First-principles Calculation Of The Effect Of B Doping On The Mechanical Properties Of Ru-Ir Alloy

In recent years,there are a large number of theoretical studies on platinum group metal boride.These results show that the Ir-B and Ru-B compounds have a high hardness,which even reached the theoretical Vickers hardness value of superhard materials.However,few researches were carried out on Ru-Ir-B system.In this paper,the structural stability,electronic properties,binding properties,mechanical properties and hardness of Ru-Ir alloys,as well as Ru-Ir-B compounds built in Ru-Ir alloys were systematically investigated by first-principles calculation based on density functional theory(DFT).According to the Ru-Ir phase diagram,it is found that these two elements form disordered solid solution instead of mesophase at room temperature,thus,the structural model is built based on the different Ru/Ir atomic ratio.The formation enthalpy and binding energy show that the Ru6Ir14 shows the best stability in the constructed Ru-Ir alloys.Based on the stability analysis,The band structure and the density of states are calculated,and there is no band gap for these three Ru-Ir alloys,illustrating metallic properties of them.The total and partial density of states(DOS)show that Ru-Ir alloys are mainly contributed by Ru-d and Ir-d orbitals,as well as few Ir-s and Ir-p orbitals near the Fermi level,forming Ru-Ir and lr-Ir bonds.Further charge occupation and population analysis calculatrions indicate that in Ru-Ir alloys,the charge transfer occurred from Ir atoms to Ru atoms.The positive population of Ru-Ir bond is more than Ir-Ir bond,illustrating that the Ru-Ir bond shows better covalent properties.The mechanical properties of Ru-Ir alloy have also been calculated.The results show that the elastic constants of three Ru-Ir alloys satisfied the Born stability criterion,illustrating that they are all stable in the ground state.The B/G values of Ru4Ir16,Ru6Ir14 and RusIr12 were found to be 1.71,1.62,and 1.63,respectively,which were lower than the critical value of 1.75,illustrating that three Ru-Ir alloys exhibited brittle property.According to the elastic anisotropy ratio and directional dependence of the Young’s modulus,it is found that the order of the elastic anisotropy of the Ru-Ir alloy is from large to small order by Ru8Ir12>Ru6Ir14>Ru4Ir16.The order of the theoretical Vickers hardness of Ru-Ir alloys is from large to small by Ru6Ir14>Ru8Ir12>Ru4Ir16,but the hardness values are lower than 30 GPa.Similarly,The relevant properties of Ru-Ir-B compounds are calculated.The binding energy and formation enthalpy show that the stability of Ru6Ir14Bx(x=1,3,5)in the Ru-Ir-B compound doped with the same content of B atom is better than that of another two Ru-Ir-B compounds.In particular,the stability up to best with x=3.The band structures of six Ru-Ir-B compounds show the same characteristics as the Ru-Ir alloys,that is zero band gap,which indicates that the Ru-Ir-B compounds still have metallic properties.The density of states near the Fermi level is mainly contributed by Ru-d,Ir-d and B-p orbitals,which forms the covalent bond between Ru-B and Ir-B.In Ru6Ir14B3,due to the hybridization of the s and p orbitals of the B,the B-B covalent bond is formed.The B atoms and Ru atoms in the Ru-Ir-B compounds obtained electrons,the Ir atoms lost electrons,and the electron transfer occurs between the Ir atom and Ru atom as well as between the Ir atom and B atom.The mechanical properties of six Ru-Ir-B compounds were calculated,only Ru6Ir14B3,Ru6Ir14B5,RusIr12B and Ru8Ir12B3 satisfied the stability criterion,and the B/G values are all greater than 1.75,which show the plastic properties,indicating that few doped B atoms can improve the plasticity of Ru-Ir alloys.It is found that the anisotropy of Ru8Ir12Bx(x=1,3)is higher than that of Ru6Ir14Bx(x=3,5)from the elastic anisotropy ratio and directional dependence of the Young’s modulus.In addition,for Ru6Ir14Bx,we studied the change of Vickers hardness under different B content.The results show that the hardness is lower than that of Ru6Ir14 alloy when the B content is very low,but with the increase of the B content the hardness increases slightly.When the B content reaches 47.36 at%,the hardness appears an obvious increase.The increased B content results in the transformation of the crystal structure.The crystal structure of the material directly affects the hardness,which can explain why there is a significant change in hardness.When the B content up to 66.66 at%,the hardness of the phase(Ir(Ru)B2)is more than 40 GPa,which indicates that it is a potential superhard material.