Relationship between layered crystal structure and mechanical properties of M₃AlN (M = Zr and Hf): A first-principles investigation

Bonding character, elastic mechanical parameters, ideal strengths, and atomistic shear deformation mechanisms of M₃AlN (M = Zr and Hf) were studied by first-principles method. M₃AlN exhibits layered chemical bonding character due to the alternately stacking of relatively soft Al–M and strong N–M covalent bonds. The second-order elastic constants and mechanical parameters of M₃AlN were reported for the first time. The stress–strain relationships for different deformation modes were studied and the ideal shear and tensile strength were obtained. M₃AlN ceramics are predicted to be “quasi-ductile” layered nitrides based on the low shear-modulus-to-bulk-modulus ratios, positive Cauchy pressure (c₁₂–c₄₄), and lower ideal shear strength compared to ideal tensile strength. Investigation of the atomistic shear deformation mechanism of Hf₃AlN shows that stretching of soft Al–Hf bonds and relatively weak bridge N–Hf1 bonds dominate the shear deformation; while the rigid N–Hf2 bonds resist against the applied shear strain. Chemical bonding characteristics and shear deformation mechanism of M₃AlN are similar with those of other “quasi-ductile” ceramics, such as MAX phases, LaPO₄ monazite, and γ-Y₂Si₂O₇. The results further suggest that M₃AlN nitrides should be quasi-ductile and damage tolerant.