Research On Microstructure And Mechanical Properties Of As-cast Mg-8Li-3Al-2Zn Alloy Containing Y With Duplex Structure

In recent years,Mg-Li alloys have drawn increasing attentions because of their impressive advantages,such as high specific strength and stiffness,good magnetic shielding and damping capacity.Therefore,Mg-Li alloys are ideal materials in the fields of aerospace,military industry and 3C?Computer,Communication and Consumer Electronics?industry.However,The research about Mg-Li series alloies is still deficient.A system which contains e×cellent microstructure and mechanical properties is still needed,meanwhile,the effect of heat treatment on the mechanical properties needs te be e×plored deeply.This research uses Mg-8Li-3Al-2Zn as base alloy,firstly,we e×plore the effect of Y content on alloy's microstructure and mechanical properties and find that the optimization effect obvious.Meanwhile,we find that solid solution treatment can improve the mechanical properties of alloy obviously.In addition,the age softening is revealed and the mechanism is proposed.At last,we give a way to relieve the age softening phenomenon,and it is meaningful to the next researcher.Firstly,The Mg-8Li-3Al-2Zn-×Y?×=0,0.5,1,1.5 wt%?alloys are made by vacuum induction melting furnace.The results of microstructure illustrate taht Mg-8Li-3Al-2Zn is primarilly composed of?-Mg and?-Li base phase,AlLi phase.After addition of Y,the needle-like?-Mg phase is significantly reformed to become the round-like structure,meanwhile,the grains are refined effectively,the grain size of Mg-8Li-3Al-2Zn-0.5Y alloy is 55?m,but the Mg-8Li-3Al-2Zn-1Y alloy is 38?m.In addition,with the increasing of Y,the amount and structure of secondary phase will change,the amount of Al₂Y increase,the structure of which transformes from tiny graininess to block.AlLi phase decreases and the?-Li phase increases.In addition,we find that Y can improve the synthetical tensile properties of Mg-8Li-3Al-2Zn alloys effectively,beacuse of grain refinement and second phase strengthening.Microstructure improvement of alloy and the refinement of grain size can both improve the strength and elongation.Al₂Y is hard phase,so it can improve the strength of alloy through hindering the dislocation slip.in summary,Mg-8Li-3Al-2Zn-0.5Y has the best synthetical mechanical properties,the yield strength,ultimate tensile strength and elongation are 153.0Mpa?218.5Mpa and 16.9%,which are improved by about 6.4%?13.2%and 13.4%compared with those of Mg-8Li-3Al-2Zn alloy,respectively.This research further raises the mechanical properties of Mg-8Li-3Al-2Zn-0.5Y by heat treatment.The solid solution treatment needs to conduct in vacuum tube furnace because of the corrosiveness of Mg-Li alloy.The parameter of solid solution treatment is300?×2h,350?×2h,400?×2h and 450?×2h.According to the microstructure results,the AlLi phase decomposes during solid solution treatment which makes the contrast between?-Mg and?-Li base phase decreases.The Al₂Y has high thermal stability,so it still distributes in the phase boundaries two base phases after solid solution treatment.After 400?×2h solid solution treatment,the?-Li phase is into the base phase basicly.In addition,we find that solid solution treatment can improve the hardness and strength heavily,with the increase of solid solution temperature,more AlLi phase get into the phase,the hardness and strength of alloy inprove gradually.But,when the temperature and time is 400?and 2h,almost all of AlLi phase has decomposed,so,with the temperature increasing further to 450?,the mechanical properties can not be improved further.In summary,the alloy after 350?×2h solid solution performs the best synthetical tensile properties,the yield strength,ultimate tensile strength and elongation are181.3MPa?263.0MPa and 11.8%,the strength are improved by 18.5%and 20.37%compared with cast alloy.The result of aging treatment illustrates that the Mg-Li alloy has the age softening obviously.in the long run,both of the RT and 75?age softening will decrease the hardness of alloy,however in short time,the RT aging alloy will appear the hardening.In summary,after 450?×2h solid solution treatment and aging at RT for 150d,the YS and UTS are 182.5 Mpa and 248.5 Mpa,improved by 19.28%and 13.72%compared with cast alloy,but the glongation rate further decreases to 7.0%.At last we deduce that increasing solid solution temperature and adding Y element may be effective way to weaken the age softening of Mg-Li alloy.