Investigation On The Phase Equilibria And Glass Formation Ability In The Al-Ce-Mn-Ni System

The Al-based amorphous alloys are attractive due to their unique properties such as the high tensile strength, high corrosion and low density as well as their potential application on aeronautics and astronautcs. The investigation on the glass forming ability (GFA), especially on the thermodynamic modeling of the amorphous forming system, is an important issue on the amorphous alloys research. The key to the thermodynamc modeling of GFA of alloys is the availability of the reliable binary and ternary system phase diagram and thermodynamics information. As major part of the project "Investigation on the relationship between formation of amorphous and quasicrystal and the phase diagram of the Al-Ce-Mn-Ni system" supported by National Natural Science Foundation of China (50571114), the phase equilibria of the Ce-Mn, Al-Ce-Mn and Al-Ce-Ni systems in the Al-Ce-Mn-Ni system have been systematically investigated by means of experimental determination and CALPHAD method coupled with First-Principle calculation of formation enthalpy of ternary compounds. Based on these results, the relationship between the GFA and the thermodynamics of the Al-Ce-Ni system was further investigated. The major work of this study is as following:(1) The phase diagram of the Ce-Mn system in the Al-Ce-Mn-Ni system was determined by XRD and SEM/EDX techniques. Based on these results and the DTA measurements for key alloys, a thermodynamic assessement of the Ce-Mn system was performed by CALPHAD approach. A set of self-consistent thermodynamic parameters has been obtained, which has been successfully utilized to perform the thermodynamic assessment of the Al-Ce-Mn system.(2) Using the XRD, SEM/EDX and EPMA techniques in combination with the calculation of lattice parameters of the identified phases, the phase equilibria at 500 and 600℃of the glass and quasicrystal forming system Al-Ce-Mn were investigated. The existence of the ternary compoundÏ„₁(Al₁₀CeMn₂) was confirmed at 500 and 600℃.According to the present work, the isothermal sections of the Al-Ce-Mn system at 500 and 600℃have been constructed. Based on the present experimental results, the thermodynamic assessment of the Al-Ce-Mn system was carried out. A set of self-consistent thermodynamic parameters has been obtained. The phase equilibria in the whole composition range were calculated based on the obtained parameters.(3) Ternary phases and the phase equilibria at 500 and 800℃in the Al-Ce-Ni system, which has the high glass formation ability among Al-Ce-TM(TM-transition metal) amorphous alloy systems, have been systematically investigated by XRD, SEM/EDX and lattice parameters calculation. The crystal structure and composition ofÏ„₇(Al₇₀Ce₁₀Ni₂₀) was determined. Besides, three new ternary compounds have been observed. There exists T₃(Al₃₅Ce_16.7Ni₄₈) at 500℃, and Tn(Al_59.8Ce_12.1Ni_28.1) andÏ„₁₂(Al_40.3Ce_30.4Ni_29.3) at 800℃. The orthorhombic crystal structure forÏ„₁₂ has been preliminarily determined experimently. According to the present work, the isothermal sections at 500 and 800℃of the Al-Ce-Ni system have been determined. Based on the reliable binary parameters of the Al-Ce, Al-Ni and Ce-Ni systems, the occurrence of liquid was analyzed by thermodynamic extrapolation, which is confirmed experimentally confirmed. The calculated result revealed that liquid appears in the Ni-rich side if the Al addition is larger than 1 at.% Al, which is confirmed by XRD, SEM/EDX and DTA. The thermodynamic assessement of the Al-rich side of the Al-Ce-Ni system has been performed supported by experiments, First-Principle calculation results. A set of thermodynamic parameters have been obtained. The calculated isothermal sections, vertical sections and invariant reactions agree well with the experimental results.(4) Based on the result from thermodynamic assessment, the relationship between the GFA and phase diagram of the Al-Ce-Ni system was investigated. It was revealed that the alloys with good glass forming ability in the Al-Ce-Ni system are in off-eutectic, indicating no deep eutectics in the Al-Ce-Ni system. It was found that the negative heats of mixing enhance the tendency to form chemical shor-range ordering (CSRO) and improve the metastability of the undercooled liquid and the GFA. The alloy with eutectic composition has poor GFA due to the larger heats of mixing. The results from melt-spinning experimental and the calculation of driving force of crystalline phases in undercooling liquid state indicated that the negative heats of mixing and minimum driving forces are the criterions for selecting the composition for formation of the Al-Ce-Ni amorphous.