| 其他摘要 | Layered ternary MAX ceramics possess unique properties combining excellent characteristics of metals and ceramics such as low density, high modulus and fracture toughness, high electrical and thermal conductivity, good machinability, damage tolerance at room temperature, and good resistance to thermal shock and oxidation. In comparison to traditional structural ceramics, MAX phases’ intrinsic ductility and good oxidation resistance have endowed these materials as potential candidate to be widely used in high-temperature, oxidation and corrosive environments. This dissertation aims to investigate the mechanical properties and defect behaviors of MAX ceramics using first-principles calculation. After establishing a solid foundation of structure-property relationship, we will earn some insight into the fundamental rules, which is in charge of optimizing the performance of materials, to contribute to experiments by predicting and providing theoretical guidance.
The deformation and failure modes of Ti2AlC and Ti2AlN were examined by using first-principles total-energy calculations. The weak Ti-Al bonds accommodated deformations by softening and breaking at large strains, and the structural instability of ternary compounds was predominated by the shear slide of Al atomic planes along the basal plane. From the stress-strain curves, it is presented that the ideal shear strengths of ternary compounds were significantly smaller than those of the binary counterparts, whereas similar ideal tensile strengths were obtained. Damage tolerance, quasi-ductility, and low Vickers hardness of these ternary compounds was suggested originating from their low resistance to shear deformation. When the deformation modes of two possible basal slip systems were concerned for Ti2AlC and Ti2AlN, the results suggested that the basal plane slip system was more active; a polymorphic phase transformation was predicted only along the shear path.
Our hypothesis on the evaluation of damage tolerant materials by judging ideal shear strength was further validated in the case of a kind typical brittle ceramic, Al4SiC4. From crystallographic point of view, Al4SiC4 can be described as Al4C3-type and hexagonal SiC-type structural units alternatively stacked along [0001] direction. Tension-induced bond-breaking of Al4SiC4 occurs inside the constitutive Al4C3-type structural unit, whereas shear-induced instability is originated from the coupling bond between Al4C3- and SiC-type structural units. The lower strain energy at maximum tensile stress, i.e. ideal strength, than the energies in shear cases, was indicative of an easier structural failure caused by tension. This result suggested a cleavage fracture mechanism and intrinsic brittle character of Al4SiC4.
An interesting bonding characteristic of Nb2AsC was reported. i.e., Nb–As bonding states locate approximately in the same energy level with those of Nb–C bonding, suggesting that Nb–As and Nb–C bonds have similar bonding strength, and thereafter the bulk modulus, shear modulus c44 and ideal shear strength of Nb2AsC are significantly enhanced. The relationship between mechanical properties and electronic structure for ternary M2AC (M = Ti, V, Cr, A = Al, Si, P, S) carbides was established. When transition metal M is fixed, bulk modulus enhances monotonously as A-element atom running across the periodic table from Al to P, but then drops obviously for S. A similar trend is observed for elastic shear modulus of Ti- and V-containing compounds. It is further suggested that tailoring the A site is more efficient toward strengthening mechanical property than M-site substitution. These results highlight possible strategies to design high strength M2AC compounds.
The formation energies of all kinds of point defects in Ti2AlC were quantitatively evaluated as a function of the atomic chemical potentials for the first time. It provides quantitative rationale of a structural tolerance to large off-stoichiometry of Al and C constituent occurring in Ti2AlC. VTi has the highest formation energy at all possible conditions. We also evaluated the atomic vacancy migration barriers and determine VAl is the most mobile one. Open-spaced region between compact TiC slabs in Ti2AlC is also capable of accommodating other defect species: TiAl/AlTi antisite show extraordinary stability depending on desirable chemical environments; Al and C atoms with small atomic sizes can occupy various interstices of high-symmetry. After that, the vacancy mechanism and associated ternary phase stability of serial Ti2AC (A = Al, Si, P, and S) materials were examined. The allowed chemical potential region for spontaneous silicon vacancy formation is close to the precipitate line of binary phase TiC. This result gives a hint to the absence of Ti2SiC compound in real material synthesis unless intermediate phase TiC can be avoidable, or a spontaneous decomposition of Ti2SiC will irreversibly occur. Additionally, the migration barrier of Si atoms by vacancy mechanism is as low as 0.78 eV, implying the diffusion of Si atom outward to the surface is kinetically available.
It was reported that impurity atoms associated with practical atmosphere, like N and O, can be easily incorporated in Ti2AlC matrix and of primary importance, inserted N or O atom obviously weaken the bonding strength of neighboring Ti-Al bond and, hence promote the formation of Al vacancy. A formalism based on the simplified consideration of the constrained thermodynamical equilibrium between the oxygen-dissolved-Ti2AlC and a given oxide was introduced. It was suggested where oxygen was incorporated in Ti2AlC was dependent on oxygen partial pressure. In the high internal oxygen partial pressure region, the incorporated-oxygen is energetically favored at the open interstitial sites and the interstitial-migration-mechanism governed diffusion barrier is low. When the oxygen partial pressure is lower, the OC substitution defect is dominant and vacancy-assisted oxygen diffusion has a lower mobility.
The stable interstitial configurations in 3C- and 4H-SiC have been characterized by formation energies, which are shown to be very delicate to the limitations in supercell size and/or k-point sampling. A new diffusion mechanism for the silicon interstitial in cubic SiC was reported, the corresponding migration energy is estimated at 0.8 eV. The ground state structure of silicon di-interstitial clusters was found in 3C-SiC. Besides of the most favorable stability, the ground state cluster also exhibits an easy reorientation, in-plan and out-of-plan, compact-to-extended transition, and self-diffusion characteristics along each specified crystallographic directions. A new kind point defect is reported to possess the stoichiometric conservation of defective compound, as well as the feature of five- and seven-member rings. |
修改评论