| Due to the importance of automobile lightweight,the heat-resistant magnesium alloy applied to automotive power system needs to have the capability of serving in high temperature for a long time.The research of magnesium alloys with high temperature and heat resistance is the key and difficult point in the research of magnesium alloys.At present,the research on high temperature heat-resistant magnesium alloys is mainly focus on the compssition design,creep properties and fracture mechanism of Mg-Al-Ca?AX system?.Mg-5Al-5Ca-xSn?x=0,0.5,1,1.5?alloys?The following is simplified as AX55-xSn?x=0,0.5,1,1.5?,wt.%?were recently studied by the author's research group.Previous studies have shown the alloy in 175-200?has good high temperature creep resistance.Therefore,the AX55-x Sn?x=0,0.5,1,1.5?alloys were chosen as the research object,using scanning electron microscope?SEM?analysis,microhardness testing and high temperature alloy tensile creep experiment to investigate the alloys:1)AX55-xSn?x=0,0.5,1,1.5?alloys after 350/32 h+500/16 h solid solution treatment+200/24 h aging treatment,as well as AX55-xSn?x=0,0.5,1,1.5?alloys after 500/24 h solid solution treatment+200/96 h aging treatment tested at175?/75MPa for 100 h creep experiments respectively;2)the high temperature creep performance of the AX55-1.5Sn alloy will be performed for 150-225?/50-80MPa for 100h high temperature creep test,and the mechanism of high temperature deformation of the alloy structure and strengthening mechanism will be studied.The research results of the high temperature creep properties of AX55-xSn?x=0,0.5,1,1.5?alloy are as follows:?1?Under the condition of two kinds of heat treatment,the main phase composition consistent of AX55-xSn?x=0.5,1,0.5?alloy are?-Mg,Al2Ca,?Mg,Al?2 Ca,CaMgSn phase.There is no CaMgSn phase in AX55 the alloy.The CaMgSn phase distributed along the skeleton structure with large volume fraction.?2?The addition of Sn elements elevated the eutectic temperature of the alloy,which lead to more Al2Ca phase precipitated in the substrates.There are more quantity of Al2Ca phase in the substrates after 500?/24 h+200?/96 h heat treatment compared to the alloy after 350/32h+500/16 h+200/24 h heat treatment.The Vickers hardness of AX55-1.5Sn reaches to 124.51Hv at room temperature.?3?After high temperature,the cracks in AX55-xSn?x=0,0.5,1,1.5?alloy occured nearby the skeleton phase and extended along the skeleton phase.The failure forms of alloy after high temperature creep is intergranular fracture.There are only few holes observed in the position near the skeleton phase.The CaMgSn phase occured nearby the skeleton facies and the cracking phenomenon could be observed between the skeleton facies and CaMgSn phase.There were few amount of skeleton phase fall off,which is considered as stress concentrations exist between CaMgSn phase and skeleton facies.?4?The AX55-xSn?x=0,0.5,1,1.5?alloy treated by 500?/24 h+200?/96 h heat treatment has lower creep rate and lower the total creep strain after creep test at 175?/70 MPa for 100 hours which means a better temperature creep resistance.The AX55-1.5 Sn alloy shows the best creep resistance in present investigation.?5?AX55-1.5Sn alloys were tested at 175?/50MPa 60MPa 70MPa 80MPa for 100h,the minimum creep rate were 1.64×10-8s-1,3.8×10-8s-1,5.21×10-8s-1,1.23×10-7s-1 respectively.The total creep variables were 0.048%,0.052%,0.066%,0.086%.AX55-1.5Sn alloys were tested at 150?175?200?225?/50MPa for 100h,the minimum creep rate were 6.01×10-9s-1,1.64×10-8s-1,5.04×10-8s-1,1.46×10-7s-1 respectively.The total creep variables were 0.037%,0.048%,0.071%,0.088%.?6?When the stress value is 50-70MPa,the stress exponent of the alloy is n=3.5.The creep deformation mechanism was mainly dominated by grain boundary sliding When the stress value is 80 MPa,alloy stress exponent of the alloy is n=6.1.The creep deformation mechanism was mainly dominated by dislocation climbing.In addition,when the temperature is 150-225?,the creep activation energy of AX55-1.5Sn alloys is Q?162.3kJ/mol. |
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