Composition Design And Strengthening And Toughening Mechanism Of Mg-Al-Zn System Magnesium Alloys With High Alloy Content

As the lightest metallic structural material,magnesium and its alloys,with combination of high specific strength,specific stiffness and machinability,have been attracting significant attention in aerospace,automobile and communication industries in recent years.Compared with the cast Mg alloys,the wrought Mg alloys processed by extrusion and rolling show better mechanical properties.Recently,numerous works focused on the Mg–Al–Zn system alloys,especially the ultrafine-grained(UFG)Mg–Al–Zn alloys prepared by severe plastic deformation(SPD),such as agitation friction,equal-channel angular pressing and high-pressure torsion,etc.The UFG Mg alloys processed by SPD show high strength,but generally poor plasticity.With increasing alloy content,the deformation of Mg alloys becomes difficult.There are relatively few studies on the plastic processing of magnesium alloy with high alloy content,while the high strength and ductility of Mg alloy with high alloy content of Mg–Al–Zn has been reported.Therefore,based on the alloy composition design and optimization,the novel Mg alloys with high strength toughness(high strength and ductility)prepared by extrusion and rolling means,will be conducive to the further development and application of Mg alloys in industry.This study designs and prepares the new-style Mg–Al–Zn system wrougtht alloys with high alloy contents.The microstructure characteristic and mechanical properties are investigated on detail,to explore the microstructure–property relationship,which further reveals strengthening and toughening mechanism.The main conclusions are as follows:(1)a fine-grained microstructure with a rather homogeneous grain size distribution was obtained in AZ75 alloy after HPR processing,which showed an average grain size of ~6 ?m.Numerous micron(1–3 ?m)/nano(50–200 nm)Mg17(Al,Zn)12 precipitates with spherical morphology were found to disperse uniformly at the microstructure.Moreover,the alloy exhibited superior mechanical properties with a simultaneous high strength and ductility at room temperature,i.e.yield strength(YS)of ~218 MPa,ultimate tensile strength(UTS)of ~345 MPa and elongation of ~19%.(2)According to the contribution of YS from fine-grain strengthening,precipitation hardening and solid solution hardening estimated by simplified model,the grain boundary strengthening is the predominant strengthening mechanism for the alloy under tension at room temperature.Moreover,the alloy exhibited the strong work-hardening capacity resulted from Zn solid solutes and the presence of numerous well-dispersed nanosized Mg17(Al,Zn)12 precipitates as well as weakened texture.(3)It revealed the deformation mechanism of AZ75 alloy under tension at different temperature.The dislocation climb creep dominates under tension deformed at 150 °C.When tested at 300 °C,the alloy exhibited an optimum superplasticity with a fracture strain of ~615%,which is attributed to the enhanced GBS,as a stable microstructure with fine grain size was well maintained during tension,due to the pinning effect by numerous Mg17(Al,Zn)12 particles segregating along grain boundaries;the decrease of fracture elongation of the alloy when tested at 350 °C was supposed to be attributed to anomaly grain growth caused by the dissolution of the fine precipitates into the matrix.(4)To compare the microstructure and tensile properties of as–extruded AT82–x Zn(x: 0~1.5 wt.%)alloys,it revealed that Zn addition refined the grain size and increased volume fraction of second phase particles(Mg2Sn and Mg17Al12)of AT82 alloy,and improved tensile properties.By optimizing better alloy component,the as–extruded AT82–1.0Zn alloy exhibited both a high strength and high ductility at room temperature,e.g.UTS of ~357 MPa and elongation of ~19%.(5)The increased YS of the AT82 alloy was mainly ascribed to the cooperation effect of enhanced precipitate strengthening from more fine Mg2 Sn and Mg17Al12 particles as well as solid solution strengthening from Zn solutes.Moreover,The solid solution Zn and the increased amount of well-dispersed fine Mg2 Sn and Mg17Al12 particles are also responsible for the strong work hardening ability,which explained the reason for high UTS and elongation of AT82–Zn alloy.