Effects Of La And Sm Microalloying On Microstructures And Mechanical Properties Of AZ31 Magnesium Alloy

In this paper,AZ3 1 magnesium alloy was used as matrix alloy,rare earth La and Sm were alloying elements,AZ31-xLa-ySm alloy with different rare-earth content was smelted and followed by hot extrusion deformation experiment.By means of XRD analysis,metallographic microscopic observation,scanning electron microscope observation,EDS spectrum analysis,Vickers hardness and mechanical properties test,the effects of La and Sm alloying on the microstructures and mechanical properties of AZ31 magnesium alloy were studied systematically.Finally,three kinds of AZ31-xLa-ySm alloys with better performance were carried out with aging heat treatment,and the effects of aging time on those three alloys microstructures and mechanical properties were studied.The results of the study show that:(1)Adding La and Sm to AZ31 magnesium alloy could refine the solidification structure of magnesium alloy significantly.When 0.3 wt.%La element was added separately,grain of alloy was refined,and ?-Mg17Al12 and Al8Mn5 phase were decreased,and rod like Al11La3 phases were precipitated at the same time.With the content of Sm was increased,the grain size was gradually refined in addition to the amount of 0.7?1.0 wt.%.After 0.7 wt.%Sm was added,the bulk-like Al2Sm phases and fine needle-like Al11Sm3 phases began to be precipitated.With the increase of Sm content Al2Sm phases gradually became larger,while Al11Sm3 phases were gradually diverged;(2)After hot extrusion deformation experiment at 360?,the alloys all undergoed dynamic recrystallization and the microstructure became finer equiaxed grains.However,the addition of La and Sm reduced the degree of dynamic recrystallization significantly and refined grains markedly;Al11La3 and Al11Sm3 phases were respectively broken into short rod-like phases and granulars distributing along the direction of the extrusion belt,but bulk-like Al2Sm phases were not broken;(3)With the addition of rare elements La and Sm and the increasing of Sm content,the yield and ultimate tensile strengths at room temperature of AZ31-xLa-ySm alloy increased gradually,and elongation increased firstly and then decreased.At 100?,the content of rare earth La and Sm had unobvious effect on tensile strength.With the addition of La and small amount of Sm,the yield strength of the alloy had unobvious change,but the content of Sm reached 1.0 wt.%,the yield strength of the alloy became higher;at 150?,the content of rare earth La and Sm had unobvious effect on the yield and tensile strengths of the alloy;at 200?,the content of rare earth La and Sm had unobvious effect on the tensile strength of the alloy.When adding La and small amount of Sm,the yield strength of the alloy has unobvious change,but when the content of Sm reached 1.7 wt.%,the yield strength of the alloy reached the highest,which was 80MPa.At the temperature range of 100?200?,the elongation of the alloy increased firstly and then decreased.The elongation of the alloy increased with the addition of 0.3 wt.%La and 0.2?0.7 wt.%Sm;(4)Results of aging heat treatment at 180? showed that the average hardness of three alloys of AZ31-0.3La,AZ31-0.3La-0.2Sm and AZ31-0.3La-1.7Sm was increased with the addition of rare earth elements.AZ31-0.3La and AZ31-0.3La-1.7Sm alloys both had two hardness peak,while AZ31-0.3La-0.2Sm alloy had only one hardness peak.With the increase of aging time,the number of precipitated phases in AZ31-0.3La alloy gradually decreased,and the grain size was growing;the number of precipitated phase in AZ31-0.3La-0.2Sm alloy increased gradually,and the grain size also grew;the number of precipitated phases in AZ310.3La-1.7Sm alloy increased gradually,but there was unobvious change in grain size.With the increase of aging time,the yield and tensile strengths of AZ31-0.3La alloy increased before 24h,but changed hardly after 24h,the elongation increased slightly with the increase of aging time;the yield and tensile strengths of AZ31-0.3La-0.2Sm alloy had also been improved a little before 24h,but there was unobvious change after 24h,and the elongation decreased.The yield and tensile strengths of AZ31-0.3La-1.7Sm alloy were improved before 10h,but there was unobvious change after 10h,and the elongation also decreased.