Effect Of Alloying On The Property Of Nb-Si Intermetallic Compounds By First Principles Study

Nb-Si intermetallic compound material is presented as a new type of hightemperature structural materials due to high melting point, high strength, lowdensity and other advantages. However, its large room temperature brittlenessand poor processability constrains its serve life and application field. In thisarticle, first principles method is used to study the crystal structures, thermal andmechanical stability, mechanical and the electronic properties of three types ofNb5Si3, by which the most stable structure of the Nb5Si3is also obtained. On thisbasis, the influence of alloy elements M(M=Ti, Cr, Al, Hf, Ga, B, C, Y, et al.)addation on the crystal structure, stability and mechanical properties of α-Nb5Si3impact is investigated, in order to provide theoretical basis for solving roomtemperature brittleness of α-Nb5Si3.By our study, the formation energies of three types of Nb5Si3are all negative,indicating the thermal stability of three structures, and α-Nb5Si3is the moststable one. The bulk moduli value of α-Nb5Si3, β-Nb5Si3and γ-Nb5Si3are192.3GPa,188.6GPa and185.9GPa. The Shear modulus value of α-Nb5Si3, β-Nb5Si3and γ-Nb5Si3are130.9GPa,111.4GPa and24.4GPa. Among them, the α-Nb5Si3has the highest volume modulus and shear modulus. According to the Pughjudgment, the B/G value is greater and the material is more toughness, while thedecreasing B/G shows the increasing brittleness of the material. The criticalvalue of B/G is1.75. α-Nb5Si3and β-Nb5Si3have large brittleness since theirB/G are less than1.75, and α-Nb5Si3is more brittle than β-Nb5Si3. In general, thelocation of the Fermi energy level and density of state at the Fermi leveldetermine the stability of intermetallic compounds. The values of density of stateat the Fermi level are14.84eV,18.89eV and23.64eV for α-Nb5Si3, β-Nb5Si3and γ-Nb5Si3, respectively. So α-Nb5Si3is the most stable structure and γ-Nb5Si3 is the most unstable one, which is in good agreement with the result of formationenergy. The density of state near the Fermi level of α-Nb5Si3, β-Nb5Si3, γ-Nb5Si3mainly are composed by Nb-4d electrons and Si-3p electrons.According to the formation energies of alloy element doping α-Nb5Si3: Ti andCr prefer to replace the Nb atoms located at4c site while Hf and Y tend toreplace the Nb atoms at16l site, Al and Ga firstly replace the Si at4a sites, but Band C primarily substitute the Si at8h site. The Ti, Hf and B aaditions enhancethe stability of α-Nb5Si3, while Cr, Al, Ga, C and Y additions reduce the stabilityof α-Nb5Si3. The B/G value is less affected for Ti addition, and the doping ofother alloying elements increase the B/G value the toughness of α-Nb5Si3isimproved. With various alloying element addation, the pseudogap near the Fermisurface appears and shows the stability of the system afer alloying. The values ofdensity of states at the Fermi level show good consistent with the results offormation energy calculations, that is the more stable structure corresponds to thelower density of states at the Fermi level.