Abstract
Due to their inherent chemical complexity and their refractory nature, the obtainment of highly dense and single-phase high entropy (HE) diborides represents a very hard target to achieve. In this framework, homogeneous (Hf0.2Nb0.2Ta0.2Mo0.2Ti0.2)B2, (Hf0.2Zr0.2Ta0.2Mo0.2Ti0.2)B2, and (Hf0.2Zr0.2Nb0.2Mo0.2Ti0.2)B2 ceramics with high relative densities (97.4, 96.5, and 98.2%, respectively) were successfully produced by spark plasma sintering (SPS) using powders prepared by self-propagating high-temperature synthesis (SHS). Although the latter technique did not lead to the complete conversion of initial precursors into the prescribed HE phases, such a goal was fully reached after SPS (1950 °C/20 min/20 MPa). The three HE products showed similar and, in some cases, even better mechanical properties compared to ceramics with the same nominal composition attained using alternative processing methods. Superior Vickers hardness and elastic modulus values were found for the (Hf0.2Nb0.2Ta0.2Mo0.2Ti0.2)B2 and the (Hf0.2Zr0.2Ta0.2Mo0.2Ti0.2)B2 systems, i.e., 28.1 GPa/538.5 GPa and 28.08 GPa/498.1 GPa, respectively, in spite of the correspondingly higher residual porosities (1.2 and 2.2 vol.%, respectively). In contrast, the third ceramic, not containing tantalum, displayed lower values of these two properties (25.1 GPa/404.5 GPa). However, the corresponding fracture toughness (8.84 MPa m1/2) was relatively higher. This fact can be likely ascribed to the smaller residual porosity (0.3 vol.%) of the sintered material.
Highlights
As a novel category of the wider family of ultra high temperature ceramics (UHTCs), high entropy (HE) transition metal diborides, often referred to as HEBs, have immediately gained a significant interest for their potential applications as structural materials in extreme environments [1]
A multiphase product was obtained after the synthesis process, which agreed with the results obtained with the other HEB systems previously investigated, i.e., of HEB_a [5] and HEB_b [8]
Detailed X-ray diffraction (XRD) analysis coupled with the Rietveld analytical procedure evidenced that reactants were only partially converted by SHS into the desired HE phase, while other individual (NbB2, etc.) and binary ((Hf0.5Ti0.5)B2, (Zr0.5Ti0.5)B2, etc.) diborides were found in the synthesized product
Summary
As a novel category of the wider family of ultra high temperature ceramics (UHTCs), high entropy (HE) transition metal diborides, often referred to as HEBs, have immediately gained a significant interest for their potential applications as structural materials in extreme environments [1]. HEBs basically originate from the combination of five or four individual diborides (ZrB2, HfB2, etc.) in near-equimolar ratio to give quinary or quaternary, respectively, crystalline solid solutions with maximum configurational entropy [2]. Three approaches are considered: (1) simultaneous synthesis and densification of the ceramic from untreated [13] or ball milled [11] elemental precursors; (2) SPS of pretreated individual borides [1,3,12]; (3) synthesis of HEB-based powders and their subsequent consolidation [4,5,6,7,8,9,10,13,14,15,16]
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