First principles calculations and synthesis of multi-phase (HfTiWZr)B2 high entropy diboride ceramics: Microstructural, mechanical and thermal characterization

Electronic and Lattice Distortions Induce Elastic Softening in Refractory Multicomponent Borides

“…Most multicomponent borides have heterogeneities consisting of B 4 C, graphite, (Zr/Hf)­O 2 , and segregates of unitary or mixed-phase borides. – These heterogeneities emerge at grain boundaries and improve the structural properties relative to those of their single-phase counterparts. These impurities get impregnated during material synthesis, typically via mechanical alloying and sintering of metal oxides in the presence of B 4 C. Monitoring and precise control of the proportion of such impurities is challenging; hence, estimating the hardness of a pure single-phase material devoid of grain boundary segregations is difficult and requires optimal synthesis methods that minimize exposure to oxide and carbonaceous contents. ,,– , A widely reported multicomponent boride (Hf 0.2 Zr 0.2 Ta 0.2 Nb 0.2 Ti 0.2 )­B 2 has been reported to possess hardness between 16.4 and 25.4 GPa from experimental measurements, ,,,,,,– with its single phase chemistry assuming 16.4 and 20.9 GPa per the literature.…”

“…The results have suggested that the bulk modulus of all the multicomponent borides is inferior to that of NbB 2 , TaB 2 , and MoB 2 . Kavak et al predict the hardness of (HfTiWZr) 1 B 2 from first-principle calculations to be 30.99 GPa, which is less than the theoretical/experimental (36/35.5 GPa) hardness value of TiB 2 . , In the same vein, molecular dynamics simulations of (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2 ) B 2 using deep learning potentials have resulted in values akin to the experiments and density functional theory (DFT) calculations. In a recent study, we find multicomponent silicides to exhibit a similar trend of reduction in bulk modulus from unitary to quinary disilicides .…”

“…While intrinsic hardening is influenced by the metal-to-boron ratio and the arrangement of boron atoms, extrinsic effects emerge at the microstructural scale and involve the grain size, number of crystallographic phases, as well as segregations present in the material. Multicomponent borides generally do not form a single phase (see Supporting Information) and are contaminated with impurities like oxides and B 4 C, – promoting segregation at the grain boundaries and hence an increased hardness due to the foregoing extrinsic effects. In contrast, and notably so, we find that single-phase multicomponent borides have lower hardness than their precursor counterparts. ,– Here, we examine the fundamental mechanisms associated with the intrinsic factors for hardness reduction in multicomponent borides.…”

Theoretical prediction of the structure and hardness of TiB4 tetraborides from first-principles calculations

“…Hard or superhard materials have been widely applied in various fundamental industries, such as petroleum, machining tools and drilling equipment. 1–8 Although diamond is a common superhard material (hardness: 60–120 GPa), poor oxidation resistance hinders its high-temperature applications because of the production of ironic carbides above 600 °C. 9 To solve the problem of diamond, therefore, the search for novel hard or superhard material is very important for the development of fundamental industries.…”

Rapid Experimental Screening of High‐Entropy Diborides for Superior Oxidation Resistance

“…For instance, Backman et al [25,26] studied the oxidation resistance of (Hf 0.2 Zr 0.2 Ti 0.2 Ta 0.2 Nb 0.2 )B 2 at 1973 K, and the results showed that the IVB elements were more preferentially oxidized than VB elements, which depressed the oxidation resistance of HEBs. Kavak et al [27] found that the oxidation resistance of (Hf, Ti, W, Zr)B 2 was worse than that of its individual diboride component HfB 2 . On the other hand, some researchers reported impressive oxidation resistance in HEBs.…”