First-Principles Study Of AL-SC-TM Random Alloys

Aluminum-scandium alloys with high density number of L12-A13Sc is of interst owing to its excellent properties with high strength, neutron irradiation injury resistance, weldability, heat and corrosion resistance, and potentially useful high-temperature strength for nuclear Engineering, military, aerospace and automotive industry applications. However, high price of scandium and the brittleness of L12-A13Sc in room temperature limit its application and development. Therefore, it is very beneficial to study partial substitution of Sc by other elements, which would not only improve ductility of L12-A13Sc but also decrease the cost of the alloys. In the present work, first-principles calculation has been performed to study the stability, elastic and electronic properties of Al-Sc-TM (TM=Y, Ti, Zr, Hf, V, Nb and Ta) random alloys using special quasi-random structures. The main contents of the dissertation are as the following:Firstly, special quasi-random structures (SQSs) with32-atom have been generated to model appropriate supercell structure of pseudo-binary random L12-Al3(Sc0.5TM0.5)(TM=Y, Ti, Zr, Hf, V, Nb and Ta) alloys. The first-principles calculation based on density functional theory has been performed to study their stability, elastic and electronic properties. The optimized lattice parameters were in good agreement with the experimental data. The obtained formation energies showed that all L12-Al3(Sc0.5TM0.5) alloys were stable from energetic point of view, and all the alloys were mechnically stable from Born ceriteria. With the TM in the same Group, L12-Al3(Sc0.5TM0.5) alloys exhibit similar properties. As the atomic radius of substitution elements TM in the same Period decreased, the elastic isotropy of L12-Al3(Sc0.5TM0.5) alloys was overall lowered and the ductility could be improved. The calculated electronic structure demonstrated bonding feature of Al-TM, which uncovered underlying mechanism for stability and elastic properties of L12-Al3(Sc0.5TM0.5) alloys.Then, first-principles calculations were performed to investigate the structural, elastic and electronic properties of L12-Al3(Sc1-xZrx)(x=0.25,0.5,0.75) based on special quasi-random structures (SQSs) with32atoms. As the content of Zr atoms increased, the lattice parameters L12-Al3(Sc1-xZrx) were increased whereas thermodynamical stability was lowered. The incompressibility along the principle axes and the resistance to shear in <001> direction on the{100} plane were lowered. The reduction of Young’s modulus along the principle axes was weaker while the bulk modulus were slightly larger. For all the L12-Al3(Sc1-xZrx) alloys, the [001] is the softest direction, and the [111] has the weakest the resistance to reversible deformations direction. The ductility was improved due to increase of the B/G ratio and Cauchy pressure. The electronic structures revealed that the dominant hybridization between the Al p states and transition-metal d states became weaker with increasing of the content of Zr, which uncovered underlying mechanism for stability and elastic properties of L12-Al3(Sc1-xZrx) alloys.