Microstructure And Texture Evolutions Of Mg-Sm Alloys During Plastic Working Processes And Their Influence On Mechanical Properties

Magnesium alloy is the lightest structural metallice material and has wide application potential in automobile,rail transit,military industry and other fields.However,magnesium alloy has poor room-temperature plasticity and low absolute strength,which largely limits its wide application as load-bearing parts.Magnesium-rare earth alloys have excellent comprehensive mechanical properties and are one of the research hotspots in recent ten years.Rare earth element Sm is relatively cheap and high solid solution in magnesium,which is expected to become an important alloying element.However,there is a lack of systematic study on microstructure,texture and mechanical properties of deformed Mg-Sm alloys.Therefore,this paper took Mg-4Sm(wt%,E4)and Mg-4Sm-0.5Mn(wt%,EM40)alloys as the research object.The microstructure and texture evolutions of the two alloys in the process of solid solution,extrusion,rolling,ageing and annealing treatment and its influence on mechanical properties were systematically studied using microanalysis techniques such as electron backscatter diffraction(EBSD)and transmission electron microscopy(TEM).The main findings are as follows:(1)The microstructures of as-cast E4 and EM40 alloys are mainly composed of?-Mg matrix and eutectic phase Mg₄₁Sm₅,and Mn solid solution in EM40 alloy matrix.The hardness of solution-treated EM40 alloy is higher than that of E4 alloy during ageing due to the addition of Mn element.(2)The extruded E4 alloy mainly contain two texture components:<(?)2(?)0>//ED extrusion texture and<(?)2(?)1>//ED rare earth texture.The extrusion texture is weakened in extruded EM40 alloy due to the addition of Mn element,and turns to be more inclined to the rare earth texture,and the grain size of the alloy is smaller.The two extruded alloys were aged at 200°C for 12 h to reach peak-aged state,and their precipitates are both prismatic precipitates with sizes in the range of tens of nanometers.(3)The effect of ageing strengthening on properties of E4 and EM40 alloys is obvious.After ageing,the tensile yield strength(TYS)and ultimate tensile strength(UTS)of aged E4 alloy are both increased by about 100 MPa,and the TYS and UTS of aged EM40 alloy increased by about 43 MPa and 69 MPa,respectively.The strength of extruded and aged EM40 alloy is lower than that of the corresponding E4 alloy due to the addition of Mn elements which weakens the extrusion texture.However,the anisotropy of EM40 alloy is small,and the tensile plasticity is better by 2?3%than that of the corresponding E4 alloy.(4)The VPSC modeling of E4 alloy confirms that the hardening effect of prismatic precipitates on prismatic slip is the best,followed by pyramidal slip and basal slip,while there is little hardening effect on twinning systems.So it is the main reason for the increase of yield asymmetry of extruded E4 alloy after ageing.(5)The microstructures of rolled E4 and EM40 alloys present dense shear bands and have precipitation of Mg₄₁Sm₅.The rolled E4 and EM40 alloys show bimodal texture offset from basal texture to rolling direction.The degree of recrystallization of EM40 annealed at 300°C is lower than corresponding E4 alloy because Mn element delayed the process.The two alloys are complete recrystallization when annealed at350°C.When annealed at 300?450°C,the grain size range of two alloys is 3?10?m,and the grains is refined by the addition of Mn element in annealed EM40 alloy.(6)The strength of rolled and annealed EM40 alloys increased by 20 MPa than that of corresponding E4 alloys due to the addition of Mn element.The TYS,UTS and elongation of rolled EM40 alloy were 283 MPa,311 MPa and 4%,respectively.With the increase of annealing temperature,the strength of the two alloys decreases gradually and the plasticity increases.The E4 and EM40 alloys annealed at 350°C and 400°C exhibit yield plateau phenomena during tension.