Materialia最新文献

Sulfur is considered an unfavorable element in metallic materials because of its potential to cause embrittlement, thereby prompting its removal. However, trace amounts of S impurities can influence the precipitation of the primary α-Cr strengthening phase in the heat-resistant 50Ni–30Cr–0.8Ti–6W–Fe alloy, and thus affect its creep strength. In this study, fine Ti₂S particles were observed in heat-treated 50Ni–30Cr–0.8Ti–6W–Fe alloys through high-sensitivity, high-resolution, and wide-field element analysis using aberration-corrected scanning transmission electron microscopy. The Ti₂S particles were distributed linearly with the adjacent α-Cr precipitates. The Ti₂S precipitates functioned as nucleation sites for α-Cr, thereby refining and increasing the hardness of the alloy. The findings of this work challenge conventional approaches to material design and emphasize the significance of design based on fundamental principles.

This study entails the fabrication of two oxide-dispersion strengthened (ODS) Hastelloy-N (HN) alloys utilizing divergent methods. The first alloy was synthesized using cryogenic attritor milling coupled with spark plasma sintering (SPS), while the second was produced via room temperature attritor milling and SPS. The ODS HN alloy derived from cryogenic milling demonstrated superior strength relative to its commercial-grade counterpart. Conversely, the alloy produced through room temperature milling exhibited lower ultimate tensile strength (UTS), attributed to manufacturing defects and the precipitation of Zr at grain boundaries. Corrosion resistance in molten FLiNaK for both ODS samples was found to be inferior compared to commercial HN. Particularly, in the room temperature-milled ODS HN, Zr present at grain boundaries appeared to dissolve more readily than in cryogenic or commercial samples, facilitating enhanced penetration by molten salt. The cryogenically-milled ODS HN contained Zr, yet it was not segregated to grain boundaries. Although the homogeneously dispersed Mo-based compound in the cryogenically-milled ODS HN augmented mechanical properties, it also accelerated corrosion propagation beyond that of the commercial-grade alloy.

Reactive spark plasma sintering of ternary Ti-Si-C system was performed using three different powder precursors systems 3Ti/Si/2C, 3Ti/SiC/C and 2Ti/TiC/Si, to explore the fundamental physics behind Ti₃SiC₂ MAX phase formation, its stability and microstructure development, and, finally linked with its hardening and contact induced damage tolerance. Phase evolution in Ti-Si-C system is a complex phenomenon, and, present experimental conditions never yield a phase pure Ti₃SiC₂ MAX phase, rather results in varying volume fractions of Ti₃SiC₂-(Ti_x,Si_1-x)C solid solution due to non-equilibrium processing conditions exerted by SPS processing which restricts coherent site specific diffusional jumps and promotes the formation of (Ti, Si)C solid-solution instead of well reported non-stoichiometric TiC_x. 3Ti/SiC/C precursor was the best candidate for processing composite with highest yields of Ti₃SiC₂. Phase evolution is guided by the free energy of formation of different phases and chemical affinity amongst the constituent elements rather than the equilibrium phase diagram of the Ti-Si-C system. Presence of free carbon, low temperature liquid phase and slow heating rate are the key requirements for forming phase pure Ti₃SiC₂, where excess free carbon reduces the stability of Ti₃SiC₂ via decarburization. Non-equilibrium processing conditions impart nano-precipitation of coherent hexagonal Ti₃SiC₂ precipitates within a cubic (Ti, Si)C matrix with a distinct orientation relation of (220)_matrix ║(0004)_precipitate and <114>_matrix ║<2–1–10>_precipitate that has never been reported, instead of growing highest density plane of hcp-on-fcc matrix. Coherency strain and fine interlocking microstructure of the as-processed composite experiences ≈36 % of enhancement in hardness followed by an improved contact damage for the as-processed composite.

Advanced cladding is critical for fast reactors with the adequate thermal conductivity, mechanical stability and radiation tolerance of the cladding base material, corrosion resistance and high temperature coolant compatibility of the cladding surface, and chemical stability of the cladding inner wall against fuel cladding chemical interaction (FCCI). The preliminary results of recent ion irradiation studies of two diffusion-couple samples of cerium (Ce)/oxide-dispersion strengthened steel (ODS) and Ce/TiN/ODS, irradiated with 80 MeV xenon (Xe) ions to 100 displacements per atom (dpa) at 500°C, are summarized. Significant Ce-Fe interaction occurred in the Ce/ODS sample, and no noticeable Ce-Fe interaction was found in the Ce/TiN/ODS sample. It shows the effectiveness of 1-µm TiN diffusion barrier coated by the pulsed laser deposition on suppressing Ce-Fe interaction, a major contributor to FCCI in cladding. Density function theory (DFT) calculations of the impurity diffusivities of Ce and Fe within the Ti sublattice of TiN were performed to assist a mechanistic understanding of the experimental results.

This study elucidated the microstructure of the Fe₄N (γ’) compound layer in specimens subjected to the nitriding to the specimens with various finished surfaces. Gyrofinishing produces a specimen surface with an ultra-fine-grained (UFGed) structure and a rough surface, whereas the buff-finished specimen exhibited a coarse grain but smooth surface. The γ’ grains formed on the gyrofinished specimens after nitriding were considerably finer and more equiaxial compared with those on the buff-finished specimen. The γ’ nucleated from the grain boundary in the matrix, indicating that the grain boundaries act as a nucleation site. Therefore, the UFGed structure promotes the nucleation of γ’ during the nitriding, resulting in fine grains. Numerous pores were formed in the γ’ layer of the gyrofinished specimen owing to the rough surface. The UFGed structure and surface roughness before nitriding are key factors for controlling the microstructure in the γ’ compound layer.

The properties of crystalline materials often depend on directionality and operating conditions. Specifically, the strength of materials can depend anisotropically on crystal direction and the loading condition. To probe these effects, a preliminary series of high strain-rate ($>10^5$/s) strength plate-impact hole closure experiments were performed on high purity single crystal tantalum cubes. The orientation of the single crystals with respect to impact/loading were varied to provide data to inform crystal plasticity modeling efforts. The experiments consist of in-situ high-resolution X-ray radiographic imaging of the hole collapse under dynamic compression conditions to infer the material strength via its resistance to closure at increasing levels of plastic strain. The experiments are compared against hydrocode simulation predictions. A comparison with simple elastic perfectly plastic strength model predictions is presented to elucidate the response of the different crystal orientations at high strain-rate and large plastic strains.

Zirconia is an engineering ceramic material with excellent comprehensive properties. However, it suffers from the inherent disadvantage of low fracture toughness. To improve its fracture toughness, Gd₂O₃ is an effective stabilizer. In the present study, Gd₂O₃–ZrO₂ composite powders were synthesized using the Mist CVD method. Following synthesis, these powders were pressed through spark plasma sintering. The crystallinity of the Gd₂O₃–ZrO₂ powders improved as the deposition temperature increased from 600°C to 900°C. The tetragonality (c/$\sqrt{2}$a) of the Gd–TZP composites increased from 0.99941 to 1.01571 as the Gd₂O₃ content increased from 2 mol% to 4 mol%, but it decreased when the content reached 5 mol%. The Gd₂O₃–ZrO₂ nanoparticles produced via the Mist CVD approach presented a unique hollow spherical structure. Under moist chemical vapor deposition (CVD) conditions with 4 mol% Gd₂O₃ at 1400°C, the Gd–TZP composites exhibited fracture toughness and hardness values of approximately 12.03 ± 0.15 MPa·m^1/2 and 12.16 ± 0.17 GPa, respectively.

Viscosupplementation by intra-articular injections of hyaluronic acid (HA) is used to treat symptomatic osteoarthritis. Exogenous HA remains in the joint cavity for a short period of time (days) while claimed pain relief period lasts over months. There is a clear lack of understanding of viscosupplementation mechanism of action. Here, we hypothesize that HA penetrates the cartilage and contributes to the restoration of its mechanical quality.

Confocal microscopy and bio-indentation were used to confirm HA penetration into cartilage and modulation of cartilage quality. Bio-indentation was performed on rat distal femurs incubated overnight in HA solutions, using phosphate buffered saline (PBS) as control. For this proof-of-concept evaluation, measurements of elastic modulus (MPa) and of maximal force (µN) were recorded before and immediately after exposure with HA, as well as after an additional washout with PBS. Cartilage thickness at the site of indentation was evaluated by contrast enhanced computed tomography with an ionic contrast agent. Indentation depths were located in the upper part of hyaline cartilage. Ostenil®, commercial product containing 1 % HA, induced a decrease in indentation depth in the range of forces influencing the whole cartilage thickness, together with an increase of the elastic modulus. Then, bio-indentation and size distribution of HA chains via SEC-SLS were assessed for a range of commercially available products. The results showed higher modulation of cartilage quality in the presence of 0.25–1 MDa HA chains.

The present in vitro study suggests that HA modulates cartilage quality and might thus explain the long-term beneficial effect of viscosupplementation.

Superlubricity, or near zero friction is a highly desired lubrication state for a wide range of practical applications. Although such application scenarios often involve complex contact geometries, solid lubricant technologies, including previous efforts on achieving superlubricity, are almost entirely in linear sliding test conditions. This report demonstrates an experimental pathway to yield superlubricity in rolling-sliding contact conditions using solid-lubricant materials. Ti₃C₂X based solid lubricant was tested under complex sliding-rolling conditions at engineering-significant contact pressures. The material's compression and inter-layer shearing result in material reconstruction to pose superlubricity. High-resolution transmission electron microscopy analysis, complemented by multi-scan Raman spectroscopy showed the formation of a robust amorphous tribolayer. This demonstration is expected to not only advance the applied aspects in the development of oil-free solid lubricants but also push the boundaries of fundamental understanding of materials’ structure-property relations across physical states.