Investigation On The Phase Diagrams And Thermodynamics Of The Mg-Nn-Fe-Ni, Mg-Mn-Ca-Zr And Mg-Mn-Al-Li Systems

Compared with steel or Al alloys, Mg alloys have some intrinsic properties, such as lowest density among structure metals, high specific strength and stiffness, good machinability and handling as well as high recycling potential. Due to the crisis of energy resources and raw materials, the systematic investigations on Mg alloys have been presenting great application values. The phase equilibria and thermodynamics on Mg alloys have become more and more important. However, due to the high evaporability and reactivity of Mg, the systematic research of the phase equilibria on Mg alloys, especially to the systems containing Ca, Mg or Mn, is a worldwide challenge.The present work is a part of the project "Experimental investigation, thermodynamic modeling and simulation of microstructural evolution of multi-component Mg-based alloy systems". The overall idea of the present thesis is:(â… ) An experimental method for the preparation and determination of the alloys containing the elements with high evaporability and reactivity, has been invented,(â…¡) by means of this method, the ternary systems containing Mg in the Mg-Mn-Fe-Ni, Mg-Mn-Al-Li and Mg-Mn-Ca-Zr system have been experimentally determined,(â…¢) and considering all the experimental data, all the ternary systems have been optimized thermodynamically. The major achievements of the present work are as follows:(1) In the research on the Fe-Mg-Mn, Fe-Mg-Ni and Mg-Mn-Ni systems the procedures for the preparation of Mg alloys using powder metallurgy method have been developed. During the investigation on the Ca-Mg-Mn and Ca-Mg-Zr systems, in view of the high reactivity of Ca, the procedure has been modified. The difficulty associated with the sample preparation resulting from the high evaporability and reactivity of Mg has been solved by the improved technique.(2) To solve the problems of the thermal analysis for Mg alloys, which is a worldwide challenge, we have invented a specially designed experimental method. This method, which has overcome the negative effects during thermal analysis resulting from high evaporability and reactivity of some elements, can be used for the investigation of the Mg alloys, Li battery and hydrogen storage materials. High accuracy and low contents of H2O and O2in the atmosphere are two significant advantages of the present method. The invented method has been used to determine the phase transformation temperatures of the Mg-Mn-Ni, Mg-Mn-Zr, Ca-Mg-Mn, Ca-Mg-Zr, Ca-Mn and Ca-Zr systems.(3) In the present work, the artificial phase relations, such as inverse miscibility gap, re-stabilization of the liquid phase at low temperature, decomposition of compounds, which are common phenomenons in thermodynamic calculations, have been discussed in details. The corresponding thermodynamic constraints have been presented to eliminate these problems. Allowing for all the thermodynamic constraints, the thermodynamic optimization has been performed for the Al-Li-Mg, Ca-Mg-Mn and Mg-Mn-Ni systems on the entire compositional and temperature ranges. The artificial phase relations have been eliminated by the thermodynamic constraints.(4) The experimental work is the foundation of the present research. By uing self-designed experimental method, the isothermal sections and phase transformation temperatures of the Fe-Mg-Mn, Fe-Mg-Ni, Mg-Mn-Ni, Mg-Mn-Zr, Ca-Mg-Mn and Ca-Mg-Zr systems have been determined. Based on the present experimental data and literature data, the thermodynamic calculation has been performed on the Li-Mg, Ca-Mg and Ca-Zr binary systems and Fe-Mg-Mn, Fe-Mg-Ni, Mg-Mn-Ni, Mg-Mn-Zr, Ca-Mg-Mn, Ca-Mg-Zr ternary systems. For the first time, most of these systems are experimentally determined and optimized in the present work. The present descriptions of all the systems are uniform. For example, the phases with the same crystal structure in various systems have been unified which is critical to the application of this database to high order systems. The thermodynamic database for Mg alloys has been expanded and developed in the present work.