| As lightweight structural materials,Mg alloys have great application potential in the fields of national defense,electronics,automobile,aerospace and so on.However,the practical applications of Mg alloys are seriously hampered by their unfavorable properties,such as lower tensile strength,yield strength and high temperature creep resistance,as well as inferior toughness and corrosion resistance,etc.To enhance the mechanical properties of Mg alloys,the addition of rare-earth(RE)elements followed by the subsequent ageing treatment has been demonstrated to be the effective method.The heavy rare earth elements Gd and Y are regarded as two effective alloying elements for solution strengthening and precipitation strengthening.Therefore,among the precipitation hardenable Mg-RE alloys,Mg-Gd/Y binary and Mg-Gd-Y ternary alloys have attracted extensive attention due to their higher strength,superior creep resistance and preferable heat resistance.Aging precipitation sequence has important reference significance for reasonably designing material composition,formulating heat treatment scheme,optimizing microstructure and improving mechanical properties.In recent years,the researches on aging precipitation sequence of Mg-RE alloys mainly focus on the prediction of precipitation sequence,crystal structure,chemical composition,atomic occupation,microscopic morphology,precipitation kinetics,strengthening mechanism and so on.However,up to now,the driving force of the aging precipitation behavior has rarely been considered from the perspective of thermodynamics.This work conducts study on the Mg-Gd/Y binary system and the Mg-Gd-Y ternary system.With the help of CALPHAD(CALculation of PHAse Diagram)technique,the effective nucleation driving forces(thermodynamic precipitation driving forces minus nucleation resistances)of precipitated phases in aging precipitation sequence are coupled to the corresponding thermodynamic parameter optimization.The self-consistent and reasonable thermodynamic parameters have been obtained again without changing the original stable phase equilibrium relations.The effective nucleation driving forces of the precipitated phases should correctly reflect the aging precipitation sequence,in order to predict the aging precipitation process.The predicted aging precipitation sequences based on the effective nucleation driving forces are compared with those reported in literatures on the Mg-Gd/Y binary system and the Mg-Gd-Y ternary system as well as experimentally observed in the present work.The main research results have been listed as follows:Firstly,the reasonable thermodynamic models are constructed on the basis of the crystal structure information of the stable phases in equilibrium phase diagram and the metastable phases in aging precipitation sequence.Considering the available experimental data of the phase equilibrium relations,the thermochemical properties and the aging precipitation sequence,and on the basis of the original stable phase equilibrium relations,the thermodynamic re-assessments of the Mg-Gd and the Mg-Y binary systems coupling with the effective nucleation driving forces of precipitated phases in aging precipitation sequence are proceeded with the help of the CALPHAD technique,the PARROT module of Thermo-Calc package and the Pandat software package.Considering that the metastable ordered phases GP zones(D019-type),Mg7Gd and Mg7Y are all the ordered parts of the disordered solid solution HCP_A3 phase,the limiting conditions of the order-disorder transitions are imposed on these metastable precipitated phases during the thermodynamic optimization process.Meanwhile,since the nucleation driving force of each precipitate phase is reduced by the influence of the nucleation resistances(strain energy and interfacial energy)based on the thermodynamic precipitation driving force,the sorting of effective nucleation driving forces(thermodynamic precipitation driving forces minus nucleation resistances)is adopted to correspond to the the aging precipitation sequence during optimization.Using the thermodynamic parameters obtained by re-assessment,the calculated Mg-Gd and Mg-Y binary phase diagrams,thermochemical properties and aging precipitation sequences of typical alloys are consistent with the experimental data reported in literatures.Based on the thermodynamic modeling and re-optimization of the Mg-Gd and Mg-Y binary systems,the same modeling and optimization evaluation are performed for the thermodynamic parameters of the stable and metastable phases in the Mg-Gd-Y ternary system.The thermodynamic parameters obtained by optimizationcan not only well reproduce the stable phase equilibrium relations(including isothermal sections,liquidus surface projection,invariant reactions and isopleth sections)reported in literatures,but also predict the effective nucleation driving forces of each precipitated phase(including metastable and stable precipitated phases)which is in good agreement with the aging precipitation sequence reported in literatures.In particular,the effects of the thermodynamic nucleation driving force and nucleation resistance are comprehensively considered during calculating the effective nucleation driving force,the gradual transition of the precipitation sequences from the Mg-Gd binary side to the Mg-Y binary side are given by predicting and analyzing the effective nucleation driving force of each precipitate phase in Mg-Gd-Y ternary alloy at the Mg-rich region.Mg-Gd-Y ternary alloys with different Gd/Y ratios are designed and prepared,in order to deeply analyze the prediction results of the gradual transition of the aging precipitation sequences from the Mg-Gd binary side to the Mg-Y binary side.The evolution rules of microstructure,phase structure,microhardness and resistivity during aging process are systematically studied,and the age hardening curve,resistivity curve and precipitated phases corresponding to each stage are obtained,and then the aging precipitation sequence of each alloy specimen is determined.It is found that the experimental observation results during aging process are well consistent with the prediction results of the effective nucleation driving forces,which provides an important attempt and useful exploration for the design and development of the precipitation hardenable Mg-RE alloys. |
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