Study On The Phase Equilibria In The Mg-Zn-Ag System

Magnesium alloys as the lowest density metal structural materials were hailed as "the 21st century green engineering materials",due to its variety of excellent properties.They have been widely applied in the scopes of aerospace,transportation,3C areas and so on.However,compared with steels and aluminum alloys,the further application of magnesium-based alloys as structural materials was limited due to the disadvantages of low absolute strength,poor heated resistance,easy corrosion and degradation.On the other hand,as the new type of degradable biomedical metal materials,magnesium alloys have been increasingly concerned and honored as "revolutionary metal biomaterials",owning to good mechanical compatibility,biocompatibility and biodegradation.Presently,the biological evaluation of Mg alloys has been mainly investigated.Mg-Zn-Ag alloys have been paid attention to with good age-hardening effect and biological properties.However,as the alloying basis,there are many controversies in the Mg-Ag binary phase diagram,at the same time,the phase equilibria at the Mg-Zn side of the Mg-Zn-Ag systemhas not been adequately studied.As a result,it cannot provide reliable scientific basis for design of composition,optimization of heat treatment process and hot working processe of Mg-Zn-Ag alloys.Therefore,in this work,the phase equilibria in Mg-Zn-Ag system have been investigated by equilibrated alloy method,and the isothermal section at 335 ? has been constructed.The main conclusions are as follows:It is testified by equilibrated alloy method that the Mg7Ag3 phase does exists in Mg-Ag binary system,and there is peritectoid transformation at 376 ?:Mg7Ag3(?)MgAg + ?-Mg3Ag,which is consistent with the results obtained by diffusion couple method.At the same time,it is showed by X-ray diffraction analysis that the structure and crystallographic parameters of the Mg7Ag3 phase are close to that of Mg7Zn3(PDF-41-1292)compound.The isothermal section of Mg-Zn-Ag at 335? is constructed through microstructures,phase constituents and compositions.There exists Mg7(Zn,Ag)3 continuous solid solution at the Mg-rich corner,which is composed of Mg7Zn3 phase and Mg7Ag3 phase.Moreover,there is the phase equilibrium between the Mg7(Zn,Ag)3 phase and ?-Mg solid solution.Zn,Ag can be simultaneously dissolved in the a-Mg solid solution.There exists Mg2Zn11+Zn+?-AgZn3 and Mg2Zn11+MgZn2+?-AgZn3 two three-phase regions at Zn-rich corner.The maximum solubility of Mg in ?-AgZn3 phase is 0.5 at.%.The maximum solubility of Ag in MgZn2 phase and Mg2Zn11 phase is 19.9 at.%and 3.3 at.%,respectively.In addition,there is a three-phase region composed of the ternary compound P1,P2 and Mg7(Zn,Ag)3 continuous solid solution in Mg-Zn-Ag system.