Oxidation Of Magnesium Alloys In The Atmospheres Containing HFC-134a And Reaction Between Molten Magnesium Alloys And Some Ceramic Materials

In this thesis, the oxidation behavior of molten magnesium and magnesium alloysin the atmospheres of air containing HFC-134a has been systematically investigated.The oxide products on the melts were examined by SEM, EDS, XRD, XPS and AES.The oxidation kinetics were examined by the weight gain method. The effects of theconcentration of HFC-134a in the gases mixture, melt temperature, exposure time andgas composition on the oxidation behavior were studied and the oxidation mechanismwas discussed. In addition, in order to solve the pollution problem of impurityelements from iron-base crucibles to molten magnesium and its alloys during melting,the reactions between molten magnesium and some ceramic materials were alsoinvestigated. The main contents and results are presented as follows:1 The microstructure of the surface films, formed on molten magnesium and itsalloys in the atmospheres of air containing HFC-134a, has been studied. The resultsshowed that the film formed on molten magnesium was composed of MgF₂, MgO andC. The film on molten AZ91 alloy was MgF₂, MgO, C and small amount of AlF₃ andAl₂O₃. The film on molten MB15 alloy was MgF₂, MgO, C and small amount of ZrF₄,and ZrO₂. The film on molten MB8 alloy was MgF₂, MgO, C and small amount ofCeF₄, AlF₃ and CeO₂. The surface morphology and thickness of the films as well asthe content of each component in the films varied with the concentration of HFC-134ain the gases mixture, melt temperature and exposure time. The distribution of eachcomponent throughout the film thickness was uneven.2 The oxidation kinetics of molten magnesium and its alloys in the atmospheres ofair containing HFC-134a were examined by the weight gain method. It was found thatthe oxidation of molten magnesium and its alloys in the atmospheres of air containingHFC-134a exhibited different kinetics modes as the concentration of HFC-134a in thegases mixture and melt temperature changed. The oxidation for molten magnesium and its alloys obeyed parabolic laws at higher HFC-134a concentration and linearlaws at lower HFC-134a concentration. Under the same HFC-134a concentration andmelt temperature, the oxidation kinetics of the melts sometimes changed from onepattern into another pattern with increasing exposure time. The oxidation rate ofmolten magnesium and its alloy decreased with increasing the concentration ofHFC-134a in the gases mixture and decreasing melt temperature.3 It was found that the film formed on molten magnesium in the atmospheres ofCO₂ containing HFC-134a consisted of MgF₂, MgO, C and traces of CO and CO₂.The oxidation kinetics of molten magnesium in the atmospheres were dependent onthe concentration of HFC-134a in the gases mixture and melt temperature. At lowertemperature, the oxidation kinetics almost followed parabolic law. At highertemperature, the oxidation kinetics followed parabolic law when the concentration ofHFC-134a in the gases mixture was higher, but followed linear law when theHFC-134a concentration was lower. The oxidation rate decreased with increasing theconcentration of HFC-134a in the gases mixture and decreasing melt temperature.4 The degradation products resulting from the interaction of the cover gasesmixture with the heated melt surface were measured by FTIR, GC-MS and GC. Theresult showed that the primary degradation products are CO₂, CO, HF. Thedegradation rate of 1%HFC-134a in air at 700℃was about 80%. HF concentrationwas about 126 ppm when using air as a diluent.5 Based on the above results and thermodynamic analysis, the oxidationmechanism of molten magnesium in the atmospheres of air containing HFC-134a wasdiscussed. The effects of alloying elements, diluent gas and melt temperature onoxidation were also studied. It was found that the effective protection of HFC-134afor molten magnesiumthe is mainly due to the formation of MgF₂ in the oxide film.6 In order to find a suitable alternative to iron-based crucible for magnesium andits alloy melting, the chemical stability of some ceramic materials in moltenmagnesium and magnesium alloy was discussed based on the results of thermodynamics calculation. The stability of Si₃N₄/SiC to liquid magnesium and itsalloy was also tested-by experiment. The experimental results showed that SiC/Si₃N₄has good stability to liquid magnesium and its alloy, so it may used as the materials ofcrucible for magnesium and its alloy melting.