The Effect of CaO on Magnesium and Magnesium Calcium Alloys

CaO is used as a cheap replacement for Ca addition to addition in Mg. CaO as an alloying element are used in the ECO-Mg (Environment COnscious magnesium) alloys. The CaO dissociation in Mg is reported in literature, but with out a theory that can which explain the dissociation of CaO in Mg alloys. The effect of Ca on the oxidation resistance was known for long time and also observed from ECO-Mg. However an explanation on how Ca or CaO is incorporated into the oxide layer of Mg-Ca has not been clarified. This thesis investigate the hypothesis that the stability of CaO and MgO in the commonly used Ellingham diagram is not accurately described. The in situ synchrotron radiation diffraction experimental investigation of the melting and solidification and thermodynamic calculation show, that the change of Gibbs free energy ∆G must be considered for the entire Mg-Ca-O system and not only the stoichiometric reactions. This investigation shows that Mg reduces CaO even in solid state, which is the opposite of the commonly ascribed view. The in situ experiments were performed on the Mg-xCaO and Mg-xCa-6CaO systems. It was possible to follower the dissociation of the phases during melting and the phase formation during the solidification. The experimental results from the solidification sequence were used to validate the thermodynamic calculations. A number of different literature values of the thermodynamic data of CaO and MgO were used to calculate the ternary phase diagram Mg-Ca-O. Based on the thermodynamic calculations a theory was developed, on how Ca is incorporated in the oxide layer. The theory used the idea of a Pilling–Bedworth ratio (PBR) for CaO on pure Mg (PBR > 1) and the M gO on pure M g (PBR < 1). At a volume fraction of CaO higher as 0.49 in the oxide layer forms with a PBR of > 1