Study On Microstructure And Mechanical Behaviors Of Near-liquidus Squeeze Cast Mg-Gd-Y-Zr Magnesium Alloy

The newly developed Mg-hREs(heavy rare-earth elements)alloys,typically the Mg-10Gd-3Y-0.5Zr(GW103K)alloy,has been proved to show good room temperature strength and excellent elevated temperature strength,and thus this series of alloys have received considerable concerns both in academic circles and industries for potential applications in auto-mobile,aero-space and military or weapon industries.The fabricating approaches for producing Mg-RE alloy components are mainly based on permanent steel mold casting or sand casting,which is either low efficiency or creating coarse grains in the casting.Although high pressure die casting(HPDC)is a good potential approach that can solve these problems,it would introduce entrapped gas into the casting,thus the HPDC casting cannot be subjected to high temperature solution heat-treatment,which limits its application in the age-hardenable Mg-RE alloys system.Therefore,it is urgent to develop new techniques and propose new theories to expand the application of Mg-RE alloy for future applications.The near-liquidus squeeze casting(NSC),based on the conventional squeeze casting(SC)process,is a newly developed liquid forming process that could potentially be applied for near-net shape forming of light weight metals.The near-liquidus squeeze casting process employs the low frequency electro-magnetic stirring process to pre-treat the alloy melt to a temperature range that is quite close to its liquidus line,and then the pre-treated alloy melts are poured into the squeeze casting mold and subj ected to applied pressure for secondary solidification.The process is believed to effectively reduce the casting porosities,refine the microstructures and enhance the mechanical performances,and therefore it could be a potentially applicable forming technique.Literatures on near-liquidus squeeze cast are mainly towards Al or Mg-Al alloys,and limited publications could be found on Mg-RE alloys.Thus,it is both theoretically and practically needed to enhance the strength of Mg-RE alloys and analysis the microstructure evolution and strengthening mechanism of squeeze cast and near-liquidus squeeze cast Mg-RE alloys.Based on aforementioned backgrounds,the currently study is mainly focused on the well-developed GW103K alloy,utilizing optical metallography(OM),scanning electron microscope(SEM),electron back-scattered diffraction(EB SD),X-ray diffraction(XRD)and transmission electron microscope(TEM)analyze the microstructures and phases.The mechanical behaviors of the studied alloys were examined by hardness test,room and elevated temperature tensile test,and elevated temperature compression creep test for samples fabricated by different casting approaches and under variant heat-treatment conditions.The materials processing procedures were systematically studied in the near-liquidus squeeze cast processes,and the related microstructure refinement mechanisms and strengthening mechanisms were well discussed.The formation mechanism and formation criterion of non-dendritic microstructure of Mg-RE alloys under LFEMS process were proposed.Moreover,the creep behavior and microstructure evolution mechanism in crept samples were clearly analyzed.The effects of applied pressure and pouring temperature on the microstructure and mechanical behavior of squeeze cast Mg-Gd-Y-Zr alloy were systematically studied,and optimum ranges of processing parameters were obtained.Under the optimized processing parameters,the microstructures were refined and homogenized and the mechanical properties were significantly improved.The grain refinement mechanism was revealed to be combined effects of decreased critical heterogeneous nucleus size and enhanced nucleation rate.The equations for critical nucleus size and nucleation rate were deduced for solidification of Mg-RE alloys under applied pressures.The relationship among critical nucleus radius(r*’),applied pressure(P)and nucleation undercooling(ΔT)should fulfill the following equation:r*’= 0.35/ΔT+0.0537P.The nucleation rate is calculated to be increased from 6.87×1011m-3 to 8.28×1012 m-3 when the applied pressure is increased from 0.1 MPa to 160 MPa.The formation mechanism of non-dendritic microstructure of Mg-RE alloys during low frequency electro-magnetic stirring(LFEMS)treatment was revealed to be that the applied low frequency electro-magnetic field accelerates the non-uniform distribution of temperature and solute at the interface of the root of secondary dendrite arms(SDAs),which promotes the necking process and leads to "fragments".The fragmented SDAs were subj ected to a relatively low cooling condition and shearing effect and progressively ripening and forming the non-dendritic microstructures.The formation criterion of non-dendritic microstructure in Mg-RE alloys treated by LFEMS was proposed to be:(a)dL>8λ2 and(b)CR≈▽T/Tλ22(1-fs)2/180fs2·ηk2U2/μo·dind·e2r(?)>1.Criterion(a)requires the penetrating depth of semisolid slurries should be 8 times larger than the secondary dendrite arm spacing,which equals to that solute concentration should be higher than a critical limit.In Mg-RE alloys,the atomic fraction of solute RE elements should be higher than～1.5%.Criterion(b)requires that the processing parameters in the LFEMS including applied voltage,rotational frequency and cooling rate should all exceed a critical value to "fragment" the dendrites.The effects of heat-treatments on the microstructure and mechanical properties of squeeze cast and near-liquidus squeeze cast Mg-Gd-Y-Zr alloy were studied.Optimized solution and aging heat-treatment parameters were obtained.Under the optimal parameters,the alloy could obtain good mechanical properties in both T4 and T6 states.The strengthening mechanisms were discussed for Mg-Gd-Y-Zr alloys made by SC and NSC under variant heat-treatment conditions,and the strength contributions for variant strengthening mechanisms were calculated,and it was found that grain boundary strengthening is the major contribution to the increased strength in squeeze cast GW103K alloy fabricated under higher applied pressures than that in lower applied pressures.The governing mechanism for improvement in UTS and Ef in squeeze cast and near-liquidus squeeze cast Mg-RE alloys were also discussed and quantified based on the classical Ghosh model by considering the role of casting pores.The related further grain refinement and strength enhancement approaches were proposed.The innovative new technique of near-liquidus squeeze cast with the aid of low frequency electro-magnetic stirring to pre-treat the alloy melt was invented.By utilizing this technique,the pure Mg,Mg-Zr binary alloys,Mg-Gd binary alloys,and Mg-Gd-Y-Zr multicomponent alloys ingots with refined,homogenized,porosity free and nearly defect free microstructure were successfully prepared.The grain size in the near-liquidus squeeze cast pure Mg was nearly the same as that in the gravity cast Mg-0.3Zr alloy.The effect of pouring temperature on the microstructure and mechanical properties of NSC fabricated Mg-Gd-Y-Zr alloy was studied,and the optimal range of pouring temperature was found to be 635 0C～660 ℃,and the consequent NSC fabricated alloy can obtain a grain size of 25～30 μm.The elevated temperature compression creep test was comparatively conducted for the gravity cast,squeeze cast and near-liquidus squeeze cast GW103K alloys in their T6 heat-treated states.The minimum creep rate was found to be the same for three different casting approaches at the testing temperature between 200 ℃ and 250 ℃ and under the applied load from 50 MPa to 120 MPa,indicating the refinement of microstructures in near-liquidus squeeze cast alloy lead to limited drawbacks to the creep resistance.The calculations of stress exponent and creep activation energy and related microstructures confirmed that the creep resistance is mainly dominated by the impeding effect of precipitates on dislocation movements and trivially influenced by grain boundary diffusion mechanism.The precipitate was evolved from 20～30 nm sized spindle-shape before the creep test to approximately 300～600 nm sized linear chain-like after the creep test.The related microstructure formation mechanism was proposed.Furthermore,the precipitation free zone(PFZ)existed in the crept samples both in areas close to or far away from the grain boundaries,which was revealed to be formed at surrounded area of residual secondary phase in grain boundaries or intermetallics.The formation mechanism was considered to be due to the repeated dislocations aggregation around the secondary phase under applied load and dislocation annihilation at elevated temperatures dissolve the precipitates to the matrix.