Study On Microstructures And Mechanical Behavior Of Sand-cast Mg-4Y-2Nd-1Gd Magnesium Alloy

The WE series magnesium alloys are the most famous and successful Mg–RE alloysdue to the remarkable aging hardening response and high specific strength at elevatedtemperature, and have promising application in industry. Effects of heat treatment on themicrostructure, mechanical properties and fracture behavior of Mg-4Y-2Nd-1Gd alloywere systematically investigated by optical microscopy, X–ray diffractometer, inductivelycoupled plasma analyzer, Differential Scanning Calorimeter, scanning electronmicroscopy with energy dispersive X-ray analyses, transmission electron microscopy,hardness and tensile tests. The microstructural evolution during aging including themorphology, size and distribution of the precipitates was studied in detail, and the effectsof precipitate on mechanical properties and fracture behavior were also studied.The results show that the microstructure of as-cast Mg-4Y-2Nd-1Gd alloy mainlyconsists of α-Mg, eutectic compound Mg₂₄Y₅and a small amount of cuboid-shaped phase,and T4alloy mainly consists of supersaturated solid solution and cuboid-shaped phase.The tensile strength of peak-aged alloy significantly decreases with increasing agingtemperature, while the elongation of peak-aged alloy increases. Considering both thestrength and elongation,525℃×8h+225℃×16h is considered the optimum T6heattreatment condition, and the corresponding yield strength, ultimate tensile strength, andelongation are180MPa,297MPa, and7.4%, respectively.The Mg-4Y-2Nd-1Gd alloys aged at200℃for8h,136h and512h all mainly consist ofβ′′and β′phase; the dominant phase of225℃/16h aged alloy are also β′′and β′phase withhigh density and even arrangement, while the alloy peak-aged at250℃mainly consistsof plate-shaped β1and β phase with large sizes. The evolution of grain boundary structure during precipitation aging is characterized by the formation of grain boundary precipitates（GBP） and precipitate free zones （PFZ）, and they grow with increasing aging time.Besides, the grain boundary of T6alloy which has larger curvature will have larger PFZ.The precipitates’ density, arrangement, and coherent degree with matrix are thedominating factors influencing the mechanical properties, and the β′′and β′phase withhigh density and even arrangement are optimum strengthening precipitates. The hightensile temperature leads to the drop of strength and the rising of ductility. The followingtwo statements result in it:（1） the high temperature causes the activation of non-basal slipsystems, especially above250℃.（2） The high tensile temperature promotes the β′→β1→βtransformations.The Mg-4Y-2Nd-1Gd alloys in different conditions show different fracture modes. Thefracture modes for the as-cast and T4solution treated alloys are intergranularquasi-cleavage and transgranular cleavage fracture, respectively. As for the alloys aged at200℃, with the aging time increased, the fracture surface shows more obviousintergranular fracture feature. The fracture modes of alloys aged at200℃/136h,200℃/512h and225℃/16h are all mixed transgranular and intergranular fracture. Throughjoint analysis of precipitates and fracture surface, the grain boundary structure andprecipitation strengthening of grain interiors cooperatively influence the fracture mode ofthe alloy.With increasing the tensile temperature, the transition of deformation mechanism frombrittle deformation to ductile deformation will emerge. The fracture surface of the alloytensile tested below200℃shows more apparent intergranular fracture feature than thattested at room temperature, and their fracture modes are still mixed transgranular andintergranular fracture. After temperature rising up to300℃, the fracture mode of the alloyis typically ductile fracture with a large amount of dimples.