Study On The Microstructure,Mechanical Properties And Anisotropy Of Large Mg-Gd-Y-Zr-Ag Alloy Conical Shell

In recent years,large-scale Mg-Gd-Y-Zr rare-earth magnesium alloy components have attracted much attention in the fields of defense,military,and aerospace due to their low density,high specific strength,and excellent mechanical properties at room and high temperature.However,large-scale magnesium alloy components often have problems such as uneven distribution of microstructure and anisotropy of mechanical properties,which often lead to the reduction of their service performance.Therefore,in this paper,the large-scale Mg-8.94Gd-1.80Y-0.42Zr-0.21Ag(wt.%)alloy conical shell die forgings are used as the research object,and are characterized by optical microscopy,scanning electron microscopy,transmission electron microscopy,electron backscatter diffraction and other characterization methods.The microstructure inhomogeneity in die forgings and its causes are analyzed,and the room temperature mechanical properties and anisotropy of die forgings are studied.At the same time,some areas of die forgings are subjected to peak aging treatment,and the effects of aging on the mechanical properties,anisotropy and fracture behavior of die forgings are explored.It has important reference significance for further optimizing the hot die forging process of magnesium alloys,so as to prepare large-scale magnesium alloy components with uniform microstructure,weak anisotropy of mechanical properties and excellent service performance.The main conclusions are as follows:(1)The inhomogeneity of temperature and strain distribution in die forgings is the main reason for the inhomogeneity of microstructure regions.Along the wall thickness direction of the die forging: the strain of the top layer and the middle layer gradually decreases from the inside to the outside,resulting in a gradual decrease in the degree of recrystallization.The strain of the bottom layer first decreases and then increases from the inside to the outside,resulting in a first decrease and then an increase in the degree of recrystallization.The average grain size in the middle and outer of the top layer is similar to that of the middle layer,while the average grain size of the middle of the bottom layer is more than 135μm due to the low strain and high temperature.Along the height direction of die forgings:under the joint influence of strain and temperature,the average grain size of each layer first decreases and then increases from top to bottom,while the degree of recrystallization gradually increases from top to bottom(except the middle of the bottom layer),Among them,the recrystallized grains in the bottom layer account for more than 90%.(2)There are significant inhomogeneity and anisotropy of mechanical properties in die forgings.There are differences in the average hardness value and the variance of hardness value in each region.The average hardness value is the highest in the middle layer because of the smallest grain size and the largest content of the second phase.The hardness variance of the bottom layer is the lowest because of the highest degree of recrystallization;There are significant differences in the tensile mechanical properties of die forgings in the axial(AD)and circumferential(CD)directions,in which the yield strength in AD direction between the middle of the top layer and the middle of the bottom layer is about 70 MPa and the tensile strength is about 50 MPa;In addition,there is obvious mechanical anisotropy in some areas of die forgings,in which the yield strength in the middle of the top layer and the middle of the bottom layer is about 40 MPa and 70 MPa respectively,while the yield strength in the middle of the middle layer is almost equal.(3)The anisotropy of mechanical properties in die forgings is closely related to the texture characteristics,and the texture characteristics have a significant impact on the initiation of the room temperature tensile deformation mechanism.When the sample in the middle of the top layer is stretched along the AD direction,its texture is beneficial to the initiation of basal slip and tensile twinning.When stretched in the CD direction,its texture facilitates the initiation of prismatic slip.When the sample in the middle of the bottom layer is stretched in the AD/CD direction,the orientation advantage of the deformation mechanism is just opposite to that in the middle of the top layer.Due to the significant difference in CRSS values between prismatic slip,basal slip and tensile twinning,there is obvious anisotropy of mechanical properties in the middle of the top layer and the middle of the bottom layer.In addition,the texture characteristics have an important influence on the actual twinning deformation behavior,and the types and numbers of activated twins of each sample show great differences due to different textures.(4)Aging treatment has a great influence on the mechanical properties,anisotropy and fracture behavior of die forgings.After peak aging treatment,the mechanical properties of die forgings are significantly improved.The yield strength of the aged samples in the middle of the top layer and the middle of the bottom layer are both above 200 MPa,and the tensile strength is increased to 340 ～ 410 MPa,which is about 100 MPa higher than that before aging.However,the strength increases in both directions of the samples before and after aging are different,and the anisotropy of mechanical properties is weakened.The increase of strength and the weakening of anisotropy are both related to the precipitation of a large number of β’ prismatic aging phases.In addition,the peak aging treatment changes the alloy from a mixed fracture mode of ductile fracture and brittle fracture to a single brittle fracture mode.The whole article contains 73 figures,10 tables,and 100 references are cited.