Study On The Microstructure And Properties Of Hot Extrusion Mg-Gd-Y-Sm-Zr Alloys

Magnesium alloy receives extensive attentions,considering its low density and high specific strength.However,conventional magnesium alloys have low mechanical properties and poor heat resistance,in order to expand application field of magnesium alloy,it is necessary to develop heat-resistant magnesium alloy with excellent high temperature mechanical properties.Mg-x Gd-4Y-1Sm-0.5Zr（x=6,8,10;wt.%）alloys were prepared by gravity casting process.The microstructure of three alloys as-cast,solid solution and hot extrusion and the room temperature tensile properties of hot extrusion alloy were systematically studied.The hot extrusion Mg-8Gd-4Y-1Sm-0.5Zr（GWS841）alloy has good microstructure and the best comprehensive mechanical properties.Then taking GWS841 alloy as the research subject,texture evolution,dynamic recrystallization and dynamic precipitation during hot extrusion were systematically studied.The effects on microstructure and mechanical properties of hot extruded GWS841 alloy were discussed,with emphasis on the precipitation characteristics and precipitates evolution mechanism during aging process.The fine microstructure evolution of hot extrusion aged GWS841 alloy during high temperature tension and its effect on mechanical properties were studied.And microstructure change,slip system activity and crack source formation during EBSD in-situ tension at 300℃were further discussed.The study on microstructure of as-cast and solid solution Mg-x Gd-4Y-1Sm-0.5Zr alloys shows that as-cast microstructure is composed ofα-Mg and the white second phase,Mg₅（Gd,Y,Sm）phase,which mainly distributes at grain boundary.After solution treatment,almost all second phases were dissolved in matrix,and then the solutioned alloys were hot extruded at 500℃.The study on microstructure and mechanical properties of hot extruded Mg-x Gd-4Y-1Sm-0.5Zr alloy shows that with Gd content increasing,the average grain size decreases gradually.The hot extruded Mg-8Gd-4Y-1Sm-0.5Zr（GWS841）alloy has the best comprehensive mechanical properties,with tensile strength and elongation of 296 MPa and 8.5%respectively.Therefore,the GWS841 alloy is selected as research object for subsequent systematic research.The study on microstructure evolution of GWS841 alloy during hot extrusion shows that dynamic recrystallized grains generated during hot extrusion do not form obvious texture,which exists in deformed grains without dynamic recrystallization.With extrusion strain increasing,the texture changes significantly:weak basal texture→{0001}//ED+?101?0?//ED→?101?0?//ED.It is due to the combined action of basal slip and prismatic slip during extrusion,which makes the grains rotate.The study on dynamic recrystallization and dynamic precipitation characteristics of GWS841 alloy during hot extrusion process shows that twin induced dynamic recrystallization,continuous dynamic recrystallization and discontinuous dynamic recrystallization occur during hot extrusion.The dynamic recrystallization mechanism mainly depends on extrusion strain and the primary contribution in this study is continuous dynamic recrystallization.The precipitation characteristic of dynamic precipitates is related to hot extrusion strain.When the strain is small,the dish shaped dynamic precipitatesβphases are first formed along original deformed grain boundary and the main factor is significant increase of dislocation density at grain boundary.With strain increasing,although dislocation density decreases,dynamic recovery occurs,a large number of sub-grains are formed,and the amount ofβphases increases,with the shape changing from dish to disc.At the maximum strain,the dislocation density decreases obviously and most dynamic precipitates dissolve into matrix.Through the systematic study for dynamic precipitates evolution,it is clarified that the extrusion temperature is primary reason for dynamic precipitates dissolution,and the fundamental reason for the inconsistent reports of dynamic precipitates in hot deformation in previous studies is well explained.The dynamic precipitates dissolve into matrix to make the composition and structure of hot extrusion alloy more uniform,which is conducive to improving comprehensive mechanical properties of hot extrusion alloy.By optimizing aging treatment process of hot extruded GWS841 alloy,the optimum aging process is 200℃×96 h,and the effects of aging time at 200℃on precipitation characteristics and precipitates evolution were systematically studied.With aging time elongating,β′phases experience nucleation and growth and gradually connect to form polygon.Under peak aged state（96 h）,The number of nanoβ′phases reaches the peak value andβ′phases connect each other as nanoscale polygon mesh with stable structure,which can greatly strengthen the alloy.The nanometerβ′phases,the shape of elliptical,distribute in three prismatic planes（1?21?0）α、（1?1?20）αand（21?1?0）α,which could hinder basal slip and strengthen alloy.With aging time elongating,after aging 120 h,there is no new phase precipitation in grain,but the coarseningβ′phase and the strip equilibrium phase ofβ-Mg₅RE about 70 nm×1μm generating along grain boundary decrease alloy hardness significantly.The study on high temperature tensile mechanical properties of hot extrusion aged GWS841 alloy at 200℃,250℃and 300℃shows that the tensile strengths are 386MPa,373 MPa and 312 MPa respectively,showing excellent heat resistance.The fine microstructure evolution of hot extrusion aged GWS841 alloy also was found during short time high temperature tension（under the joint action of temperature field and stress field）,which mainly shows the change of precipitated phase.After tension at 250℃,the nano triangles and nano dish shapeβphases are formed at grain boundary and the1.6 nm width nano precipitation bands are formed along grain boundary.After tension at 300℃,βphases and nano precipitation bands are also formed at grain boundary,with growth trend contrast to 250℃.The formation mechanism and its influence on tensile properties at high temperature were also systematically analyzed.The study on microstructure characteristics of hot extrusion aged GWS841 alloy during EBSD in-situ tension at 300℃shows that at 6%strain,among the slipping grains,94.2%grains are basal slip grains,2.9%grains are prismatic slip grains,and2.9%grains are slip transfer grains.The primary deformation mechanism is basal slip and the non-basal slip is minimal proportion,which form the abnormal activation phenomenon of slip system.This shows that the precipitates in grain could effectively prevent non-basal slip.At 6.8%strain,the grains activated base slip form a special phenomenon of kinking.This is because basal?(6?dislocation pair slides along opposite direction of base plane and then the base plane bends to form kink band.The kinking improves the plasticity of alloy.At 7.2%strain,micro-cracks mainly form at trigeminal grain boundary and the grain boundary perpendicular to tensile direction,and there are non-slipping grains around micro-cracks,which initiate cracks due to non-unform deformation at these grain boundaries.