Study On The Microstructure, Mechanical Properties And Precipitated Phase Transformations Of Mg(-Gd)-Sm-Zr Alloys

WE43and WE54, based on the Mg-Y-Nd system, become more and more attractivefor aerospace and automotive industries and the most successful commercialmagnesium alloys developed to date, because of their high room temperaturemechanical properties, excellent high temperature creep resistance and good corrosionresistance. The strength of these alloys is achieved essentially via precipitationhardening. Gd and Sm belong to the same subgroups as Y and Nd with highermaximum solubility in solid magnesium than Y and Nd, respectively. Therefore, it isreasonable to anticipate a higher precipitation hardening effect in Mg-Gd-Sm than thatin Mg-Y-Nd alloys, which may result in a higher strength. However, there is littleresearch on Mg-Sm binary and Mg-Gd-Sm ternary alloys. It is necessary to dosystematic research on the promiseful Mg（-Gd）-Sm alloys.Several Mg-（6-10）Gd-（2-6）Sm-Zr and Mg-4Sm-Zr（wt.%） alloys were prepared.Effects of variant contents of Gd and Sm, heat treatment and thermal-mechanicalprocess on the microstructure, mechanical properties and fracture behavior ofMg-xGd-2Sm-0.4Zr（wt.%, GSx2K, x=6,8,10） and Mg-6Gd-ySm-0.4Zr（wt.%,GS6yK, y=2,4,6） alloys were investigated, by using inductively coupled plasmaanalyzer（ICP）, optical microscope（OM）, differential thermal analyzer（DTA）, X-raydiffractometer（XRD）, scanning electron microscope（SEM） and transmission electronmicroscope（TEM）. The strengthening mechanisms in the alloys were discussedsemiquantitatively. The evolution of precipitates during ageing was studied in detail.Furthermore, the microstructure, mechanical properties and fracture behavior ofMg-4Sm-0.4Zr（wt.%） alloy were investigated for the first time. The purpose of thepresent work is to provide practical and theoretical information for the development onhigh performance magnesium-rare earth alloys.The as-cast microstructure of the Mg-Gd-Sm-Zr alloys consists of primary α-Mg solid solution and β-Mg5（Sm, Gd） eutectic compounds. The eutectic compound inGS64K was characterized to have face-centered cubic （fcc） structure with a=2.288nmand a stoichiometry of Mg_6.2(Sm_0.56Gd_0.44), which is isomorphous with the equilibriumphase in Mg-Gd binary alloy. The eutectic compound in S4K alloy was characterized tohave body-centered tetragonal （bct） structure with a=1.509nm, c=1.029nm and astoichiometry of Mg_7.7Sm.Microstructural evolution of Mg（-Gd）-Sm-Zr cast alloys during optimized solutiontreatment involves the dissolving of primary eutectic phases, coarsening of grains andappearance of petal-like Zr-rich particles within grains and RE-rich particulate phasesfcc, a=0.5502nm) on grain boundaries （GB）. The particulate phase was probablyinherited from the master alloys, formed during solidification of Mg（-Gd）-Sm-Zr alloysand grew during solution treatment. It will weaken mechanical properties and should bereduced in the mater alloys.The age-hardening curves of GSx2K alloys exhibit two stages with an incubationperiod while those of GS6yK alloys one stage without incubation period. The peakhardness and peak-ageing time rises and shortens, respectively, with increasing Gd andSm contents, and both increase with decreasing ageing temperature. The age-hardeningeffect of Sm is higher than that of Gd. The ageing curves of S4K alloy shows one stageand lower peak hardness and less peak-ageing time than those of GS6yK alloys. Theextruded alloys have similar age-hardening curves with cast alloys with lower hardnessincrement. Lowering extrusion temperature enhances the hardness of as-extruded alloysbut has little effect on ageing curves.The decomposition of α-Mg supersaturated solid solution （S.S.S.S., cph） in GS64Kcast alloy during isothermal ageing at225℃is as follows: S.S.S.S.（cph）â†'β″(D0₁₉)â†'β′（cbco）â†'β₁（fcc）â†'β（fcc）. Fine plate-like β″and ellipsoidal β′precipitatesformed on {1120}αat the early stage of ageing, and β″possessed a higher volumefraction than β′. When peak-aged, both β″and β′precipitates grew in size and β′phasebecame the predominant precipitates. During over-aged stage, β″and β′phasesdiminished gradually, whilst β₁phase precipitated on{1100}αplanes. After long-termageing, the β₁phase transformed in situ to the equilibrium β phase.The decomposition of S.S.S.S. in S4K cast alloy during isothermal ageing at200℃is as follows: S.S.S.S.（cph）â†'β″(D0₁₉)â†'β′（fcc）, without the β′（cbco） phase inGS64K alloy, which resulted from the difference in composition （6wt.%Gd） but not theageing temperature. Fine β″precipitates formed on {1120}αat peak-aged stage anddiminished gradually during over-aged stage, whilst {1100}αβ′plates precipitated and remained up to3208h. The equilibrium β phase was not found. The crystalstructure, lattice parameters and orientation relationship with the matrix of β″and β′phases in S4K cast alloy are the same as those of β″and β₁phases in GS64K cast alloy.β′（cbco） and β″(D0₁₉) phases are the main strengthening precipitates in peak-agedMg-Gd-Sm-Zr and Mg-4Sm-Zr alloys, respectively.The atomic models indicate that the stacking type of atoms in β″(D0₁₉) and β′（cbco）phases are the same as that in the hcp matrix, and the precipitation makes progresssimply by long-period ordering of RE atoms such as Gd and Sm. Both of them keep aperfect coherency with the matrix. The β′phase can generate from the β″lattice by themigration of RE atoms on the（0001）β’’plane in the3aα [1100]αdirection viavacancy mechanism, and then grow in the<1100> β’’directions to form a triangulararrangement on {1120}αprismatic planes. The thickening of β′phase in the<1120> αdirections can be considered to take place via a successive operation of twoopposite shears of magnitudeaα <1120> β’’on the（1100）β’’plane. The phasetransformation process is controlled by diffusion. The nucleation and growth of β′fromβ″phase result in the incubation period on ageing curves. The β₁（fcc） phase maynucleate and grow from β′（cbco）, and transformed in situ to the equilibrium β phase.The β′（fcc） phase in S4K alloy may generate and grow from the β″(D0₁₉) phase.In general, the room temperature （RT） strength and elongation of as-cast, solution-treated and peak-aged cast/extruded alloys rises and descends, respectively, withincreasing Gd and Sm contents or lowering extrusion temperature. For cast alloys, thepeak ultimate tensile strength （UTS） is360MPa and obtained in GS62K alloy whoseyield strength （YS） is low （204MPa）, thus, the best RT properties are achieved inGS102K such that YS237MPa, UTS347MPa and elongation3.2%. The appropriatecomposition range of alloy is6-10wt.%Gd and2-4wt.%Sm. The possible highest RTproperties may be gained in Mg-10Gd-4Sm-0.4Zr cast alloy optimally solution-treatedand peak-aged at225℃. For extruded alloys, the best RT properties are obtained inGS102K such that YS315MPa, UTS410MPa and elongation4.9%. The instant tensilestrengths of cast/extruded alloys, except GS62K, are higher than those of cast/forgedWE54alloys at the temperatures from RT to300℃. The temperature range of use forthe extruded alloys is from RT to200℃, and that for cast alloys is from RT to250℃.The rupture of as-cast, solution-treated and aged cast/extruded alloys mainly belongsto （quasi-）cleavage fracture. The brittle eutectic compounds, particulate phase and theintersection of mechanical twin and GB, precipitates and precipitate-free zones （PFZ） on GB are the main crack sources in as-cast, solution-treated to peak-aged, over-agedalloys, respectively. From RT to250℃, the rupture of peak-aged cast/extruded alloys ismainly transgranular quasi-cleavage fracture; the microcavities coalescing becomes themain fracture mechanism over250℃.Precipitation strengthening is the predominant contribution to YS in peak-aged castalloy. The relationship between solid solution strengthening σssand atomicconcentration c is such that σ ss=3646c. After extrusion and peak-ageing, strengthen-ing contribution of GB due to grain refinement is enhanced. There is a little decrease inabsolute value of precipitation strengthening, the contribution percentage is reduced.The proportion of solid solution strengthening decreases a little and that of texturestrengthening is lowest.