Study On Composite Strengthening And Toughening Of Microalloyed Mg-Gd(-Y)-Zn Alloy

Magnesium alloy is the lightest metal structural material in industrial applications.It has the advantages of low density,high specific strength and specific stiffness,good thermal conductivity and electrical conductivity,good damping and shock absorption,and good machinability.Rare earth magnesium alloys containing long-period stacking ordered?LPSO?and aging precipitation strengthening phases are widely used in the aerospace,defense,military and automotive industries due to their good room temperature and high temperature mechanical properties.Mg-Gd-Y-Zn alloy is the best magnesium alloy system in research at home and abroad.Its strength can reach more than 500MPa,but its elongation is generally low?usually less than 5%?.It is difficult to obtain high strength and high plastic toughness at the same time,which seriously restricts the development of alloys.Microalloying is extremely important in the field of materials or metallurgical research by adding an element to the alloy to change its structure and properties.Therefore,in this paper,the microalloying design of different strengthening factors in Mg-Gd?-Y?-Zn alloy is carried out to clarify the strengthening and toughening mechanism of the alloy and to master the evolution of its strength and plastic toughness.In order to provide theoretical guidance for the design and application of high strength and toughness magnesium alloy.The main contents are as follows:?1?The LPSO phase and the aging precipitation phase are firstly distinguished.The effects of different heat treatment processes on Mg-Gd-Y-Zr alloy strengthened by single aging precipitation phase and Mg-Gd-Y-Zn-Zr alloy strengthened by single LPSO phase were investigated.The experimental results show that the optimal solution treatment process of Mg-10Gd-3Y-0.6Zr alloy is 500°C for 8h,and then the optimal aging treatment process is220°C for 16h.The alloy under this heat treatment process obtained a tensile strength of345.6MPa and an elongation of 7.2%due to the formation of a large amount of?'precipitate.With the prolongation of solution treatment time at 520°C,the morphology of LPSO phase in Mg-8Gd-5Y-2.5Zn-0.6Zr alloy will be layered,rod-shaped and massive,and the content will increase in a certain processing time.The formation of a large number of LPSO phases can promote the reduction of the total stacking fault energy of the alloy,which makes the non-base slip system easier to be driven,thereby greatly improving the plastic toughness of the alloy.The rod-like LPSO is most advantageous for the mechanical properties of the alloy,so that the tensile strength,yield strength and elongation of the alloy are 252MPa,214MPa and 17.2%,respectively.?2?A trace amount of Zn element gradient was added to the Mg-Gd-Y-Zr alloy to study the effect of the combined strengthening of the LPSO phase and the aging precipitation phase on the properties of the alloy.The results show that the Zn element can reduce the stacking fault energy in the alloy,thus promoting the formation of stacking faults and LPSO phases.The formation of?'precipitated phase and the high temperature stable 14H-LPSO have a competitive demand for rare earth elements.The aging effect of the alloy gradually decreases with the increase of Zn element.The formation of the?'precipitate phase can greatly increase the tensile strength of the alloy,but reduce the plastic toughness of the alloy.Due to the strengthening of the LPSO phase and the?'precipitate phase,the peak-aged Mg-10Gd-3Y-0.5Zn-0.6Zr alloy obtained the best mechanical properties.?3?The effect of Sn element on the microstructure and properties of Mg-Gd-Y-Zn-Zr alloy was studied.The results show that there are few Zn elements in the alloy,there are many Gd elements,and there is no LPSO phase formation in the alloy.The brittle high temperature stable phase Sn₃RE₅ increases with the increase of Sn element,and the small granular Sn₃RE₅has a significant fine crystal effect.The Young's moduli of Sn₃Gd₅ and Sn₃Y₅ phases are109.0GPa and 119.2GPa,respectively,which promotes the elastic modulus of the alloy increases with the increase of Sn element,and the elastic modulus of the peak-aged Mg-12Gd-3Y-0.5Zn-0.6Zr-2Sn alloy reached 51.3GPa.The Sn element can reduce the stacking fault energy of the alloy and promote the precipitation of a fine circular?'strengthening phase in the peak aged alloy.The peak-aged Mg-12Gd-3Y-0.5Zn-0.6Zr-1Sn alloy with an appropriate amount of Sn₃RE₅ phase,fine recrystallized grains,strong basal texture and a large number of fine?'precipitation strengthening phases obtained the optimal mechanical properties.?4?The effect of Al on the microstructure and properties of Mg-Gd-Zn alloy was investigated.The results show that the addition of 0.4wt%Al promotes the formation of the LPSO phase(Mg₁₂Gd?Al,Zn?phase)in the as-cast alloy.As the Al element continues to be added,the LPSO phase disappears in the alloy,and a large amount of Al₁₁Gd₃ and Al₂Gd phases are formed.After high temperature homogenization treatment,the?Mg,Zn?₃Gd phase,the 18R-LPSO phase and some Al₁₁Gd₃ phases can be transformed into the 14H-LPSO phase,while the bulk Al₂Gd phase has good thermal stability.Due to the formation of a large amount of 14H-LPSO and Al-Gd phases,the aging effect of the as-rolled alloy gradually decreases as the Al element increases.The peak-aged Mg-15.3Gd-1Zn-0.4Al alloy containing a fibrous LPSO phase and a large amount of?'precipitated strengthening phase obtained the optimal mechanical properties.?5?The effect of Ti on the microstructure and properties of Mg-Gd-Zn?-Al?alloy was investigated.The results show that the Ti element has good grain refinement effect on the as-cast and as-homogeneous Mg-Gd-Zn?-Al?alloys.The Ti element uniformly distributed in the matrix not only has the effect of solid solution strengthening,but also hinders the dynamic recrystallized?DRXed?grains growth.The addition of Ti does not affect the ageing behavior of Mg-Gd-Zn?-Al?alloy,but can significantly improve the hardness and tensile mechanical properties of the alloy.The peak-aged Mg-15.3Gd-1Zn-0.4Al-0.6Ti alloy obtained excellent comprehensive mechanical properties,and the tensile strength,yield strength and elongation were 478MPa,394MPa and 7.7%,respectively.