Study On Manufacturing Technology,microstructures And Properties Of Low Alloyed Mg-Zn-Gd Alloy Reinforced By Quasicrystal

Magnesium alloys are widely used in transportation,electronic 3C and other fields due to its advantages such as low density and high specific strength.However,the applications of magnesium alloys are limited because of the poor deformability and low strength at room temperature.Mg-Zn-RE quasicrystals can not only effectively strengthen magnesium alloys,but also have a great influence on the dynamic recrystallization of magnesium alloys during plastic deformation due to its special structure and properties.In this paper,Magnesium alloys with different volume fraction of quasicrystal and their nominal composition is Mg-1.5Zn-0.25Gd(at.%),Mg-3Zn-0.5Gd(at.%)and Mg-6Zn-1Gd(at.%)were melted and the microstructure and properties of as-cast,as-extruded,as-semi-solid and as-secondary extruded were systematically studied.In order to obtain high strength quasicrystal reinforced magnesium alloy with low alloy content,the quasicrystal phase was dispersed in magnesium matrix and the matrix structure was controlled to be bimodal structure by semi-solid +secondary extrusion process.The main results are as follows:Mg-6x Zn-x Gd(x=0.25,0.5 and 1)alloys with quasicrystal phase were obtained by conventional casting method.The microstructure and mechanical properties of as-cast,asextruded and as-secondary extruded alloys were characterized.The second phase mainly existed in the form of lamellar eutectic(I-phase+?-Mg)at the grain boundary in as-cast MgZn-Gd alloy.With the increase of quasicrystal phase content,the grain size of as-cast Mg-Zn Gd alloy decreases,the strength increases but the plasticity decreases.The lamellar eutectic in as-cast Mg-Zn-Gd alloys was broken and distributed in the matrix in strip shape along the extrusion direction.This leads to the strength and elongation of the alloy were significantly improved.After secondary extrusion,the grain size and second phase size of Mg-Zn-Gd alloy were further refined,and the strength and elongation of the alloy were also slightly improved.However,due to the coarseness of the second phase in the initial alloy,it is difficult to refine the second phase effectively by conventional extrusion.In order to further improve the comprehensive properties of Mg-Zn-Gd alloy,the microstructure of Mg-Zn-Gd alloy with low alloy content was controlled by semi-solid +secondary extrusion process.The nano-lamellar eutectic(I-phase +?-mg)can be formed at the grain boundary of Mg-Zn-Gd alloy after semi-solid and quenching treatment.The thickness of Mg-1.5Zn-0.25 Gd alloy and Mg-3Zn-0.5Gd alloy are 80±10 nm and 85±14 nm.After the secondary extrusion,the nano-lamellar eutectic was broken into nano particles and micro particles which distributed in strip shape along the extrusion direction and the matrix exhibited bimodal structure.With the formation of bimodal structure and the breaking of nano lamellar eutectic,the strength of Mg-Zn-Gd alloys were increased significantly.The yield strength(YS),ultimate tensile strength(UTS)and elongation(EL)of Mg-1.5Zn-0.25 Gd alloy are 407±0.7MPa,443±2.1 MPa and 11±0.7%.The YS,UTS and EL of Mg-3Zn-0.5Gd alloy are 356±0.7MPa,413±1.4 MPa and 12±1.3%.The main reasons for the significant improvement of YS and UTS of Mg-Zn-Gd alloy are as follows:(1)Fine grains provide grain boundary strengthening,while unrecrystallized grains have strong basal texture,which can produce significant strengthening effect by inhibiting basal slip.(2)The refinement and dispersion distribution of quasicrystal phase in the matrix can exhibit a better dispersion strengthening effect.In order to study the effect of extrusion parameters on bimodal structure formation and lamellar eutectic refinement,the extrusion speed,extrusion temperature and extrusion ratio of secondary extrusion were studied.The results show that with the increase of extrusion speed and temperature,the fraction of crystallization of the alloy increases.When the extrusion ratio is 12.25: 1,the alloys exhibited a low recrystallization fraction.In addition,the extrusion parameters also have a significant effect on the second phase morphology after extrusion.At high temperature or high temperature caused by friction and severe deformation,the brittleness of quasicrystal phase will decrease,resulting in the weakening of the crushing effect of plastic deformation on lamellar eutectic.Therefore,the better combination of strength and elongation of Mg-1.5Zn-0.25 Gd alloy can be obtained at low temperature(? 300?),low speed(? 1 mm/s)and moderate extrusion ratio(12.25: 1).