arXiv subjects

Sashank Shivakumar

Publications and source records attributed to Sashank Shivakumar.

3 recordsLinked to original sources

arXiv2026

Fabrication of Dense Ultrafine-Grained MoW, MoWNb, and MoWNbTa Alloys: Influence of Cobalt Doping on Sintering and Grain Growth

Dense ultrafine-grained (UFG) refractory MoW, MoWNb, and MoWNbTa alloys were fabricated by combining high-energy ball milling (HEBM) and spark plasma sintering (SPS), achieving ~92-96% relative densities and ~70-180 nm grain sizes. The effects of 2 at.% cobalt (Co) addition on sintering behavior and high-temperature grain growth resistance were investigated as a function of compositional complexity. Activated sintering was observed, with 2 at.% Co addition increasing relative densities from ~92-96% to ~96-98%. Isothermal grain growth experiments at 1200 {\deg}C and 1300 {\deg}C showed that Co doping suppressed the relative grain growth rate, despite a modest initial grain size increase due to Co-activated sintering, with the effect becoming more pronounced in compositionally complex alloys. The observed trend is consistent with the recently proposed high-entropy grain boundary (HEGB) effect. Notably, Mo24.5W24.5Nb24.5Ta24.5Co2 achieved a 96.4% relative density and maintained an ultrafine grain size, increasing only slightly from ~122 nm to ~127 nm after 5 h annealing at 1200 {\deg}C. Scanning transmission electron microscopy (STEM and energy-dispersive X-ray spectroscopy (EDS) confirmed strong Co segregation at grain boundaries, accompanied by minor depletion of Ta and W, supporting a recently proposed grain boundary segregation model for high-entropy alloys and HEGBs.

cond-mat.mtrl-sci↗

arXiv2022

High-temperature stable refractory high-entropy nanoalloys with enhanced sinterability

Nanocrystalline alloys (nanoalloys) are prone to grain growth. It is known that grain boundary segregation and precipitation can stabilize nanoalloys, but the stabilization becomes less effective at high temperatures and adding grain growth inhibitors often reduces sinterability. Herein, we have simultaneously achieved improved sinterability and exceptional high-temperature stability for a class of MoNbTaTiW-based refractory high-entropy nanoalloys (RHENs). Bulk pellets of RHENs were fabricated through planetary ball milling and spark plasma sintering, achieving 93-96% relative densities with 50-100 nm grain sizes for three compositions. For example, Mo17.8Nb17.8Ta17.8Ti17.8W17.8Ni6Zr5 sintered at 1300 {\deg}C attained ~96% relative density with ~55 nm mean grain size. Moreover, these RHENs exhibited exceptional stability at 1300 {\deg}C. Both Ti17.8Nb17.8Mo17.8Ta17.8W17.8Ni6Zr5 and Mo18.8Nb18.8Ta18.8Ti18.8W18.8Ni6 retained <150 nm grain sizes with >96% of the theoretical densities after five hours annealing at 1300 {\deg}C. Notably, the addition of Ni, a well-known sintering aid for activated sintering of refractory metals such as W and Mo, in high-entropy MoNbTaTiW can promote sintering while maintaining high-temperature stability against rapid grain growth, which can be explained by hypothesized effects of high-entropy grain boundaries. These RHENs possess some of the highest temperature stability achieved for nanoalloys and ultrafine-grained metals.

cond-mat.mtrl-sci↗

arXiv2021

A New Type of Compositionally Complex M5Si3 Silicides: Cation Ordering and Unexpected Phase Stability

A new type of compositionally complex (medium- or high-entropy) M5Si3 silicides is synthesized. Both (V1/5Cr1/5Nb1/5Ta1/5W1/5)5Si3 and (Ti1/5Zr1/5Nb1/5Mo1/5Hf1/5)5Si3 form single-phase homogenous solid solutions. Notably, (V1/5Cr1/5Nb1/5Ta1/5W1/5)5Si3 forms the hexagonal gamma (D88) phase, while all its five constituent binary silicides, V5Si3, Cr5Si3, Nb5Si3, Ta5Si3, and W5Si3, are stable in the tetragonal alpha (D8l) or beta (D8m) phases. Annealing at 1600C demonstrates that this hexagonal gamma phase is stable. Comparison of the experimental and calculated X-ray diffraction patterns, Rietveld refinements, and analysis of aberration-corrected scanning transmission electron microscopy high-angle annular dark-field images suggest cation ordering, which reduces the configurational entropy. This work expands the field of high-entropy and compositional complex ceramics by not only discovering a new compositional complex silicide phase but also demonstrating the cation ordering and unusual phase stability. These compositionally complex silicides can be combined with refractory high-entropy alloys to make the high-entropy counterparts to the Nb-silicide and Mo-Si-B composites.

cond-mat.mtrl-sci↗