Research Of Formation Enthalpies And Defects Of Rare Earths-Aluminum Alloys

As important non-ferrous structure materials, aluminum alloys are irreplaceable in the civilian application and national defense. However, similarly to other alloys, aluminum alloys have been impacted continuously by new inorganic materials and polymer materials. The special properties of aluminum alloys should be improved and applications should be extended. Thus, it is very important to develop some new aluminum alloys. The rare earths-aluminum alloys just are new promising aluminum alloys with good mechanical and physical properties.In this doctoral thesis, the formation enthalpies and defects of rare earths-aluminum alloys were investigated, and the application, improvement, evaluation of macro thermodynamic model-Miedema's model and comparison with other calculation methods were emphasized. Based on Miedema's model, back-propagation neural network and the first principle calculation, thermodynamic properties of rare earths-aluminum alloys, in particular, the formation enthalpies and defects were comprehensively studied and systematically discussed. These main conclusions were presented as follows.1. Based on the Miedema's model and Toop's geometric model, the thermodynamics behavior of erbium in aluminum alloys including silicon was investigated. The formation enthalpies, excess entropies, excess Gibbs free energy, and the activity of all components of Al-Si-Er ternary alloys and their sub-binaries were calculated. The results showed that their values were all negative. Compared to an ideal solution, the activity had a larger negative bias, which indicated that erbium had stronger interaction with aluminum and silicon. According to the binary phase diagram of Al-Er and Si-Er, the formation energy of Al3Er and Er5Si3were deduced. The formation energy of Al3Er was lower than that of Er5Si3. It suggested that Er5Si3formed priority to Al3Er, and then the excess erbium reacted with aluminum and formed Al3Er. These conclusions were confirmed by our experiments. A "fishbone" structure a(Al)+Er5Si3which was a typical primary eutectic texture microstructure formed during the solidification process in Al-Mg-Si alloy with erbium.2. The glass forming ranges of Al-Ni-RE (RE=Ce, La, Y) systems and their sub-binaries were successfully predicted based on Miedema's and Toop's geometric model. The glass forming ranges of Al-Ni-RE (Ce, La, Y) systems were almost identical, and the same to their sub-binaries of these ternary alloys. The calculated glass forming ranges agreed with experiments well. The mixing enthalpy and mismatch entropy were also calculated. The results showed that the mixing enthalpies of Al-Ni-RE (Ce, La, Y) ternary systems ranged mainly from-5to-45kJ/mol, and the normalized mismatch entropies ranged from0.1to0.9, especially0.1to0.3near the Al-rich corner. These conclusions indicated the glass forming abilities of Al-Ni-RE (Ce, La, Y) systems were very good. The enthalpy change from amorphous phase to solid solution in the glass forming ranges was calculated, and the results suggested that the glass forming abilities of these three Al-Ni-RE systems would follow the sequence was GFAAl-Ni-Y> GFAAl-Ni-La> GFAAl-Ni-ce, which was in accordance with the experimental results.3. A back-propagation artificial neural network (ANN) was established to predict the formation enthalpies of Al2X-type intermetallics as a function of some physical parameters. These physical parameters included the electronegativity difference, the electron density difference, the atomic size difference, and the electron-atom ratio (e/a). Some trends of formation enthalpies for Al2X-type intermetallics were also observed from the ANN. The results showed that formation enthalpies decreased with increasing electronegativity difference, which was in accordance with Pauling's scheme of formation enthalpy. The relationships between formation enthalpies and other factors, such as the size factor and valence factor, were also revealed. The values calculated by the ANN method agreed with experiments well. The method comparison based on the predicted formation enthalpies suggested that our ANN method was superior to Miedema's model in precision, but it could not describe a clear physical picture of mechanisms.4. Miedema's model failed to predict formation enthalpies of binary transition metal compounds with adequate accuracy. A new extended Miedema's model for binary transition metal alloy systems was proposed, in which the effects of the atomic size difference between two dissimilar transition metals could be taken into account more appropriately. Compared to other available Miedema-based models models, our present model was fairly simple, and could provide more insightful physical meaning and higher prediction precision on formation enthalpies.5. Based on the Miedema's model, the formation enthalpies of four binary Al-X (Sc, Er, Zr, Li) systems were calculated. The results showed that the formation enthalpies of Al-Sc system were similar to those of Al-Er system. It indicated that scandium had the same metallurgical chemical behavior to erbium in aluminum alloys. According to the vacancy formation theory, in the full composition range, the single vacancy formation energies of these AxBy intermetallics for four binary Al-X (Sc, Er, Zr, Li) systems were calculated. The vacancy formation energies of X elements were all higher than those of aluminum element, and it indicated that aluminum vacancies formed priority to X (Sc, Er, Zr, Li) vacancies in Al-X (Sc, Er, Zr, Li) systems. This conclusion agreed with the first principles calculations.6. These lattice constants, bulk modulus and formation enthalpies of A13X (Sc, Er, Zr, Li) intermetallics and their constituents were calculated by the first-principles method, and there calculations were in accordance with the experiments. These properties of point defects for A13X intermetallics were investigated, and bonding charge density maps were described. According to vacancy-induced charge density, Sc dz2-Al pz orbital hybridization of Al3Sc was depicted. The vacancy-induced charge density of Al3Zr was more complex than that of Al3Sc, and Zr4d-Al3p hybridization of Al3Sc was revealed.7. The surface energies, atomic geometry and electronic structures of A13X (Sc, Er, Zr, Li) intermetallics surfaces were studied by the first-principles calculations. It was shown that some surfaces of A13X (Sc, Er, Zr, Li) intermetallics exhibited significant oscillatory geometric relaxation and anti-symmetric relaxation. It is found that the oscillatory geometric relaxation was caused by the electrostatic repulsion of those same kind neighbor atoms. And the oscillatory geometric relaxation could generate surface ripples, decrease surface energy, and stabilize the surface structure. According to the calculated surface energy, the structural stability of A13X surfaces from strong to weak followed the sequence was (111),(100) and (110), which was in accordance with the basic law of face-centered cubic metal surfaces. The results also showed that the structural stability of D023-Al3Zr surfaces from strong to weak followed the sequence was (114),(100),(110) and (012), which agreed with the close-packed general rule of square structure metals surfaces.8. According to the calculated surface energy of L12-A13X (Sc, Er, Zr, Li), the structural stability of A13X surfaces from strong to weak followed the sequence was Al3Zr, Al3Sc, Al3Er and Al3Li, which was basically in accordance with the sequence of formation enthalpies for Al-X (Sc, Er, Zr, Li) systems calculated by Miedema's model. The formation enthalpy could reflect the strength of cohesive energy to a certain extent. It indicated that the surface energy was roughly proportional to its cohesive energy evaluated by formation enthalpy.