Research On The Phase Equilibria Of Ternary Compounds And The Relative Thermal Stability In Mg-Zn-Gd System

In the past few years,magnesium alloy has been widely concerned due to its great potential in the field of automotive industry,aerospace,medical equipment,national defense,3 C industry,etc.Magnesium alloy has many characteristics such as low density,high specific strength,good thermal conductivity,good electromagnetic shielding and radiation resistance.It is also a new type of green engineering material.However,the thermal stability of magnesium alloy and its low strength are limited by the application of magnesium alloy.For a large number of solid solution elements Mg,it can serve to strengthen the role of magnesium alloy.For example,rare earth elements areused as the most effective and most valuable elements to improve the heat resistance of magnesium alloy,as study has found.Studies show that in Mg-Zn alloy,the addition of Gd element can effectively improve the mechanical properties of the alloy and its corrosion resistance.But as the thermal stability of Mg-Zn-Gd alloys being showed on the basis of the phase diagram and phase,there is also a need to make further research.Therefore,the thermal stability of the Mg-Zn-Gd series compounds and its related phase equilibrium are being studied.In the experiment,7 Mg-Zn-Gd alloys are prepared,and the heat treatment was carried out at 400?/5-7 weeks,440?/2 weeks and 475 ?/1 week.The alloy equilibrium structure by the method of analysis.By using the means of SEM,TEM,EPMA,XRD and DSC analysis method,the experiment has carried on the research of Mg-Zn-Gd ternary system which is rich in Mg alloy angle transformation and phase equilibrium relationship,and has won the 400 ?,440 ? and 475 ? isothermal section diagram.The results show that the Mg phase,W phase,H phase and Z phase of the three compounds are the coexistence of the rich Mg angle in the Mg-Zn-Gd three element system.The components of Z phase were Mg:30-37,Zn:54-61,Gd:6.9-8.1(mole fraction),with six square crystal structure and its lattice constant was a=b=1.46330nm,c=0.87610nm.W phase nucleation is divided into Mg:26-32;Zn:48-50;Gd:21.5-25.5,with face centered cubic crystal structure and the lattice constant a=b=c=0.69138nm;H phase nucleation is divided into Mg:27-34;Zn:54-60;Gd:11.6-13.3,may with hexagonal crystal structure.The Z phase was stable before 427.5 ??and the phase H phase is changing with the increase of Z phase.The H phase can be stably exist at 443.2 ? and the temperature can be changed to W phase.The W phase can be stably existed at 480.5 ?,and the temperature increases gradually to the liquid phase..When the Mg-Zn-Gd three is rich in Mg angle at 400?,the isothermal cross section map exist two three-phase-regionsa-Mg+L+Z and a-Mg+Z+H,two two-phase-regionsa-Mg+H and a-Mg+Z.At 440 ?,the isothermal cross section map exists for two three-phase-regions,a-Mg+H+W anda-Mg+L+H,one two-phase-region a-Mg+H,and the Z phase can not coexist with the-Mg equilibrium..At 475?,the isothermal section diagram shows a three phase region a-Mg+L+W;the H phase can not coexist with the a-Mg equilibrium.