In this study, we report a comprehensive assessment on the elastic and electronic properties of 792 possible MAX (M nþ1 AX n ) phases with n ¼ 1-4 using ab initio methods. These crystals are then screened based on their elastic and thermodynamic stability resulting in a large database of 665 viable crystals. All the experimentally verified MAX phases passed the screening. Various correlations among and between them are fully explored. In particular, the key elements in the interdependence between the elastic properties together with mechanical parameter derived from them and the electronic structure are identified. Detailed analysis of various correlation plots shows that there is a clear correspondence between bulk modulus K and total bond order density (TBOD). Calculations show a marked difference between the carbides and nitrides. This database is also used to test the efficacy of data mining algorithms for materials genome. We further identified several thermodynamically stable new MAX phases with unusual mechanical parameters that have never been synthesized in the laboratory or theoretically investigated. The complete database on the elastic and electronic structure together with the mechanical parameters for these 665 MAX phases compounds are included in the Supplementary Materials and fully accessible.

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Boron carbide undergoes an amorphization transition under high velocity impacts causing it to suffer a catastrophic loss in strength. The failure mechanism is not clear and this limits the ways to improve its resistance to impact. To help uncover the failure mechanism we used ab initio methods to carry out large-scale uniaxial compression simulations on two polytypes of stoichiometric boron carbide (B 4 C), B 11 C-CBC, and B 12 -CCC where B 11 C or B 12 is the 12-atom icosahedron and CBC or CCC is the three-atom chain. The simulations were performed on large supercells of 180 atoms. Our results indicate that the B 11 C-CBC (B 12 -CCC) polytype becomes amorphous at a uniaxial strain s=0.23 (0.22) and with a maximum stress of 168 (151) GPa. In both cases, the amorphous state is the consequence of structural collapse associated with the bending of the three-atom chain. Careful analysis of the structures after amorphization shows that the B 11 C and B 12 icosahedra are highly distorted but still identifiable. Calculations of the elastic coefficients (C ij ) at different uniaxial strains indicate that both polytypes may collapse under a much smaller shear strain (stress) than the uniaxial strain (stress). On the other hand, separate simulations of both models under hydrostatic compression up to a pressure of 180 GPa show no signs of amorphization in agreement with experimental observation. The amorphized nature of both models is confirmed by detailed analysis of the evolution of the radial pair distribution function (RPDF), total density of states (TDOS), and the distribution of effective charges on atoms. The electronic structure and bonding of the boron carbide structures before 2 and after amorphization are calculated to further elucidate the mechanism of amorphization and to help form the proper rationalization of experimental observations.(PACS NO: 61.50. Ks, 83.10.Tv, 81.05Je, 82.40.Fp)

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.

The alumino-silicate series (Al 4+2x Si 2À2x O 10-x , x = 0-1), is an important class of structural ceramics with many applications. Except for the end member (x = 0), which is the crystalline phase sillimanite, the reported crystal structures of the other phases, called mullites, all have partially occupied sites which makes any theoretical calculation a formidable task. In this article, we describe a systematic and detailed theoretical investigation of the structures and properties of the phases in this series. We constructed stoichiometric supercell models for the four well-known mullite phases 3Al 2 O 3 ·2SiO 2 , 2Al 2 O 3 ·SiO 2 , 4Al 2 O 3 ·SiO 2 , 9Al 2 O 3 ·SiO 2 , corresponding to x = 0.25, 0.40, 0.67, and 0.842. The construction of the models began with experimentally reported crystal structures followed by systematic removal of selected atoms at the partially occupied sites to maintain charge neutrality. A large number of models were built for each phase and fully relaxed to high accuracy using the Vienna ab initio simulation package program. The model with the lowest total energy for a given x was chosen as the representative structure for that phase. Together with sillimanite (x = 0) and the silica free ι-Al 2 O 3 (x = 1), this series' electronic structure and mechanical properties were studied via first-principles calculations. Their elastic coefficients and mechanical properties (bulk modulus, shear modulus, Young's modulus, and Poisson's ratio) were evaluated. The electronic structure, effective charges, bonding, and optical properties of these mullite phases were calculated using the orthogonalized linear combination of atomic orbitals method. These first-principles results provide the basis for an explanation of the experimentally observed structure and properties of mullite phases and their trends with x at the fundamental level.