Effects Of Ag On Microstructures,Mechanical Properties And Corrosion Behaviour Of Mg-Gd(-Y)-Zr Alloys

As the lightest structural metallic material,Mg alloys have attracted widespread attention in scientific and industrial areas due to the great social and economic benefit from weight and energy saving.So far,Mg-Gd-Ag-Zr casting alloys that previously developed by our research group obtained the highest room temperature tensile yield strength(328 MPa)and ultimate tensile strength(423 MPa).However,the effects of Ag on microstructures and mechanical properties of Mg-Gd(-Y)-Zr alloys are still not systematically investigated thus far.There also lacks comprehensive characterisation of precipitates in Mg-Gd(-Y)-Ag-Zr alloys.Furthermore,the Ag addition can significantly decrease the corrosion resistance of Mg-Gd(-Y)-Ag alloys,but the effect of Ag on the corrosion behaviour of Mg-Gd(-Y)-Zr alloys is unclear.At present,the limited studies on the microstructures,mechanical properties and corrosion behaviour of Mg-Gd(-Y)-Ag-Zr alloys greatly hinder the development and application of advance Mg alloys with high strength and high corrosion-resistant.Thus,it is necessary to carry out further study.In this project,six alloy compositions were designed:Mg-2.8Gd-0.1Zr(VK,at.%),Mg-2.4Gd-0.4Ag-0.1Zr(VQK,at.%),Mg-2.8Y-0.1Zr(WK,at.%),Mg-2.4Y-0.4Ag-0.1Zr(WQK,at.%),Mg-1.6Gd-0.8Y-0.4Ag-0.1Zr(VWQK21,at.%)and Mg-0.8Gd-1.6Y-0.4Ag-0.1Zr(VWQK12,at.%).Firstly,the effects of Ag on microstructures and mechanical properties of Mg-Gd(-Y)-Zr alloys were systematically studied.Then,the morphologies,structures,compositions and transformation of precipitates in the VQK alloy were comprehensively investigated.Finally,the VK and VQK alloys were selected and the role of Ag in the corrosion behaviour of Mg-Gd-Zr was studied.The main contents and conclusions in the present study are summarised below.Ag plays obviously different roles in the age-hardening response of Mg-Gd and M-Y alloys.When 0.4 at.%Gd is substituted by Ag,the VQK alloy shows a stronger age-hardening response than the VK counterpart.In contrast,when 0.4 at.%Y is replaced by Ag,the WQK alloy exhibits a weaker age-hardening response than the WK counterpart.This is because the partial replacement of Gd by Ag leads to the microstructure of VQK consisting of prismatic and basal strengthening precipitates.Compared with the VK counterpart,which only contains prismatic precipitates,the age-hardening response of VQK is enhanced.In contrast,the basal precipitates in the WQK alloy are formed at the expense of prismatic precipices after the partial replacement of Y by Ag.Consequently,the age-hardening response of WQK is weakened.The partial replacement of Gd by Ag can not only increase the tensile yield strength(YS)and ultimate tensile strength(UTS),but also improve the elongation to fracture(EL)of the Mg-Gd alloy.The room temperature YS,UTS and EL of the 200 ? peak-aged VQK are 271 MPa,414 MPa and 2.7%,respectively,which are increased by 10%,41%and 575%in comparison with those of the 200 ? peak-aged VK counterpart.The main reason accounts for the improvement in YS of the 200 ? peak-aged VQK alloy is the combined strengthening effect from the basal and prismatic precipitates.The Ag addition also leads to the formation of particle free zones(PFZs)with 30-60 nm width along the grain boundaries in the 200 ? peak-aged VQK.The PFZs are posited to accommodate the plastic deformation to some extend during the tensile testing,which is assumed to be the main reason for the improved EL of the 200 ? peak-aged VQK alloy.The improved EL of the 200 ? peak-aged VQK further help it achieve much higher UTS than that of the 200 ? peak-aged VK counterpart due to the contribution from work hardening.The number densities of prismatic and basal precipitates in Mg-Gd-Y-Ag-Zr alloys are effectively adjusted by the relative contents of Gd and Y.Compared with VWQK12(Gd/Y atomic ratio 1:2),the 200 ? peak-aged VWQK21(Gd/Y atomic ratio 2:1)has higher number densities of both basal and prismatic precipitates.Compared with the VQK alloy,the partial replacement of Gd by Y leads the resultant VWQK21 to obtain a good combination of strength and ductility.The YS,UTS and EL of the 200 ? peak-aged VWQK21 are 260 MPa,402 MPa and 4.2%,respectively.The precipitation in VQK includes ? and ?-type precipitates.The ?-type precipitation sequence is proposed as:super-saturated solid solution(S.S.S.S.)?ordered solute clusters?zigzag GP zones??'??F'??1??.Compared with the precipitation sequence of Mg-Gd system,the proposed ?-type precipitation sequence includes ordered solute clusters,zigzag GP zone and ?F',but excludes ?".The ordered solute clusters comprise 1-6 solute-rich atomic columns along[0001]?,in which hexagonal-shaped clusters are the most common type and have the best thermal stability than other types of clusters.The zigzag GP zones form as disks on{1010}? and tend to distribute in pairs in the microstructure.In addition,the zigzag GP zones act as building blocks in the construction of the ?'phase.The ?F' phase forms in the over-aged stages and inevitably distributes attached to the coarsened ?'The strain associated with the coarsening of ?' is assumed to provide sufficient driving force to overcome the strain energy barrier to the ?F' formation.Furthermore,the ?F' phase provides a structurally and chemically favourable site for the ?i formation.The ?1 phase may involve from the ?F' phase.The ?-type precipitation sequence is established as:S.S.S.S.?basal GP zone ??"'??"??.The basal GP zones form as mono-layer plates on(0001)? with a hexagonal structure(a=0.548 nm).Besides,the basal GP zones act as building blocks for the ?"' phase.The ?"'phase forms as three-layer plates with an orthorhombic lattice:a=0.548 nm,b=0.949 nm,c=0.546 nm.The ?" phase also forms as three-layer plates,but has a hexagonal structure:a=0.548 nm,c=0.417 nm.Moreover,the ?'" phase is found to locate at the edges of the ?" phase.It is posited that the ?"' phase may transform into the ?" phase in an in-situ manner.The y phase shows an irregular shape and has a diamond cubic structure(a=1.6 nm).The Ag additions result in great decrease in the corrosion resistance of Mg-Gd alloys.Using a quasi-in-situ STEM/EDS approach in the present study,the segregation of oxygen(O)at the interface between the a-Mg matrix and the ? phase and the formation of Ag-rich particles during the corrosion process are directly observed,which provide direction evidences for micro-galvanic corrosion.Since the enrichment of Ag in the ? phase and the Ag-rich particles formed during the corrosion process can enhance corrosion,the VQK alloy exhibit poor corrosion resistant.The present study systematically investigates the effects of Ag on microstructures,mechanical properties and corrosion behaviour of high-strength Mg-Gd(-Y)-Zr alloys.The findings obtained in the present study provide theoretical and experimental basis for the design and development of advance Mg-RE alloys with higher strength.