The Thermal Properties Of Al-Mg-TM Alloys And The 24R-type LPSO Structure In Mg₉₇Zn₁Y₂ Alloys

As lightweight structural materials with a good combination of mechanical properties such as low density, good stiffness, the highest strength-to-weight-ratio, especially the significant advantages of easy-recycling, Mg alloys and Al-Mg alloys have been applied extensively in the field of automotive, aerospace and microelectronic industries. However, applications of these alloys are still restrained due to the limited mechanical properties at high temperature. In order to further improve their properties, many experimental investigations have been performed. However, the theoretical study is very scarce. In this dissertation, First principles calculations have been carried out for study of thermal properties of Al-Mg-TM(TM=Sc,Zr) alloys and the microstructure of 24R-type LPSO phase. The main contents of the dissertation are as following:Ab inito density functional theory (DFT) and density function perturbation theory (DFPT) have been used to investigate the thermal properties of the face-center-cubic (fcc) Al₃X (X=Mg,Sc,Zr) alloys over a wide range of pressure and temperature, in comparison with fcc Al. Phonon dispersions were obtained at equilibrium and strained configurations by density functional perturbation theory. Using the quasiharmonic approximation for the free energy, several thermal quantities of interest such as thermal Grüneisen parameter, heat capacity, thermal expansion coefficient, and entropy, were calculated as a function of temperature and pressure, as well as adiabatic bulk modulus and isothermal bulk modulus of Al₃Zr, and the variation features of these quantities were discussed in details. The present investigation provides useful information for design and applications of technologically relevant Al-based alloys.Ab initio density functional theory (DFT) and density function perturbation theory (DFPT) have also been used to investigate the thermal properties of the Al-Mg-Sc,Al-Mg-Zr and Al-Mg-Sc-Zr alloys over a wide range of temperature and pressure. Phonon dispersions were obtained at equilibrium and strained configurations by DFPT. Using the quasiharmonic approximation (QHA) for the free energy, several physical quantities of interest such as thermal Grüneisen parameter, heat capacity at constant pressure and at constant volume, thermal expansion coefficient, entropy, adiabatic bulk modulus and isothermal bulk modulus as a function of temperature and pressure are calculated and discussed. The present results show that the thermal expansion coefficient of the Al-Mg-Sc-Zr is far lower than that of Al-Mg-Sc and Al-Mg-Zr, and the variation feature in the adiabatic bulk modulus and isothermal bulk modulus for the Al-Mg-Sc-Zr are also very different from that of Al-Mg-Sc and Al-Mg-Zr.The first-principles calculation based on density functional theory has been carried out to study the microstructural feature of the novel 24R-type long period stacking ordered structure in Mg₉₇Zn₁Y₂ alloy. The lattice positions of the Y and Zn atoms are determined theoretically, it is shown that the additive atoms are firstly enriched in the stacking fault layers at the two ends, a small amount are distributed in the interior stacking fault layers of the structure. And the arrangement of these Y and Zn atoms trends to be along the diagonal line of the unit cell. The structural stability is analyzed and the electronic density of state is discussed as well as.