Experimental and thermodynamic study on the Mg-X (X: Ag, Ca, In, Li, Na, Sn, Sr and Zn) multicomponent systems.

摘要

Magnesium alloys, as the current lightest structural metallic materials, have been widely used in automotive, electronic consumer, and aerospace industries. This interest in the use of magnesium alloys arises from their low density and potentially high strength/weight ratios, good processing properties, as well as the possibility of nearly complete recycling. Hitherto, several series of magnesium alloys have been developed for different applications. These alloys include Mg-Al, Mg-Zn, Mg-Mn, and Mg-Rare-earths, etc. However, magnesium alloys display shortcomings. Among them are poor corrosion behavior, low creep resistance at elevated temperatures, and low strength. Consequently, improvements are still needed in the properties of current alloys and in the development of new Mg-based alloys to meet the requirements of industry. Microalloying has been widely used for improving the mechanical properties of alloys. Current research shows that additions of Li, Na, Ca, Zn, Ag, In, Sr, and Sn can improve the mechanical properties of Mg-based alloys, by forming secondary precipitates in the Mg matrix.;As part of ongoing projects in our group to develop a thermodynamic database for Mg-based multicomponent alloys, the main objective of the present work is to establish such a database with added Ag, Ca, In, Li, Na, Sn, Sr and Zn. This is expected to provide a guide for magnesium alloy design, through computational modeling and experimental investigations. Numerous binary and ternary systems in the Mg-X (X: Ag, Ca, In, Li, Na, Sn, Sr and Zn) multicomponent system have been critically evaluated and systematically optimized in the present work.;First, phase equilibria measurements on the five ternary systems: Mg-Sn-X (X: Ag, In, Zn, Ca, and Sr) system have been carried out. Phase equilibria in the Mg-rich region of the Mg-Sn-In (415 and 330 °C), Mg-Sn-Ag (415 and 330 °C), and Mg-Sn-Zn (300 °C) ternary systems were investigated by quenching, electron probe micro-analysis (EPMA), and X-ray diffraction (XRD). Ternary isoplethal sections at constant compositions were inverstigated using differential scanning calorimetry (DSC). These were Mg-In-Sn (5 at. %, 10 at. % Sn), Mg-Sn-Ag (10 at. % Sn, 30 at.% Ag0 and Mg-Sn-Zn (10 at.% Sn). No ternary compounds were found in these three isothermal sections. Iisothermal sections of the Mg-Sn-Ca and Mg-Sn-Sr systems in the Mg-rich region (350 and 415 °C) were inverstigated by quenching key samples. SEM and EDS were used for phase composition analysis. The existence of the ternary phases MgSnCa and MgSnSr was confirmed, and two new ternary phases (Mg5Sn 3Sr and Mg25Sn24Sr14) were found in Mg-Sn-Sr isothermal sections at 415 and 350 °C.;Second, thermodynamic descriptions of 19 binary systems (Mg-In, Mg-Ag, Ag-Zn, Ag-Ca, Ag- Li, In-Na, Na-Sn, Li-Sn, Na-Zn, In-Zn, Sn-Sr, Ca-Li, Ca-Sn, In-Sn, Ca-In, Ca-Na, Ag-In, Ag-Na and Ag-Sn) and 12 ternary systems (Mg-Sn-X, X: Ag, In, Li, Zn, Ca, and Sr, Mg-Zn-In, Mg-Ag-In, Mg-Ca-Li, Mg-Ca-Sr, Mg-Sn-In, and In-Sn-Zn) have been carried out in the present work. These were based on literature review of the solid solutions including intermetallic compounds (crystal structures, melting points, enthalpies of formation, transformation temperatures, etc.). The same has been done for liquid solutions (liquidus curves, integral enthalpies of mixing, partial enthalpies of mixing, activities of the components, heat capacities, etc.). The Modified Quasichemical Model in the Pair Approximation (MQMPA) was used for modeling the liquid solution, which exhibits a high degree of short-range order. The solid phases are modeled with the Compound Energy Formalism (CEF). A self-consistent thermodynamic database was constructed for the Mg-X (X: Ag, Ca, In, Li, Na, Sn, Sr and Zn) multicomponent system.;In developing new magnesium alloys, it is important to understand their constitution (microstructure) and thermodynamic behaviour. Obtaining such information solely through experiment is cumbersome and costly. Phase diagrams, as a visual representation of the state of equilibrium in a system as a function of temperature, pressure and component composition, have been proved to be a useful aid for materials design and processing. Thermodynamic modeling on multi-component systems by the CALPHAD approach has been shown to be a very efficient way of investigating phase equilibria. With the help of computational thermochemistry, not only binary and ternary systems, but also multi-component systems, can be investigated properly.;Third, some applications with the complete thermodynamic database are shown. A study of alloy design for Mg-based multicomponent systems was carried out. The effects of In, Li and Na additions on the properties of Mg-Sn based alloys was also studied.;Futhermore, an experimental study of metallic glass formability in selected Mg-Zn-X ternary systems have been done with the collaboration of Mr. Yi-Nan Zhang of Concordia University. Phase equilbria in the Mg-Zn-Sr ternary system, at 300 °C in the composition range 0-30 at. % Sr, were measured in the present work using key samples and the diffusion-couple technique. Four new ternary compounds were found in this isothermal section. The glass formability of two series of Mg-Zn-Sr alloys of compostions Mg88-xZnxSr 2 (28≤x≤38) and Mg85-yZnySr5 (23≤y≤37) were studied experimentally in the present work.;Use of the thermodynamic database of the Mg-X (X: Ag, Ca, In, Li, Na, Sn, Sr and Zn) multicomponent system provides clear guidelines for selection of Mg-based alloys for design, thereby avoiding tedious and time-consuming experiments.