Influence Of Al Alloying On The Properties Of Mosi₂: First-principles Calculation And Experiment Study

MoSi₂is a potential candidate for high-temperature structural applications, primarily due to itshigh melting temperature, relatively low density, good oxidation resistance, and metal-like thermaland electrical conductivities. Unfortunately, its low toughness at ambient temperature remains a majorobstacle to a wide range of practical applications. Alloying by chemical substitution for both Mo andSi sites is regarded as an effective strategy to address this deficiency. Therefore, in this paper, theeffect of Al alloying on the properties of MoSi₂were studied by experiment and first-principlecalculation. The first-principle calculation shows that the lattice constants expand with the increase inAl content for both C11_band C40structured MoSi₂. In addtion, for the C11_bstructure, the Al additionlowers the bulk modulus （B）, shear modulus （G） and Young’s modulus （E）. Interestingly, for the C40structure, the Al addition reduces bulk modulus （B）, but enhances shear modulus （G） and Young’smodulus （E）. On the other hand, the change of ductility of the C40and C11_bstructured MoSi₂withAl addition is explained according to G/B ratio, Poisson’s ratio and Peierls stress. The results showthat the substitution of Si by Al can enhance the ductility of C11_bMoSi₂, but has a deleterious effecton that of C40MoSi₂. Electronic structure analysis indicates that the covalent Mo-Si bonds in MoSi₂with C11_band C40structures are mainly responsible for the high G/B value. For the ternary C40MoSi₂, the presence of Al into MoSi₂leads to the rise in brittleness of C40MoSi₂due to theformation of strong Al-Si ionic-covalent bonds.Experimentally, in the present work, four kinds of nanocrystalline Mo(Si_1-xAl_x)₂（x=0.045,0.07and0.165） films have successfully been prepared on Ti6Al4V （TC4） alloy by a double cathode glowdischarge technique. The XRD results showed that nanocrystalline Mo(Si_1-xAl_x)₂films were consistedof single Mo（Si,Al）₂phase. The cross-sectional SEM morphology indicated that the films composedof two sublayers: an outer deposited layer and inner diffusion layer. TEM image of nanocrystallineMo(Si_1-xAl_x)₂films shows that the average grain size of crystallites is about5nm. Nanoindentationtests showed that with increasing Al content, the hardness and the elastic modulus of thenanocrystalline Mo(Si_1-xAl_x)₂films increased. Wear experiments showed that the wear resistance ofnanocrystalline Mo(Si_1-xAl_x)₂films were improved by the Al alloying. Moreover, electrochemicalmeasurements indicated that corrosion resistance of the films is proportional to the Al content of theas-deposited films. XPS analysis demonstrates that the passive layers formed on the Mo(Si_1-xAl_x)₂（x=0.045,0.075and0.165） films are highly enriched in SiO₂with smaller concentrations of MoO₄^2-, MoO₂, SiO_xas well as Al₂O₃, while the passive layer formed on the MoSi₂film is essentially the sameas the former but without Al₂O₃. Theoretical studies on the corrosion resistance of Mo(Si_1-xAl_x)₂filmswere carried out using cohesive energy and Mulliken populations based on the first-principlesdensity-functional theory. The calculation results showed that the weaker strength of Si-Si bond （III）and Mo-Si （III） bond within the （001） plane of MoSi₂crystal is responsible for the corrosion damagein MoSi₂crystal. On the contrary, the weakest Mo-Al covalent bonds in Al-doped MoSi₂arepreferentially broken, which promotes the rapid formation of Al₂O₃incorporated into passive layer.