Intrinsic Mechanical Properties of 20 MAX-Phase Compounds

The intrinsic mechanical properties of 20 MAX-phase compounds are calculated using an ab initio method based on density functional theory. A stress versus strain approach is used to obtain the elastic coefficients and thereby obtain the bulk modulus, shear modulus, Young's modulus, and Poisson's ratio based on the Voigt-Reuss-Hill (VRH) approximation for polycrystals. The results are in good agreement with available experimental data. It is shown that there is an inverse correlation between Poisson's ratio and the Pugh ratio of shear modulus to bulk modulus in MAX phases. Our calculations also indicate that two MAX compounds, Ti2AsC and Ti2PC, show much higher ductility than the other compounds. It is concluded that the MAX-phase compounds have a wide range of mechanical properties ranging from very ductile to brittle with the A in the MAX phase being the most important controlling element. The measured Vickers hardness in MAX compounds has no apparent correlation with any of the calculated mechanical parameters or their combinations.