Atomistic deformation modes and intrinsic brittleness ofAl4SiC4: A first-principles investigation

Abstract

From crystallographic point of view, ${\mathrm{Al}}_{4}\mathrm{Si}{\mathrm{C}}_{4}$ can be described as ${\mathrm{Al}}_{4}{\mathrm{C}}_{3}$-type and hexagonal SiC-type structural units alternatively stacked along [0001] direction. However, relationship between this layered crystal structure and mechanical properties is not fully established for ${\mathrm{Al}}_{4}\mathrm{Si}{\mathrm{C}}_{4}$, except for the reported bulk modulus locating between those of ${\mathrm{Al}}_{4}{\mathrm{C}}_{3}$ and SiC. Based on the first-principles pseudopotential total energy method, we calculated the elastic stiffness of ${\mathrm{Al}}_{4}\mathrm{Si}{\mathrm{C}}_{4}$, and reported on its ideal tensile and shear stress-strain relationships considering different structural deformation modes. Elastic properties of ${\mathrm{Al}}_{4}\mathrm{Si}{\mathrm{C}}_{4}$ are dominated by the ${\mathrm{Al}}_{4}{\mathrm{C}}_{3}$-type structural units and exhibit similar results with those of ${\mathrm{Al}}_{4}{\mathrm{C}}_{3}$. Furthermore, the atomistic deformation modes of ${\mathrm{Al}}_{4}\mathrm{Si}{\mathrm{C}}_{4}$ upon tensile and shear deformations are illustrated and compared with ${\mathrm{Al}}_{4}{\mathrm{C}}_{3}$ as well. Since the tension-induced bond breaking occurs inside the constitutive ${\mathrm{Al}}_{4}{\mathrm{C}}_{3}$-type unit, the ternary carbide has similar ideal tensile strength with ${\mathrm{Al}}_{4}{\mathrm{C}}_{3}$. On the other hand, despite the softening of strong coupling between ${\mathrm{Al}}_{4}{\mathrm{C}}_{3}$- and SiC-type structural units is involved in shear, the shear strength for ${\mathrm{Al}}_{4}\mathrm{Si}{\mathrm{C}}_{4}$ is, however, lower than the tensile strength, since $p$-state involved Al-C bonds respond more readily to the shear deformation than to tension. In addition, based on the comparison of strain energies at the maximum stresses, i.e., ideal strengths, for both tension and shear, we suggest that structural failure occurs in tensile deformation firstly and, thus confirms an intrinsic brittleness of ${\mathrm{Al}}_{4}\mathrm{Si}{\mathrm{C}}_{4}$. For crystal structure arranged in alternatively stacking configuration, such as ${\mathrm{Al}}_{4}\mathrm{Si}{\mathrm{C}}_{4}$, mechanical properties can be traced back to the constituent units, and are also related to the coupling strengths between each constituent unit. The results might provide a computational method to predict ductile or brittle response of a solid to applied deformations.