Fabrication and Characterization of Quinary High Entropy-Ultra-High Temperature Diborides

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.

Processing Conditions Optimization for the Synthesis and Consolidation of High-Entropy Diborides

“…As reported in two recent works [24][25], the synthesis of (Hf 0.2 Mo 0.2 Zr 0.2 Ti 0.2 Ta 0.2 )B 2 and (Hf 0.2 Mo 0.2 Zr 0.2 Ti 0.2 Nb 0.2 )B 2 by SHS according to Eqs. ( 1) and (2), respectively, did not go to completion.…”

“…More recently, a novel subclass of UHTCs, known as High Entropy Borides (HEBs), has been developed [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]. According to the general definition of high-entropy alloys [31], HEBs consist This paper is devoted to the celebration of 75 years' jubilee of Professor Zulkhair Mansurov.…”

“…Examples of these ceramics are (Hf 0.2 Nb 0.2 Zr 0.2 Mo 0.2 Ti 0.2 )B 2 , (Hf 0.2 Mo 0.2 Zr 0.2 Ti 0.2 Ta 0.2 )B 2 , (Hf 0.2 Mo 0.2 Zr 0.2 Ti 0.2 Nb 0.2 )B 2 , etc. As for their fabrication in bulk form, Spark Plasma Sintering (SPS) is the main consolidation technology utilized to this aim [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. On the other hand, different approaches are proposed to synthesize the corresponding powders.…”

“…The most widely employed routes are based on the borothermal or boro-carbothermal reduction of metal oxides [17-18, 20, 26-29]. Alternatively, the co-milling of individual borides [14,22] and the self-propagating high temperature synthesis (SHS) from elemental constituents [15][16][24][25]] are adopted instead. In this regard, even if SHS did not lead to single phase high entropy products, it allowed for the fast (order of seconds) obtainment of multiphase ceramics, with elemental metals well mixed across each grain [15][16][24][25].…”

“…Alternatively, the co-milling of individual borides [14,22] and the self-propagating high temperature synthesis (SHS) from elemental constituents [15][16][24][25]] are adopted instead. In this regard, even if SHS did not lead to single phase high entropy products, it allowed for the fast (order of seconds) obtainment of multiphase ceramics, with elemental metals well mixed across each grain [15][16][24][25]. The latter feature is extremely important to promote the formation of a single-phase product when SHS powders are processed by SPS.…”