Transformation of metastable dual-phase (Ti0.25V0.25Zr0.25Hf0.25)B2 to stable high-entropy single-phase boride by thermal annealing

Abstract

Transition metal borides have a unique combination of high melting point and high chemical stability and are suitable for high temperature applications (>2000 °C). A metastable dual-phase boride (Ti0.25V0.25Zr0.25Hf0.25)B2 with distinct two hexagonal phases and with an intermediate entropy formation ability of 87.9 (eV/atom)−1 as calculated via the density functional theory (DFT) was consolidated by pulsed current sintering. Thermal annealing of the sintered dual-phase boride at 1500 °C promoted the diffusion of metallic elements between the two boride phases leading to chemical homogenization and resulted in the stabilization of a single-phase high-entropy boride. Scanning electron microscopy, in situ high temperature x-ray diffraction, and simultaneous thermal analysis of the as-sintered and annealed high-entropy borides showed the homogenization of a dual-phase to a single-phase. The experimentally obtained single-phase structure was verified by DFT calculations using special quasirandom structures, which were further used for theoretical investigations of lattice distortions and mechanical properties. Experimentally measured mechanical properties of the single-phase boride showed improved mechanical properties with a hardness of 33.2 ± 2.1 GPa, an elastic modulus of 466.0 ± 5.9 GPa, and a fracture toughness of 4.1 ± 0.6 MPa m1/2.

Electronic and Lattice Distortions Induce Elastic Softening in Refractory Multicomponent Borides

“…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. 14,24,[28][29][30][31]43 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, 14,21,22,24,29,31,44−46 with its single phase chemistry assuming 16.4 46 and 20.9 GPa 43 per the literature. Similarly, the hardness of another multicomponent boride (Hf 0.2 Zr 0.2 Ti 0.2 Ta 0.2 Cr 0.2 )B 2 is recorded to range from 19.9 to 29.3 GPa, while its single-phase structures have a hardness between 19.9 and 25.4 GPa.…”

“…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. Similarly, the hardness of another multicomponent boride (Hf 0.2 Zr 0.2 Ti 0.2 Ta 0.2 Cr 0.2 )­B 2 is recorded to range from 19.9 to 29.3 GPa, while its single-phase structures have a hardness between 19.9 and 25.4 GPa. ,,, A case in point is that single-phase multicomponent borides have a lower hardness relative to their precursor metal diborides.…”

“…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. While the literature extensively reports on diborides, we expand our analyses to monoborides and dimetal borides as well.…”

Anisotropic thermal expansion in high-entropy multicomponent AlB2-type diboride solid solutions

“…However, few studies have been dedicated to this topic [6,12]. TE investigations of non-oxide HE ceramics like borides [14][15][16][17] or carbides [18][19][20] have been limited and most studies have only speculated about the effects. Computational studies suggest that the compositional diversity may affect TE [15,20].…”

“…Computational studies suggest that the compositional diversity may affect TE [15,20]. However, experimental studies of the TE of HE borides and carbides remain sparse at best [15][16][17]. Iwan et al investigated the equation of state of (Hf,Nb,Ta,Ti,Zr)B 2 up to 2000 K 14 and found a non-linear volumetric thermal expansion with coefficients…”