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,204 北京科技大学学报 第29卷 Al Sr:creep resistance,mechanical properties,and microstrue- alloy.Scripta Mater.2000.43(11):1009 tmre.J0M,2003,55(11):34 [10]Bradai D.Kadi H M.The kinetics of the discontinuous precipi- [6]Liu M P.Wang Q D.Zeng X Q.Mechanical properties and creep tation and dissolution in Mg rich Al alloys.J Mater Sci,1999. behavior of Mg Al-Ca alloys//Ke W.Han E H.Han Y F,et 34,5331 al.Materials Science Forum.Beijing:Trans Tech publications, [11]Xue S,Sun YS.Ding S S,et al.Effects of calcium additions on 2005:763 microstructure and creep behaviour of AE(42)alloy.Mater Sci [7]Pekguleryuz M O,Kaya AA.Creep resistant magnesium alloys Technol,.2005,21(7):847 for powertrain applications.Adv Eng Mater.2003.5(12):866 [12]Min X.Sun Y,Xue F,et al.Analysis of valence electron struc" [8]Blum W.Watzinger B.Zhang P.Creep of die-cast light-weight tures (VES)of intermetallic compounds containing calcium in Mg-Al-base alloy AZ91hp.Adv Eng Mater.2000.2(6):349 Mg-Al-based alloys.Mater Chem Phys.2003.78:88 [9]Guangyin Y,Yangshan S.Wenjiang D.Effects of Sb addition on [13]Regev M.Aghion E.Rosen A.Creep studies of AZ91D pres- the microstructure and mechanical properties of AZ91 magnesium sure die casting.Mater Sci Eng A.1997:123 Microstructures and creep properties of high performance heat resistant magnesium- alkaline earth alloys BAI Jing,SUN Yangshan),XUE Feng),YAN Jingli),QIANG Jing),TAO Weijian2) 1)Department of Materials Science and Engineering,Southeast University,Nanjing 211189.China 2)Welbow Metals Co.Ltd.,Nanjing 211221.China ABSTRACT The microstructures of Mg-4Al alloys containing alkaline earth Sr and Ca were investigated by optical microscope (OM),scanning electronic microscope (SEM)and transmission electron microscope (TEM), and creep properties were also tested.The as-cast microstructure of the studied alloys consists of dendritic Mg and grain boundary second phases.Divorced eutectic and lamellar eutectic Al4Sr,and bulky ternary t phase are observed along grain boundary with 2%of Sr addition to the based alloy.2 of Ca addition results in the for- mation of lamellar eutectic Mg2Ca at grain boundary and AlCa particles in grains.In the Mg 4Al-2Sr1Ca al- loy,grain boundary phases are t phase and lamellar eutectic Mg2Ca,and Al2Ca particles are also precipitated in grains.With the increase of Al content in the Mg4Al-2Sr-Ca,the coarse irregular-shaped (Mg,Al)2Ca eutectic forms along grain boundary and its volume fraction gradually increases,meanw hile Mg2Ca and t phase gradually decrease.The new fine lamellar AlSr appears when the Al content reaches to 7%.The additions of Sr and Ca improve the creep resistance of Mg Al alloys significantly,and the Mg 5Al-2Sr-1Ca and Mg 6Al-2Sr -1Ca alloys indicate the best creep properties in all studied alloys.According to the power -law equation,under conditions of 175C/50~80 MPa and 70 MPa/150~200C,the creep behavior of the Mg-4Al-2Sr alloy is dif- fusion controlled dislocation climb at lower stresses(<60 MPa)and shows the breakdown of power-law at high- er stresses.The creep mechanism of the Mg4Al-2Sr-1Ca alloy seems to be the combined effect of diffusion controlled dislocation movement and grain boundary sliding. KEY WORDS magnesium alloys;strontium;calcium;microstructure;creep resistanceAl－Sr：creep resistancemechanical propertiesand microstructure．JOM200355（11）：34 ［6］ Liu M PWang Q DZeng X Q．Mechanical properties and creep behavior of Mg－Al－Ca alloys∥Ke WHan E HHan Y Fet al．Materials Science Forum．Beijing：Trans Tech publications 2005：763 ［7］ Pekguleryuz M OKaya A A．Creep resistant magnesium alloys for powertrain applications．Adv Eng Mater20035（12）：866 ［8］ Blum WWatzinger BZhang P．Creep of die-cast light-weight Mg－Al－base alloy AZ91hp．Adv Eng Mater20002（6）：349 ［9］ Guangyin YYangshan SWenjiang D．Effects of Sb addition on the microstructure and mechanical properties of AZ91magnesium alloy．Scripta Mater200043（11）：1009 ［10］ Bradai DKadi H M．The kinetics of the discontinuous precipitation and dissolution in Mg-rich Al alloys．J Mater Sci1999 34：5331 ［11］ Xue SSun Y SDing S Set al．Effects of calcium additions on microstructure and creep behaviour of AE（42） alloy．Mater Sci Technol200521（7）：847 ［12］ Min XSun YXue Fet al．Analysis of valence electron structures （VES ） of intermetallic compounds containing calcium in Mg－A-l based alloys．Mater Chem Phys200378：88 ［13］ Regev MAghion ERosen A．Creep studies of AZ91D pressure die casting．Mater Sci Eng A1997：123 Microstructures and creep properties of high-performance heat-resistant magnesiumalkaline earth alloys BAI Jing 1）SUN Y angshan 1）XUE Feng 1）Y A N Jingli 1）QIA NG Jing 1）TAO Weijian 2） 1） Department of Materials Science and EngineeringSoutheast UniversityNanjing211189China 2） Welbow Metals Co．Ltd．Nanjing211221China ABSTRACT The microstructures of Mg－4Al alloys containing alkaline earth Sr and Ca were investigated by optical microscope （OM）scanning electronic microscope （SEM） and transmission electron microscope （TEM） and creep properties were also tested．The as-cast microstructure of the studied alloys consists of dendriticα－Mg and grain boundary second-phases．Divorced eutectic and lamellar eutectic Al4Srand bulky ternary τphase are observed along grain boundary with2％ of Sr addition to the based alloy．2％ of Ca addition results in the formation of lamellar eutectic Mg2Ca at grain boundary and Al2Ca particles in grains．In the Mg－4Al－2Sr－1Ca alloygrain boundary phases are τphase and lamellar eutectic Mg2Caand Al2Ca particles are also precipitated in grains．With the increase of Al content in the Mg－4Al－2Sr－1Cathe coarse irregular-shaped （MgAl）2Ca eutectic forms along grain boundary and its volume fraction gradually increasesmeanwhile Mg2Ca and τphase gradually decrease．The new fine lamellar Al4Sr appears when the Al content reaches to7％．The additions of Sr and Ca improve the creep resistance of Mg－Al alloys significantlyand the Mg－5Al－2Sr－1Ca and Mg－6Al－2Sr －1Ca alloys indicate the best creep properties in all studied alloys．According to the power-law equationunder conditions of175℃／50～80MPa and70MPa／150～200℃the creep behavior of the Mg－4Al－2Sr alloy is diffusion controlled dislocation climb at lower stresses （＜60MPa） and shows the breakdown of power-law at higher stresses．The creep mechanism of the Mg－4Al－2Sr－1Ca alloy seems to be the combined effect of diffusion controlled dislocation movement and grain boundary sliding． KEY WORDS magnesium alloys；strontium；calcium；microstructure；creep resistance ·204· 北 京 科 技 大 学 学 报 第29卷