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

TL;DR

This study demonstrates that cobalt doping enhances densification and suppresses grain growth in ultrafine-grained MoW-based refractory alloys, with evidence of Co segregation at grain boundaries supporting high-entropy grain boundary effects.

Contribution

It introduces the effect of cobalt doping on sintering and grain growth in complex MoW-based alloys, highlighting the role of high-entropy grain boundary effects and segregation phenomena.

Findings

Cobalt addition increases relative density during sintering.

Cobalt doping suppresses grain growth at high temperatures.

Strong cobalt segregation at grain boundaries observed.

Abstract

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.