Investigation Of The Phase Relations In The Mg-RE Alloys

Magnesium alloys have been attracting many attentions in recentyears due to their properties such as low density, high specific strength,good castability, etc. Rare earths (RE), as an important class of alloyingelements to magnesium-based alloys, are often added in Mg matrix toenhance the high temperature properties, corrosion-resistance and castingcharacteristics of the alloys.To improve our understanding about the precipitating process anddesign alloy compositions, knowledge of phase diagrams andthermodynamic data of the involved systems are crucially necessary. Inthis paper experimental investigation and thermodynamic calculation(CALPHAD) were carried out on the phase relations of somemagnesium-RE systems as described in the following.(1) The phase relations in the Mg-rich part of the Mg-Nd-Y systemat 773 K were investigated by scanning electron microscopy (SEM),X-ray diffraction (XRD) and electron-probe microanalysis (EPMA). Theexperimental results show that there are three three-phase regions,(Mg)+Mg₂₄Y₅+β,(Mg)+Mg₄₁Nd₅+β,β+Mg₂Y+Mg₃Nd, and fivetwo-phase regions, (Mg)+β,(Mg)+Mg₄₁Nd₅,Mg₂₄Y₅+β,Mg₄₁Nd₅+β,β+Mg₃Nd. Both ternary phaseβand binary phase Mg₃Nd have largelyextended homogeneities along Nd to Y.Thermodynamic assessment and optimization of the Mg-Nd systemwas carried out. Gibbs energies of the disordered bcc_A2 and orderedbcc_B2 phases were modeled with a single expression. Liquid andterminal solutions were modeled as substitutional solutions, and otherintermediate phases were treated as stoichiometric compounds.Reasonable agreement between the calculated and experimental data wasachieved.A minor revision for the bcc_B2 phase in the Mg-Y binary systemwas carried out to reduce the orders of the parameters. Calculated resultsbased on this revision are in good agreement with the experimental data.Based on the experiment results in the present work, the Mg-Nd-Yternary system was optimized. Reasonable agreement of the phase equilibria in the Mg-Nd-Y ternary system between thermodynamiccalculation and experiments was achieved.(2) The Ce-Y system was critically evaluated and optimized. Allphases were treated as substitutional solution phases. It results in aconsistent formulation of the Gibbs energies of all phases in this system.In order to treat the ordered and disordered transaction betweenbcc_B2 phase and bcc_A2 phase, the ordered phase bcc_B2 in the Mg-Cebinary system was described with a 2-sublattice model, instead ofstoichiometric compound model in the literature. The model parameterswere optimized in the present work. In comparison with experimentaldata, the present results are more reasonable.Based on the reported data, the Mg-Ce-Y temary was optimized. Thecalculated phase relations are in accord with experimental ones.(3) A thermodynamic description of the Al-Yb binary system wasfirstly developed based on critically evaluated experimental data. Twodifferent thermodynamic models were applied to two different types ofphases in this system, i.e., solution phases and stoichiometric compounds.Calculated results from the obtained model parameters are in goodagreement with the selected experimental data.Combined with the optimized parameters in the literature for theAl-Mg and Mg-Yb binary systems, the Al-Mg-Yb ternary system wasextrapolated. The liquid project and several vertical and isothermalsections were calculated.(4) Using the present database and the one reported in the literature,the equilibrium solidification and the solidification according to theScheil model were calculated for the alloys Mg-19.2 wt.ï¼…Y-8.03 wt.ï¼…Ce, Mg-23 wt.ï¼…Ce, Mg-8 wt.ï¼…Al-1 wt.ï¼…Yb and Mg-6.4 at.ï¼…Zn-1.1at.ï¼…Y. The calculated results were successfully applied to understand theexperimental microstructures reported in the literature for the abovementioned Mg-Ce-Y alloys and Mg-Zn-Y alloy. The solidificationsimulation for the Mg-Al-Yb alloy can help to understand the resultingmicrostructure during solidification or annealing.