Effect Of Zn-Mg-Zr-B Master Alloy On Microstructure And Mechanical Properties Of Mg-Zn-Y-Mn Alloys

Magnesium and magnesium alloys with small density,high specific strength,good processing performance,as well as easy recovery have attracted much attention in recent years,which have a wide application prospect on automobile,military industry,aerospace and biomaterials.Nevertheless,the drawbacks of magnesium alloys such as low absolute strength and poor plasticity limit their widespread engineering application.The Mg-Zn-Y long-period magnesium alloy with long period stacking ordered(LPSO)phase has become a research hotpot due to its excellent properties.However,the expensiveness of the rare earth Y entails the biggest obstacle to the civil industrial application of long-period magnesium alloys.Therefore,the vital problem that have plagued researchers in recent years is how to obtain outstanding mechanical properties of Mg-Zn-Y alloys at low Zn/Y atomic ratios.The key to obtain excellent mechanical properties of long-period magnesium alloy is to refine the grain size and promote the formation of LPSO phase through inoculation.In this paper,Zn-Mg-Zr-B master alloy,which was prepared by the traditional casting process and hot extrusion for the first time,then was added to the Mg-Zn-Y-Mn alloys as inoculant to refine the grain size.The effects of Zn-Mg-Zr-B master alloy on the grain refinement mechanism and mechanical properties of as-cast Mg-Zn-Y-Mn alloys were investigated.Then the effects of solution treatment time and Zn-Mg-Zr-B master alloy on the microstructure evolution of the second phase and mechanical properties of the alloy were also studied.And the mechanism of W phase spheroidizing and LPSO phase transformation was explored.After solid-solution treatment,the extruded treatment was performed on the alloys to investigate the effects of extrusion temperature and Zn-Mg-Zr-B master alloy on the microstructure and mechanical properties of the alloys.Additionly,systematical analysis on the dynamic recrystallization behavior of the alloys was conducted.The main results are as follows:(1)Zn-Mg-Zr-B master alloy,in which Zn as base metal,was prepared by conventional casting method for the first time.Then the alloy was subjected to extrusion deformation,resulted in further refinement of the grains,the optimal microstructure of the alloy can be achieved when the extrusion temperature is 250°C.(2)The results show that Zn-Mg-Zr-B master alloy is effective inoculant of Mg-Zn-Y-Mn alloys and its additions can lead to significant refinement of the as-cast alloy,promote the formation of 18 R LPSO phase and inhibit the precipitation of W phase.With 2 wt.% Zn-Mg-Zr-B master alloy addition,the as-cast alloy exhibited optimal microstructures and mechanical properties,with the yield strength of 198 MPa,the ultimate tensile strength of 226 MPa and the elongation of 11.5%.(3)After solution treatment at 500?for 40 h,the fishbone W phase occurred spheroidization and the 18 R LPSO phase transformed into lamellar 14 H LPSO phase which precipitated in the matrix.The Zn-Mg-Zr-B master alloy can promote the precipitation of the 14 H LPSO phase which is benefit to the mechanical properties of the solid-solution treated alloy.With Zn-Mg-Zr-B addition,the mechanical properties of the solid-solution treated alloy improve significantly,with the tensile strength reaches 228 MPa and the elongation increases to 16.5%.(4)The extrusion temperature has significant effect on the microstructure and mechanical properties of the alloy.Within a certain range of temperature,the higher the extrusion temperature,the more complete the DRX is,the larger the grain size,the strength of the alloy decreases and the plasticity increases.(5)The Discontinuous dynamic recrystallization and continuous dynamic recrystallization behaviors were observed during extrusion process of the alloy.The Zn-Mg-Zr-B master alloy can delay the DRX process and refine the DRXed grains effectively.The extruded alloy with Zn-Mg-Zr-B master alloy addition exhibits excellent mechanical properties with the yield strength of 345 MPa,the ultimate tensile strength of 350 MPa and the elongation reaches 15%,which are attributed to the grain refinement strengthening and LPSO phase strengthening.