First-principle Investigations On The Properties Of Cr₂AlC And Its Solid Solution

Compared with the most widely studied Ti3SiC2and Ti3AlC2in MAX family, Cr2AIC possesses excellent oxidation resistance and corrosion resistance. It is a promising candidate as high-temperature structural components and coating materials. However, the strength of Cr2AIC is not so high compared to that of the conventional structural ceramics such as Alumina and Silicon Carbide. Thus, improvement of Cr2AIC with tunable properties by solid solution strengthening is a very interesting task for its potential applications.Based on density functional theory, first-principle has become a popular method for the computation of materials. In the present study, the structure, formation energy, mechanical and electronic properties of Cr2AIC as well as Cr2AlSiC solid solution have been investigated by using the VASP package of the first-principle. In addition, the solid strengthening mechanism has also been explained in calculation theory.The research firstly focused on the construction of Cr2AIC and Cr2AlSiC solid solution models and calculation of the structure and the formation energy. At first, we chose a Cr2AIC primitive cell, optimized its lattice constants. Final models have been constructed by replacement of Si atoms with Al in Cr2AIC using the minimum energy law to determine the location of the Si atoms solute. The structure and formation energy of those Cr2AlSiC models were calculated. The results show that that the lattice constant a keeps unchanged, while c decreases lineally. The decrease of c is mainly resulted from that the bond length of Cr-Si is shorter than that of Cr-Al. In addition, all of the calculated formation energies of Cr2AlSiC solid solution are negative, indicating the stability of those solid solutions. We synthesized the corresponding Cr2AlSiC solid solution, and proved the conclusion of the calculation. In addition, the lattice constants of a=2.85A, c=12.83A, which are close to the calculated values of a=2.85A, c=12.58A. The deviation between calculated and experimental values is smaller than5%, indicating the reliability of the model.The calculation of elastic modulus, density of states, and electron density distribution of Cr2AIC and Cr2AlSiC solid solution has been further performed to analyze the performance of Cr2AIC. The result shows that the high elastic modulus and high mechanical strength results from the strong Cr-C covalent bonds via the comprehensive calculation of density of states and electron density distribution. Its excellent high-temperature oxidation resistance and corrosion resistance is due to the weak bonds between Al layers and the Cr6C octahedrons. These weak bonds make Al atoms easily escape to the surface of materials under some special conditions, which will formulate a dense Al2O3layer and protect the substrate from further oxidation and corrosion. Compared the results of Cr2AIC and Cr2AlSiC, it can be concluded that the solid solution strengthening mechanism of Cr2AlSiC is due to the extra electron of Si atom than Al atom, which increases the outermost electron bond filling and the total charge density of Cr2AIC. The above mechanism improves the bond strength, and further enhances the strength of Cr2AIC.Based on the calculation of Cr2AIC and Cr2AlSiC solid solution, a comprehensive understanding of the relationship between their structural and characteristics has been made, and solid strengthening mechanism has been obtained. This research has guidance for designing other MAX materials.