Abstract
First-principles calculations based on density functional theory (DFT) have been performed to explore the effects of Si, Cr, W, and Nb elements on the stability, mechanical properties, and electronic structures of MoAlB ternary boride. The five crystals, with the formulas of Mo4Al4B4, Mo4Al3SiB4, Mo3CrAl4B4, Mo3WAl4B4, and Mo3NbAl4B4, have been respectively established. All the calculated crystals are thermodynamically stable, according to the negative cohesive energy and formation enthalpy. By the calculation of elastic constants, the mechanical moduli and ductility evolutions of MoAlB with elemental doping can be further estimated, with the aid of B/G and Poisson’s ratios. Si and W doping cannot only enhance the Young’s modulus of MoAlB, but also improve the ductility to some degree. Simultaneously, the elastic moduli of MoAlB are supposed to become more isotropic after Si and W addition. However, Cr and Nb doping plays a negative role in ameliorating the mechanical properties. Through the analysis of electronic structures and chemical bonding, the evolutions of chemical bondings can be disclosed with the addition of dopant. The enhancement of B-B, Al/Si-B, and Al/Si-Mo bondings takes place after Si substitution, and W addition apparently intensifies the bonding with B and Al. In this case, the strengthening of chemical bonding after Si and W doping exactly accounts for the improvement of mechanical properties of MoAlB. Additionally, Si doping can also improve the Debye temperature and melting point of the MoAlB crystal. Overall, Si element is predicted to be the optimized dopant to ameliorate the mechanical properties of MoAlB.
Highlights
Binary transition metal borides are regarded as the superior candidates for high-temperature structural ceramics, due to their outstanding properties, such as high melting temperature, high hardness, and thermodynamic stability [1,2,3]
In MAB phases, single or double Al atomic layers are interleaved into a transition metal boride sublattice
The structures of MAB phases are analogous to the previous MAX phases, which show significant advantages for both metal and ceramics with good heat and electricity conduction, and considerable ductility and damage tolerance [6,7,8,9,10]
Summary
Binary transition metal borides are regarded as the superior candidates for high-temperature structural ceramics, due to their outstanding properties, such as high melting temperature, high hardness, and thermodynamic stability [1,2,3]. The inherent brittleness, poor oxidation resistance, as well as low damage tolerance have impeded the application of binary borides To address these limitations, the researchers have tried to introduce an Al atomic layer into transition metal borides to form so-called MAB phases [4,5]. The structures of MAB phases are analogous to the previous MAX phases, which show significant advantages for both metal and ceramics with good heat and electricity conduction, and considerable ductility and damage tolerance [6,7,8,9,10] In this case, MAB phases are expected to possess the excellent properties of both metals and borides
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