Novel processing route for the fabrication of bulk high-entropy metal diborides

Abstract

A single high-entropy phase material with hexagonal structure is produced by a two-steps processing method. Elemental reactants are first remarkably converted by Self-propagating High-temperature Synthesis (SHS). The completion of the chemical transformation to the desired (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2 phase and its concurrent consolidation up to 92.5% relative density is achieved by processing the SHS powders at 1950°C via Spark Plasma Sintering. It is clearly evidenced that the use of the SHS technique is extremely beneficial to promote the formation of high-entropy ceramics, as compared to the time consuming ball milling treatment alternatively adopted.

Two‐step synthesis process for high‐entropy diboride powders

“…Heating to 2000°C also further reduced the oxygen content to 0.3 wt%, while the carbon content increased slightly to 0.3 wt% (Table ), presumably because diffusion of carbon from the graphite crucible and furnace became active at higher temperatures. The presence of oxide phases has been reported for HEB powders in previous studies. In the present study, (Hf,Zr,Ti,Ta,Nb)B 2 powders with higher purity were produced by the optimization of batch composition and reaction temperature resulting in lower oxygen (0.3 wt%) and carbon (0.3 wt%) contents than for previously reported processes.…”

“…The surface oxides present in commercial powders and additional oxide impurities induced during HEBM suppressed the densification of HEB ceramics. Tallarita et al produced (Hf,Mo,Ta,Nb,Ti)B 2 ceramics with a relative density of 92.5% by a two‐step process consisting of self‐propagating high‐temperature synthesis (SHS) followed by SPS at 1950°C. HfO 2 impurities, and unreacted B that remained in SHS powders, suppressed the densification of HEB ceramics.…”

“…Several process for the synthesis of HEB powders have been reported . Tallarita et al synthesized the precursors of (Hf,Mo,Ta,Nb,Ti)B 2 by self‐propagating high‐temperature synthesis (SHS) using elemental metal and B powders as the starting materials. HfO 2 impurities and unreacted B were detected in the synthesized powders and attributed to the uncontrolled SHS reaction.…”

“…1-3 ZrB 2 and HfB 2 are the most studied boride UHTCs due to their melting temperatures in excess of 3000°C, thermal conductivities that can be over 100 W/m K at 25°C, superior mechanical properties, and good chemical stability. [4][5][6] Recently, high-entropy ultra-high temperature ceramics (HE-UHTCs), including high-entropy boride (HEB), [7][8][9][10][11][12][13] high-entropy carbide (HEC), [14][15][16][17][18] and high-entropy nitride (HEN) 19 ceramics have been reported. HEB materials are solid solutions of individual transition metal diborides that have attracted increasing interest due to the high thermal and electrical conductivities, and chemical stability.…”

Synthesis of high‐purity high‐entropy metal diboride powders by boro/carbothermal reduction

“…3,13,14 As a member of new emerging high-entropy ceramics, high-entropy metal diborides have recently been attracting great research attentions for potential applications in extended fields, such as aerospace, cutting tools, microelectronics, and nuclear reactors, owing to the unique combination of high-melting point (>3273 K), tailorable thermo-mechanical performance, good chemical and high-temperature stability of their individual metal diboride components. 3,13,14 Synthesis of high-entropy metal diboride powders is critical to implementing their extensive applications. Recently, a simple borothermal reduction method has been proposed to successfully synthesize the high-entropy (Hf 0.2 Mo 0.2 Ta 0.2 Nb 0.2 Ti 0.2 )B 2 powders.…”