Property Prediction Of Ternary Layered Cr-Al-B Ceramics Using First Principles

As an important member of Ultra-high temperature ceramics,binary transition metal borides have broad application prospects in hypersonic vehicles and thermal structural components of supersonic ramjet engines.However,the inherent brittleness and poor oxidation resistance of such ceramics limit their pot ential applications.The excellent performance of the ternary layered machinable ceramic MAX phase has prompted the formation of a ternary transition metal boride MAB phase by adding one or two Al atoms to the binary boride lattice to improve its brittlene ss and oxidation resistance.Cr-A-B MAB phase have become a hotspot of recent research due to the possibility of forming dense oxides and thus having excellent oxidation resistance.In this dissertation,the primary compounds Cr2 Al B2,Cr3 Al B4 and Cr4 Al B6 existed in this system were predicted by the first-principles method based on density functional theory to predict the ground state properties and lattice dynamics of the material,and by comparing a hypothetical compound Mo Al B-type compound Cr Al B,the influence of the crystal structure of the MAB phase on the material properties was investigated.In this paper,the first-principles method is used to calculate the crystal structure of Cr-A-B ceramics.The calculated atomic configurations match well with experiment and the metal-like electronic structure is consistent with the high electrical conductivity of the Cr-Al-B borides.In the Cr Al(Cr B2)n series strong covalent bonding is present between the B and Cr atoms with,significantly,much weaker metallic Cr-Al and B-Al bonds,suggesting similar unusual properties to the MAX phases.The relative stiffness of the weakest and strongest bonds hint at similar unusual properties to the MAX phases with superior damage tolerance expected for hypothetical Cr Al B,as evidenced in the lowest Al-Al bond stiffness.The layered nature and metallic bonding,associated with crack deflection,delamination and crack bridging are expected to result in high fracture toughness and damage tolerance.Anisotropic compression is demonstrated,with the stiffest axes along the direction of the B-B zigzag-/hexagonal-chains and the softest axes determined by an interplay between the soft metallic interlayers and the rigid covalent bonds.In general the elastic moduli in Cr Al(Cr B2)n increase as a function of n,however,without the price of an increase in density.In this paper,the lattice vibration and phase stability of Cr-Al-B ceramics are discussed.Dispersion curves and phonon density studies of phonons for Cr-Al-B ceramics show that for Cr Al B,Cr2 Al B2,and Cr4 Al B6,all frequencies are positive(real numbers),indicating the dynamic stability of these phases.Cr3 Al B4 has an imaginary frequency under the harmonic approximation.High-frequency phonons are almost occupied by B atoms,due to the strong chemical bonding and low atomic mass of B atoms.The frequencies of Cr and Al phonon states are always below 20 THz.The study of the stability of Cr-Al-B ceramics using linear programming shows that the hypothetical Cr Al B is not thermodynamically stable,and the stability of Cr2 Al B2 is the best and the stability of Cr3 Al B4 is poor.Cr3 Al B4 and Cr4 Al B6 compete with each other.In this dissertation,the lattice dynamics and related thermodynamic properties of Cr2 Al B2 are studied in detail,including thermal expansion,heat capacity and thermal conductivity.Due to the highly ordered stacking of Cr2 Al B2,Cr2 Al B2 has the highest thermal conductivity.The Cr4 Al B6 bridged by the curved B-Cr-B bond has a relatively low intrinsic thermal conductivity.Cr2 Al B2 has a significant anisotropic expansion.The thermal expansion in the b direction is significantly greater than the values in the a and c axis directions.The heat capacity of Cr2 Al B2 at room temperature to 2300 K can be accurately expressed by the formula.The heat capacity increases rapidly at lower temperatures and then linearly increases.At low temperatures,the bulk modulus decreases slowly,and as the temperature increases,the bulk modulus decreases almost linearly.