Effect Of Extrusion Process And Thermomechanical Treatment On Microstructure And Properties Of Mg-Gd-Y-Zr Alloy

In recent years,with the rapid development of aeronautics and astronautics,there has been an urgent requirement for light-weight high strength magnesium alloys with favorable heat resistance in these fields.Mg-Gd-Y alloy is the most promising candidate due to the remarkable effect of age hardening and excellent mechanical properties at both room temperature and high temperature.In general,in order to develop the Mg-Gd-Y alloy with high strength,the relatively high contents of Gd and Y are added,which leads to the high density and the serious deterioration of the plasticity.How to improve the mechanical properties of such low-density alloy is a topic worthy of study.Based on it,this paper takes Mg-6Gd-1Y-0.5Zr alloy as the research object,and makes alloy plates by homogenization and hot extrusion（420℃/extrusion ratio 24.1-1#,355℃/extrusion ratio 24.1-2#,355℃/extrusion ratio 10.2-3#）.And the hot extrusion sheet with the best mechanical properties was selected as the research object,and the thermomechanical treatment（cold rolling deformation+peak aged treatment and pre-aging+cold rolling deformation+peak aged treatment）was carried out to study the effect of thermomechanical treatment on aged behavior and microstructure and properties of the alloy.And the related mechanism was discussed.The main research contents and conclusions of this paper are as follows:（1）The effects of three extrusion processes on the microstructure,aging behavior and mechanical properties of Mg-6Gd-1Y-0.5Zr alloy were investigated.The results show that the microstructure of the three hot extrusion alloys is typical dynamic recrystallization.And the grain size of alloy 3#is the smallest,at 6.5μm.The second phase of the three hot extrusion alloys areβ-Mg₅（Gd,Y）and the rare-earth rich square phase Mg₂Y₃Gd₂.The grain size of alloy 1#,2#and 3#are 10.9μm,10.3μm and 6.5μm,respectively,indicating dynamic recrystallization.The results show that the peak aging hardness of the three extruded alloys decreases with the increase of aging temperature（175℃,190℃,205℃）,while the peak aging time decreases with the increase of aging temperature.The optimum aging temperature of the three extruded alloys is 175℃.After peak aging,the hardness of alloy 3#is the highest,reaching 123HV.And its tensile strength is 375MPa at room temperature and 292MPa at 240℃.The contributions of different strengthening mechanisms to yield strength were calculated.The results show that the contributions of grain boundary strengthening,solution strengthening,precipitation strengthening and dislocation strengthening is 39.8%,24.4%,24%and 11.8%,respectively.（2）The effects of pre-deformation and aging treatment on microstructure,aged behavior and mechanical properties of Mg-6.0Gd-1.0Y-0.5Zr alloy were investigated.The results show that the dislocation density increases and the peak-aged time decreases with the increase of pre-deformation amount.When the pre-deformation increases from 0%to 15%,the aged time of the alloy decreases from 272h to 200h.When the strain rate increases from 0.1s^-1 to 10s^-1,the peak aged time of the alloy decreases from 224h to 208h.The peak aged strength increases gradually with the increase of deformation,while the elongation decreases.With the 10%pre-strain and1s^-1of strain rate is,the peak-aged mechanical properties of the alloy are the best,with yield strength of 344 MPa,tensile strength of 400 MPa and elongation of 6.1%.（3）The effects of pre-aging on the aging behavior and mechanical properties of Mg-6.0Gd-1.0Y-0.5Zr alloy were investigated.According to the aging hardness curve of 3#alloy at 175℃,60h with faster hardness rise was selected as the pre-aging time.The results show that the peak aged time is greatly shortened and the mechanical properties are effectively improved by the pre-aging process.The peak aging time of the alloy by 175℃/60h pre-aging+15%cold rolling reduction+peak aging treatment is 60h.So,the aging treatment is 120h in total.Considering strength and elongation,the optimal thermomechanical treatment is 175℃/96h with 10%rolling reduction after pre-aging at 175℃/60h.The tensile strength,the yield strength and the elongation to rupture of the peak aged alloy at room temperature are 419 MPa,352 MPa and 5.3%,respectively.