The Study On Microstructures And Mechanical Properties Of Mg-Sm(-Ce/Y)-Zn-Zr Alloys

In this thesis,effects of samarium(Sm)content(0,2.0,3.5,5.0,6.5 wt%)on microstructure and mechanical properties of Mg-0.5Zn-0.5Zr alloy under as-cast and as-extruded states were thoroughly investigated.Microstructural evolution of as-cast Mg-Sm-Zn-Zr alloy under dififerent heat treatment conditions was studied in detail.Strengthening mechanism of the extruded Mg-3.5Sm-0.6Zn-0.5Zr alloy with excellent properties was explored.Simultaneously,influences of extrusion ratio on the microstructure and mechanical properties of the optimum alloy were studied.In addition,effects of cerium(Ce)content on microstructure and mechanical properties of Mg-3.5Sm-0.5Zn-0.5Zr alloy under as-cast and as-extruded states were investigated systematically.The last but not the least,a novel high-strength Mg-3.5Sm-2.0Y-1.5Ce-2.0Zn-0.5Zr alloy with low RE content has been developed by the synergistic effect of light and heavy RE elements.The research results are as follows:The chemical composition and crystal structure of the dominant intermetallic phase will be affected by different content of Sm addition.The dominant intermetallic phase changes from Mg3Sm(face-center cubic,a = 0.7371 nm)to Mg4:Sm5(body-centered tetragonal,a=1.476 nm c= 1.039 nm)till Sm content exceeds 5.0 wt%.Mg3Sm phase was transformed into Mg41Sm5 phase during the heat treatment of Mg-3.5Sm-0.6Zn-0.5Zr alloy at 350 ??The orientation relationship between Mg41 Sm5 phase and Mg3Sm phase was revealed as:(110)Mg41Sm5//(110)Mg3Sm,[1 1 0]Mg41Sm5//[1 1 1]Mg3Sm.This indicates that Mg41Sm5 phase was precipitated along the(110)plane of Mg3Sm phase.In addition,both Mg3Sm phase and Mg41Sm5 phase were precipitated along the(0001)Mg plane.The orientation relationships between them and Mg matr:ix were revealed as:(110)Mg3Sm//(0001)Mg,[111]Mg3Sm//[1 2 10]Mg and(211)Mg41Sm5//(0001)Mg,[1 20]Mg41Sm5//[1 2 10]Mg.The intermetallic phases formed during solution treatment in Mg-5.0Sm-0.6Zn-0.5Zr alloy were studied using transmission electron microscopy.Four types,namely the plate phase,the spindle-like phase,the nano-scale granuliform phase and the rod-like phase were revealed.The first one is MgZn2(close-packed hexagonal)phase,the other three ones are Zn-Zr phase.In addition,the granuliform phase presents quadrate-shaped or lath-shaped and was identified as Zn2Zr3(primitive tetragonal).Furthermore,three kinds of rod-like phases were observed:one across the grain boundary but only coherent with one grain,one covered by jagged Mg3Sm precipitates and one with no surficial phase.The former twos are Zn2Zr3 while the later one is Zn2Zr(face-center cubic).Finally,this work indicates that only the plate MgZn2 phase and the rod-like Zn2Zr3]phase in the grain interior will act as heterogeneous nucleation sites for the Mg3Sm precipitates,thus influencing the precipitation.The dynamically precipitated intermetallic phase during hot-extrusion in all Sm-containing alloys is Mg3Sm.The intermetallic particles induced by Sm addition could act as heterogeneous nucleation sites for dynamic recrystallization during hot extrusion.Sm addition can significantly enhance the strength of the as-extruded Mg-0.5Zn-0.5Zr alloy at room temperature,with the optimal dosage of 3.5 wt%.The optimal yield strength and ultimate tensile strength are 368 MPa and 383 MPa,which were enhanced by approximately 23.1%and 20.8%compared with the Sm-free alloy,respectively.Numerous nano-scale plate ?'particles and lamellae ?"particles homogeneously precipitated in the matrix during artificial aging treatments.Therefore,the studied alloy exhibits very high ultimate tensile strength and yield strength,whose maximum values at room temperature are approximately 427 MPa and 416 MPa,respectively,the dominant strengthening mechanisms are revealed to be grain boundary strengthening and precipitation strengthening.The microstructure and mechanical properties of Mg-3.5Sm-0.6Zn-0.5Zr alloy with different extrusion ratio were also investigated.The results indicate that the dominant phase was changed from Mg3Sm to Mg41Sm5 when the extrusion ratio was 10.4 and 17.6.With the increase of extrusion ratio,the fraction of recrystallization region increased,the grain size also increased gradually,and the base texture was weakened effectively,the corresponding yield strength was decreased finally.After Ce addition into Mg-3.5Sm-0.6Zn-0.5Zr alloy,blocky Mg12RE phase was observed.With the increase of Ce content,the volume fraction of Mg3RE decreased gradually,at the same time,both the volume and size of Mg12RE increased obviously.The extruded Mg-3.5Sm-xCe-0.6Zn-0.5Zr alloy contains fine recrystallized grains and relatively coarse non-recrystallized regions.The volume fraction of dynamic precipitates Mg3RE increased with the increasing of the amount of Ce.After aging treatment,the yield strength of extruded alloys was improved to a certain extent.The volume fraction of bulk phase increased with the increase of Ce addition.The yield strength was enhanced with the increase of Ce addition,while the elongation decreased.W phase and Mg 12RE phase distributed at grain boundary were observed in as-cast Mg-3.5Sm-2.0Y-1.5Ce-2.0Zn-0.5Zr alloy,also lamellar 14H-LPSO phase in grain.The extruded alloy consists of fine recrystallized grains and relatively coarse non-recrystallized zone.Non-recrystallization zone contains lots of small angle grain boundaries,indicating high density dislocations in it.The results of EBSD analysis indicated typical<1010>fiber texture and relatively weak<1124>rare earth texture were in extruded alloy.Apart from W phase and Mg12RE phase,18R-LPSO phase appeared at grain boundaries in extruded alloy,which has a certain orientation relationship with Mg12RE:(0001):8R-LPSO about 6.80°from(210)Mg12RE,[1210]18R-LPSO//[1 2 3]Mg12RE.Numerous lamellar 14H-LPSO phase were observed in the inter:ior of the grains,and the orientation relationship with Mg matrix was:(0001)14H-LPSO//(0001)Mg,[1210]14H-LPSO//[1210]mg.After aging treatment,numerous lamellar y precipitates aligned on base planes.The tensile yield strength and compressive yield strength of extruded alloy are 438 MPa and 413 MPa,respectively.After ageing treatment,they are 482 MPa and 457 MPa,respectively.The excellent properties of alloys come from various strengthening mechanisms,including fine grain strengthening,dislocation strengthening and precipitation strengthening.