|
The general trends on the mechanical properties, ranging from elastic shear modulus c44 to ideal shear strength far beyond elasticity, of M2AC (M = Ti, V, Cr, A = Al, Si, P, S) compounds are studied based on first-principles total energy method. When transition-metal element is fixed, bulk modulus enhances monotonously as A-group element running across the periodic table from Al to P, but then drops for S. A similar trend is observed for elastic shear modulus c44 as well in Ti- and V-containing compounds. In contrast to the case of Al-containing M2AlC (M = Ti, V, Nb, and Cr) ceramics, wherein shear modulus c44 reaches maximum when the valence electron concentration (VEC) is in the range of 8.4-8.6, bulk modulus, c44 and ideal shear strength of V-containing V2AC (A = Al, Si, P, and S) compounds saturate to their maximums simultaneously when the VEC varies from 9.3 to 9.6. V2PC, with a VEC of 9.5 having higher bulk modulus B, shear constant c44, and ideal shear strength compared to other studied M2AC compounds. The obtained trends in mechanical properties are well explained in terms of electronic bonding characteristics. As more p electrons provided by the A-group element, the V d-A p bonding states shift to lower energy level and give rise to enhanced mechanical properties. In addition, the V d-P p bonding states locates in the same energy range as those of V d-C p bonding, which indicates that V-P bond has similar bonding strength as the V-C bond does, and thereafter, V2PC shows superior mechanical properties. As A-element changes from P to S, the V2SC suffers less effective covalent V-S bonding, and the mechanical properties decrease as well. Atomistic deformation modes investigation demonstrates that V2PC undergoes similar deformation mechanism with other MAX phases, which provides its intrinsic damage tolerance. The surface propterties of the M2AC phases are also calcualted and are compared with the oxidation behavior. |