The fatigue crack propagation behavior of an experimental fully ferritic high-chromium steel HiperFer 17Cr2 was investigated at elevated temperatures of 650 °C and 675 °C at loading frequencies of 20, 5, and 0.05 Hz, motivated by the demand for advanced high-temperature materials capable of improving the thermodynamic efficiency of future thermal energy conversion systems and reducing greenhouse gas emissions. The widely used 9Cr-1Mo-V-Nb ferritic-martensitic steel P91 was examined in parallel at 650 °C for benchmarking purposes. Complementary microstructural analyses were performed to characterize frequency- and temperature-dependent damage mechanisms. At 650 °C, the stress intensity required for the initiation of crack propagation was substantially higher in HiperFer 17Cr2 than in P91 across all tested frequencies. Furthermore, crack growth rates were up to half an order of magnitude lower in HiperFer 17Cr2. At 675 °C, frequency-dependent damage mechanisms were identified, including dynamic recovery, subgrain formation, and pipe diffusion-assisted redistribution of Cr and Nb, promoting formation of the metastable C14 Cr2Nb Laves phase at grain and sub-grain boundaries. These precipitates effectively impeded crack progression, while crack-tip blunting reduced the local driving force for crack propagation. The results indicate that HiperFer 17Cr2 is suitable for continuous service at 675 °C under high-cycle fatigue conditions in the frequency range from 5 to 20 Hz.
Dynamic thermo-mechanical stresses caused by sudden temperature changes and molten steel impact, etc., accelerate the degradation of Al2O3-C refractories during service. To investigate the dynamic degradation behavior, dynamic mechanical tests were conducted using the Split Hopkinson Pressure Bar (SHPB), systematically examining the effects of partial substitution of flake graphite by expanded graphite and thermal degradation. The results show that the Al2O3-C refractories exhibit a significant strain-rate hardening effect, with strength increasing with impact velocity and the failure mode progressively transitioning from crack propagation to pulverization. Cyclic prolonged thermal exposure to 1500 °C contributes to the SiC whiskers formation and densification, and results in the increase strength and brittleness. The phenomenon of specimen after 5 cycles having the optimal impact resistance proves the both the strength and energy dominated failure process. The introduction of expanded graphite effectively suppresses crack propagation and enhances energy dissipation capacity through interlayer sliding and stress buffering related to the myrmekitic texture, which provides a rationale for the development of low-carbon materials.
In the present study, lamellar hydrates were designed via curing regimes and additives. Structural and high-temperature fracture behavior analyses were employed to elucidate the influence of initial lamellar hydrates on microstructural evolution and thermal stress resistance at elevated temperatures. Key findings reveal that: Pure CAC cured at 25 °C for 24 h predominantly forms metastable CAH10 and C2AH8, whereas under the two-step curing regime, the hydration products are granular C3AH6 and lamellar AH3. Incorporating CaCO3 and MgO under two-step curing promotes the simultaneous generation of micro/nano-lamellar C4AcH11 and Mg-Al Hydrotalcite (M-A-H). The enhanced extent of hydration and pore-filling effect of M-A-H refines matrix porosity, increasing the volume fraction of 5–100 nm pores and elevating the fractal dimension (Ds). This microstructural optimization improves bonding strength, as evidenced by an 82% increase in demolding strength in the designed samples compared with the R samples. C4AcH11 and M-A-H serve as reactive CaO and MgO sources, respectively, facilitating the interlocking distribution of in-situ CA6 and MgAl2O4 at 1600 °C and optimizing pore structure. The hierarchical pore structure and refined crystals synergistically enhance thermal stress resistance by increasing crack deflection, dissipating energy, and improving plastic deformation capacity.
Driven by the goals of carbon peaking and carbon neutrality, the resource utilization of solid waste has attracted considerable attention due to its scientific importance and environmental benefits. For this purpose, MgO-MgAlON-BN refractories with enhanced thermal shock and slag resistance were designed and successfully fabricated by introducing BN solid waste as an additive. The results indicate that the sample containing BN additives exhibits superior overall performance compared with those without additives. At a BN content of 3.26 wt.%, the cold compressive strength increases by 21.20%, and the contact angle at 1723 K increases by 50.01%. Further analysis reveals that the improved wetting behavior is mainly attributed to the introduction of BN additives, which promote the formation of numerous micro-convex structures. These structural characteristics restrict slag infiltration and inhibit slag penetration into the refractory matrix, thereby enhancing slag resistance. Consequently, a cost-effective and feasible reinforcement strategy for MgO-MgAlON-BN refractories is proposed, providing a promising pathway for the high-value utilization of solid waste resources.
The high-temperature gas-cooled reactor (HTGR), as a preferred reactor type for Generation IV nuclear energy systems, widely employs nuclear-grade graphite in its core as the moderator, structural material, and matrix of fuel elements. The friction coefficient between graphite components and the contact stiffness of spherical fuel elements directly affect the structural integrity of the core, the flow characteristics of the pebble bed, and the design of the fuel handling system, serving as critical mechanical parameters for ensuring reactor safety and economic operation. This paper systematically reviews the preparation processes and multi-scale microstructural features of nuclear graphite for HTGRs and summarizes the current research status in two directions: the friction coefficient and the contact stiffness of nuclear graphite. In the field of tribology, the influencing mechanisms of intrinsic factors such as grain size and porosity, as well as extrinsic factors such as ambient atmosphere, temperature, load, and sliding velocity, on the friction behavior of graphite are analyzed with emphasis. The controversies and applicable conditions of the surface energy mechanism and the dangling bond mechanism are discussed. In terms of contact mechanics, the development of asperity models from the Hertz contact and Abbott-Firestone (AF) model to the Kogut-Etsion finite-element based fitting is summarized, along with the evolution from the Greenwood-Williamson (GW) statistical model to the Majumdar-Bhushan (MB) fractal contact model. Experimental methods for measuring stiffness coefficients—namely, the interface displacement method, the impact method, and the acoustic method—are reviewed. On this basis, the severe scarcity of nuclear graphite mechanical data under in-core conditions, such as high temperatures and inert atmospheres, is highlighted. Two systematic experimental works dedicated to graphite materials for HTGRs are integrated: the first measurements of the static and dynamic friction coefficients of isostatically pressed nuclear graphite BG80 in a high-purity helium environment from 25 to 1300 °C, revealing a four-stage variation of the friction coefficient with temperature; and the first acquisition of the temperature dependence and irreversible evolution characteristics of the stiffness coefficient of spherical fuel elements in high temperature helium. These works fill the gaps in fundamental data and can provide quantitative parameter inputs and theoretical support for HTGR core design, pebble-bed flow simulations, and safety analyses.
Al2O3–TiO2–CaO-based ferrotitanium slag is a waste slag generated during ferrotitanium-alloy smelting. At present, the TiO2 resource (about 15 wt.%) in ferrotitanium slag has not been effectively utilized. In this study, aluminothermic reduction was used to extract Ti and prepare a Ti–Si alloy, while the low-density Al2O3–CaO-based molten tailing slag floating on the Ti–Si melt was separated to fabricate CA–CA2 tailing-slag cement. The TiO2 content decreased from 14.42 wt.% in the ferrotitanium slag to 1.63 wt.% in the tailing slag. The Ti–Si alloy was mainly composed of Ti5Si3 and Ti5Si4. The mineral composition and hydration behavior of the CA–CA2 tailing-slag cement were similar to those of commercial calcium aluminate cement Secar71. Under the adopted preparation and testing conditions, the mechanical strength of the tailing-slag cement paste reached approximately 85% of that of Secar71, indicating its potential as an alternative refractory binder. The co-production of Ti–Si alloy and CA–CA2 tailing-slag cement provides a potential route for the value-added utilization of ferrotitanium slag.
