Thermodynamic Investigation Of Systems Concerning Heat-resistant Aluminum Alloys Strengthened By L1₂Phase

Aluminum alloys have been playing a significant role in aerospace, electric-utility industry, auto industry and other fields by virtue of light weight, high specific strength and good electrical and thermal conductivity. Although they tend to face the challenge of titanium alloys and composite materials in the application of aircraft, the principal position as structural materials is still unshakable. Nowadays, an important research direction of Al alloys goes to the increase of their strength at elevated temperatures and the development of heat-resistant Al alloys. As is well known, the addition of a rare-earth element, scandium, could improve the heat resistance effectively due to the formation of Ll2_Al3Sc phase. However, for its high price, the Al alloys containing Sc is destined to be used in military filed or high-grade products for civil use. Finding out other alloying elements to substitute Sc partially or even completely has a great value for technology and economics. For now, there are two design concepts for heat-resistant Al alloys with less or no Sc based on the core idea of strengthening by L12_A13X phase. One is to devitrify the pre-formed amorphous alloy to form an L12phase and the other is to have an Ll2phase precipitated in the matrix through rapid solidification or powder metallurgy process. In this thesis, several representative systems containing rare-earth elements and transition metal elements were selected and investigated from the perspective of thermodynamics and diffusion kinetics. It is hoped to provide theoretical and experimental basis for the development of novel heat-resistant Al alloys strengthened by Ll2phase. The main work is as follows.(1) The phase diagram of the Al-Cu-Ce, Al-Cu-Zr and Al-Cu-Sc ternary systems were first thermodynamically optimized using CALPHAD (CALculation of PHAse Diagram) method and an accurate and reliable thermodynamic description has been established. With the optimized thermodynamic parameters, the onset driving forces for the precipitation of crystalline phases in undercooled liquid of each composition were calculated and then the projection of driving force surface over the entire composition was constructed. Based on the driving force rule, the glass-forming ability in both systems was analyzed and composition range with high glass-forming ability was predicted.(2) Employing individual alloys, the isothermal section at873K, vertical sections and liquidus projection of the Al-Sc-Zr and Al-Sc-Y ternary systems were experimentally studied by means of Scanning Electron Microscope/Energy Dispersive Spectrometer (SEM/EDX), X-Ray Diffraction (XRD) and Differential Scanning Calorimetry (DSC). With the structure of hcp_A3solid solution in Sc-Zr system simulated by SQS (Special Quasirandom Structure) supercell, the enthalpy of mixing of this phase was calculated using first-principles method and the binary phase diagram was then optimized. Moreover, the phase diagram of the Al-Y binary system was re-optimized. On the basis of above work, the thermodynamic descriptions of the Al-Sc-Zr and Al-Sc-Y systems have been developed for the first time coupling first-principles calculation with the CALPHAD method.(3) Similarly, using individual alloys, the isothermal section at873K, vertical sections and liquidus projection of the Al-Dy-Zr and Al-Gd-Zr ternary systems were experimentally investigated by SEM/EDX, XRD and DSC. The enthalpy of mixing of hcp_A3solid solution in the Dy-Zr and Gd-Zr binary systems was calculated by first-principles method. On the basis of this result, evaluated phase diagram and experimental information reported in literatures, the two binary phase diagrams were optimized. Besides, the phase diagrams of Al-Dy and Al-Gd binary systems were re-optimized. Integrating experimental study and first-principles calculation with the CALPHAD method, the thermodynamic descriptions of the Al-Dy-Zr and Al-Gd-Zr systems were established.(4) The atomic mobilities of Al and Zr in the solid solution phases fcc_Al, bcc_A2and hcp_A3have been optimized. Introducing a narrow homogeneity range into the ZrAl3compound, the diffusional growth of it at the interface of Zr/Al diffusion couple has been simulated using the interdiffusion coefficient concept. By fitting the thickness of the ZrAl3layer, the interdiffusion coefficient of the ZrAl3phase was obtained and its dependence of temperature has been determined. A reasonable explanation was given for the difference in diffusion rate of this phase observed in separate experiments.