| Based on the study of EW75(Mg-7Gd-5Y-1.2Nd-0.5Zr) alloy, this research aimed at the addition of trace Al element on EW75and its influence on microstructure and mechanical properties. By using the method of OM, SEM, TEM, hardness, tensile test, creep experiment, recycling the waste of EW75with Al is discussed and new Mg-Gd-Y-Al alloy is developed.It was discovered that AbZr, which is a primary phase and formed by the reaction between Al and Zr, then segregated to the bottom of crucible seriously. EW75-xAl(x=0,0.2,0.4,0.7wt.%) alloys were prepared by smelt technology, when under the as-cast condition, the grain size is about40μm-80μm, the microstructure is composed of a-Mg, MgsRE eutectic and Al2RE phase, meanwhile its tensile strength range from175MPa to215MPa, yield strength is from159MPa to187MPa, ductility is about1%to3.5%at room temperature. When it experienced homogenization treatment by545℃/16h, the eutectic structure is completely dissolved into matrix, but Al2RE phase is seldom dissolved, the grain size is protect from growing under high temperature. By the way, its strength remains stable after this heat-treatment.The performances of EW75-xAl alloy are changed under the condition of extruding, the grain size ranges from8μm to13μm, tensile strength is from328MPa to344MPa, yield strength is about230MPa to252MPa and ductility remains9.2%to15%. The strength tends to be first decreases, then increases and last decreases again with different additives of Al, the highest strength is EW75-0.4A1. By the T5treatment with the process of225℃/12h, the strength is increased about100MPa compared to the extrusion state alloy, tensile strength ranged from406MPa to460MPa, yield strength is from321MPa to340MPa and plasticity is about1.2%to3.3%. The strength of EW75-0.4A1alloy is still the highest. The alloy under T5condition, high temperature properties at200℃are more considerable than that of250℃. The specifications is that tensile strength ranges from365MPa to403MPa, yield strength is from315MPa to347MPa at200℃, while the temperature increases to250℃, its tensile strength changes from327MPa to361MPa and yield strength is from287MPa to311MPa.The creep behaviour of EW75-xAl alloys under aging state of T5were studied, the creep condition is temperature of200℃,225℃,250℃and loads of100MPa, the steady creep rate shows8.06E-09,6.61E-08and8.41E-07(s-1), and it will improve an order of magnitude by temperature rising. Results show that when the temperature is250℃, creep test specimen fracture with time of70h and total creep variable of44.74%. When creep condition is temperature of200℃and loads of100MPa,125MPa,150MPa, the steady creep rate present8.06E-09,1.25E-08and2.44E-08(S-1). Seldom changing indicates that the influence of temperature on creep performance plays more important role than that of stress, that is, creep performance is insensitivity to stress. By the way, creep activation energy of alloy under T5state is190.7kJ/mol, stress exponent is2.7. Research show that alloys under T5state with temperature ranging from200℃to250℃and loads ranging from100MPa to150MPa, its mechanism of creep is dislocation creep, and creep rate is mainly controlled by the dislocation viscous movement. |
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