Crystals

Nickel-based superalloys are extensively used in fabricating high-temperature gas turbine components, owing to their superior high-temperature strength, excellent structural stability, and remarkable hot corrosion resistance. The influence of impurity sulfur content on their hot corrosion performance is a core scientific issue in hot-end component compositional design and smelting. This study investigated chromium (Cr)-rich nickel-based superalloys with sulfur (S) contents of 3 ppm, 16 ppm, and 42 ppm via XRD, SEM, and an EPMA, focusing on their hot corrosion behavior under a 100% Na₂SO₄ deposit at 900 °C. The results indicated that their hot corrosion products were basically identical, forming a Cr-dominated outer oxide layer rich in Ti, Co, and Ni, an Al₂O₃-based inner corrosion zone, and a CrS_x-dominated sulfide layer. With increasing sulfur content, the outer layer thickness decreased from approximately 30 μm to less than 20 μm, pores in the outer oxide layer increased in quantity and size, and internal sulfides and nitrides accumulated. The average depth of spallation increased from 55 μm for the S3 alloy to 80 μm for the S16 alloy, with the S42 alloy showing even more extensive spallation. The alloy’s hot corrosion performance deteriorated notably with increasing S content. The mechanism of sulfur’s effect on hot corrosion behavior is that sulfur in the alloy segregates at oxide film defects, enhancing defect stability and increasing their quantity and size. These defects serve as rapid diffusion channels for corrosive media, thereby accelerating the alloy’s hot corrosion rate.

We present a modular building block strategy for synthesizing phosphonated polyaromatic systems as an alternative to the conventional late-stage phosphonation of prefabricated aromatic scaffolds, which often requires harsh conditions and has limited tolerance for functional groups. A monophosphonated biphenyl building block was obtained via nickel-catalyzed phosphonation of dibromobiphenyl at 170 °C for three hours. This synthesis is more economical and milder than typical high-temperature palladium systems. In parallel, a borated pyrene derivative was prepared by Suzuki–Miyaura borylation. The final palladium-catalyzed Suzuki cross-coupling reaction produced the target compound, octaethyl(pyrene-tetrakis(biphenyl))tetrakis(phosphonate), Et₈-PyTPPE. Single-crystal X-ray diffraction reveals a centrosymmetric molecule that crystallizes in the triclinic space group P–1, with the inversion center located at the central C–C bond of the pyrene core. The pyrene unit is essentially planar, while the biphenylphosphonate arms are highly twisted relative to the core and to each other. The crystal packing is dominated by weak intermolecular interactions, and no significant π–π stacking is observed. Hirshfeld surface analysis shows that H···H (60.5%) and C···H (22.5%) contacts predominate, while O···H interactions (14.4%) with phosphoryl oxygen atoms represent the most relevant directed contacts. From photophysical investigations, Et₈-PyTPPE exhibits blue fluorescence (λ_em. = 452 nm) in solution and aggregation-induced red-shifted emission with nanosecond lifetimes in the solid state, confirming purely fluorescent behavior.

Additive manufacturing (AM) of Ti-6Al-4V alloy is widely used in aerospace and medical fields due to its excellent strength and corrosion resistance. However, the microstructural heterogeneity induced by the AM process often results in fatigue properties inferior to those of their forged counterparts. Synchrotron Radiation Computed Tomography (SR-CT) was employed to conduct an in situ three-dimensional investigation of fatigue damage evolution in Ti-6Al-4V alloy fabricated via laser powder bed fusion (LPBF). Experimental results revealed phenomena of crack bridging and deflection, accompanied by the consistent presence of local high-density zones (LHDZs) throughout the fatigue damage progression. Combined with quantitative analysis of crack propagation rates, the influence of LHDZs on fatigue damage evolution was analyzed, and the relationship between AM processes, LHDZs, and fatigue damage was discussed. The results indicate that the basket-weave α-phase microstructure in Ti-6Al-4V prepared by LPBF exhibits a high correlation with the distribution of LHDZs, and the orientation of LHDZs aligns with the crack propagation direction. By adjusting process parameters such as cooling rate and temperature gradient, the formation of LHDZs can be modified, thereby influencing the fatigue properties of the material. This provides theoretical support for achieving process optimization of the fatigue properties of Ti-6Al-4V alloy prepared via LPBF.