Microstructural Analysis of Mechanically-Alloyed Tungsten-Titanium-Chromium and Carbon-Based Ultra-High Temperature Ceramics

Rising global energy demands, in conjunction with increased awareness of the climate crisis, have instituted a need for reliable, clean energy, which can be sourced through nuclear fusion power. However, it is necessary to develop materials resistant to the extreme radiation doses and high-temperature environments intrinsic to nuclear fusion reactors. Previous research has shown that a material’s microstructural properties such as grain size and grain boundary density can impact its ability to shield against neutron irradiation. Hence, the objective of this research is to analyze the microstructural characteristics of two candidate material classes fabricated through direct current sintering: firstly, ultra-high temperature ceramics (UHTCs) tungsten carbide (WC) and vanadium carbide (VC) with and without silicon carbide (SiC) additives, and secondly, mechanically-alloyed tungsten-titanium-chromium (W-Ti-Cr) with varying levels of chromium concentration.<br/><br/>Four UHTC specimens (WC, WC with 4% SiC by concentration, VC, and VC with 4% SiC by concentration) and three W-Ti-Cr specimens (W–Ti–Cr with 10% Ti by concentration and 5%, 10%, and 15% Cr by concentration) were sintered. To prepare for microstructural characterization, the samples were sequentially polished with SiC polishing paper of grits P800, P1200, and P2400, then fine polished with diamond suspension solutions of sizes 15 microns, 9 microns, 3 microns, and 1 micron. The samples were then ultrasonicated in isopropanol for 5 minutes and analyzed using X-ray diffraction (XRD) with a Copper K-α source, and atomic force microscopy (AFM) at a 5 μm by 5 μm field of view. XRD results were further examined using TOPAS (Bruker) Rietveld refinements for quantitative phase analysis, while AFM images were segmented using Trainable Weka Segmentation in FIJI and Watershed Segmentation in Gwyddion to determine mean grain size.<br/><br/>XRD analysis revealed the UHTC specimens were primarily single-phase, while the W-Ti-Cr samples were multi-phase. Both specimen sets contained minor surface contamination from the SiC polishing. TOPAS refinements indicated two trends: in the UHTC specimens, similar lattice parameters and coherent mean crystallite sizes were found between the WC and VC and their SiC-containing counterparts, while in the W-Ti-Cr specimens, increased chromium concentration corresponded with systematic lattice contraction with decreased mean crystallite sizes. Within the W-Ti-Cr specimens, diffraction peaks from a chromium phase were not observed, indicating that the chromium was effectively incorporated within the tungsten host matrix as a substitutional dopant and potentially concentrated at grain boundaries. AFM segmentation revealed consistent mean grain sizes across the UHTC samples, while lateral force imaging showed the segregation of chromium at W-Ti-Cr grain boundaries with clear depressions in force traces across.<br/><br/>Our results indicate the stability of the UHTC microstructure and the incorporation of chromium within the W-Ti-Cr host matrix. In future studies, the stability of the grain size and chemical alloying distribution of our samples after neutron irradiation and high-temperature exposures will be investigated, electrical properties of the W-Ti-Cr specimens will be measured using kelvin probe force microscopy, and XRD and AFM methods will be utilized to quantify changes post-exposure.<br/><br/>We acknowledge the Louis Morin Charitable Trust for funding.