep condition, the degradation of mechanical properties of P91 steel was mainly caused by the reduction of dislocation strengthening which was followed by grain boundary strengthening. [ABSTRACT FROM AUTHOR]

[Display omitted] • A method utilizing sol–gel assisted carbothermal reduction was developed to facilitate the synthesis of multi-branched HfB 2 with high crystallinity and aspect ratio. • By modulating the atomic molar ratio of Hf: B: C in the reactants, the aspect ratio of HfB 2 microrods can be tailored from 9.52 to 12.68. • Oriented attachment and reactive templated growth of the growth mechanisms of HfB 2 microrods would guiding significance for one-dimensional growth of ceramic powders. • The fracture toughness of the sample was significantly enhanced upon the addition of 6 wt% HfB 2 microrods, increasing from 3.98 MPa m1/2 to 7.24 MPa m1/2. Development of strategies to toughen HfB 2 ceramic, improve their mechanical properties is the key to promote their applications in extreme thermal environments. This study explores a kind of self-toughening method of HfB 2 ceramic by incorporation the highly crystalized multi-branched HfB 2 microrods. The multi-branched HfB 2 microrods were synthesized by sol–gel process and carbothermal reduction, exhibiting length of 6–13 μm and diameter of 0.60–1.50 μm, respectively. Detailed analysis of the formation mechanism of HfB 2 microrods reveals that the HfB 2 multi-branched structure is a collaborative consequence of orientation attachment mechanism and reactive templates growth, in which process monoclinic HfO 2 crystals transform into crystalized HfO 2 microrods at ∼1000 °C, and worked as template to induce one-dimensional HfB 2 microrods. Furthermore, spark plasma sintering experiments show that the optimal mechanical properties were attained by 6 wt% addition of HfB 2 microrods, comparing with HfB 2 ceramics free of self-toughening agent, the hardness and fracture toughness of self-toughened HfB 2 ceramic improved by 8.82 % and 81.91 % respectively, demonstrating the effectiveness of self-toughening of the microrods without sacrificing the hardness of HfB 2 ceramic. Thus, it can be seen that the work has important reference value for the synthesis of HfB 2 powder with different morphologies and the toughening of HfB 2 ceramics. [ABSTRACT FROM AUTHOR]

[Display omitted] • Adding nanoparticles boosts ARMOX 500 T's weld heat-affected zone hardness by up to 16.6 % and 5.28 %. • Among the three applied nanoparticles, tungsten carbide 8% exhibited the highest hardening effect. • Nanoparticle flux induces arc plasma contraction and deflection, refining microstructure. Given the direct correlation between the hardness of high-hardness armor (HHA) steel and its ballistic performance, mitigating softening in the heat-affected zone (HAZ) becomes essential. In this study, welding was performed using a nanoparticle flux composed of Tungsten Carbide (WC), Titanium Carbide (TiC), Silicon Carbide (SiC) nanoparticles, and ethanol. Subsequently, the shape and hardness distribution of each weld bead were comprehensively analyzed. The analysis revealed an increase in penetration depth in the order of 0.37 mm for WC 8 %, 0.31 mm for SiC 8 %, and 0.12 mm for TiC 2 %. Furthermore, the hardness evaluation results demonstrated a significant improvement at the fusion line (FL) when nanoparticle flux was utilized. Specifically, hardness values of 654.0 HV and 590.4 HV were observed at 8 % WC and 8 % SiC, respectively, representing increases of 16.6 % and 5.28 % compared to the absence of nanoparticle flux. Additionally, further investigations were conducted to elucidate the mechanism behind the improvement in hardness. This experimental evidence confirms the effectiveness of incorporating WC 8 % and SiC 8 % nanoparticles into the welding process of ARMOX 500 T in preventing hardness degradation due to softening. [ABSTRACT FROM AUTHOR]

[Display omitted] • We demonstrate a new way of inducing local recrystallization, using a selective laser-based heat treatment technique. • Synchrotron X-ray diffraction investigates recrystallization kinetics in cold-rolled 316L steel after such laser treatments. • The evolution of full width at half maximum of diffraction peaks can be directly correlated to the successive recrystallization stages. • Recrystallization stages were characterized by Transmission Electron Microscopy and Scanning Electron Microscopy. Laser based heat treatments have multiple applications for local tuning of microstructures. They are particularly relevant in the context of laser-based additive manufacturing (LAM), in order to relieve residual stresses or introduce equiaxed structures through recrystallization. In 316L, high stored energy inherited from the high cooling rates of laser processing creates strong crystallographic textures and stable cellular walls, which challenges the possibility of inducing fast recrystallization. In a first step, we perform here a laser heat treatment on a highly deformed 316L steel, and show that in-situ X-ray Diffraction (XRD) is a viable tool for the monitoring of fast recrystallization. The evolution of Bragg peak width over time is connected to the evolution of dislocation density and grain size, thanks to Electron Back-Scattered Diffraction (EBSD) and Transmission Electron Microscopy (TEM) characterization. X-ray signals are shown to reveal different stages of recrystallization, in particular the development of dislocation cells and the onset of nucleation. This study also demonstrates how local heat sources such as lasers may help designing composite-like materials for grain size, texture and hardness optimization. [ABSTRACT FROM AUTHOR]