Effect Of ZrB₂ On Microstructures And Properties Of Mg-Zn-Y-Mn Alloys With Long Period Stacking Ordered Structures

As the common engineering materials,the main characteristic of magnesium and magnesium alloys is light weight.In addition,magnesium alloys also have a series of advantages such as low density,high specific strength,specific stiffness and easy recovery,which have a wide application prospect in the field of military industry,aerospace and transportation.However,due to its low absolute strength and other shortcomings,the application is limited to some extent.Therefore,the preparation of high strength and toughness magnesium alloys has become a research hotspot.At present,it has been found that magnesium alloys with long period stacking order structure?LPSO?have unique microstructure and excellent properties.As a typical representative of magnesium alloys with LPSO structure,Mg-Zn-Y alloys have rapidly become the focus of scholars'research.In this paper,Mg-Zn-Y-Mn alloys with different ZrB₂ content were prepared by conventional casting method.The effects of trace addition of ZrB₂on the microstructure and mechanical properties of as-cast Mg-Zn-Y-Mn alloys were studied.Then the electromagnetic stirring was applied to the preparation of as-cast Mg-Zn-Y-Mn alloy containing 0.0075 wt.%ZrB₂ and the influence was explored.Subsequently,the alloys were subjected to solid solution treatment and forward extrusion deformation.And the effects of solution treatment processes and forward extrusion processes such as extrusion speed and temperature on the microstructural evolution and mechanical properties of the alloys were investigated.The transformation of LPSO phase and W phase in solid solution alloys as well as the dynamic recrystallization behavior in extruded alloys were analyzed comprehensivly.The main results are as follows:?1?The microstructure of the as-cast alloy is composed of?-Mg matrix phase,18R-LPSO phase and reticulated W phase.The addition of ZrB₂ does not change the phase composition of the alloy.Micro addition of ZrB₂ can remarkably refine the grain size of the Mg-Zn-Y-Mn alloy and promote the formation of 18R-LPSO phase.When 0.0075 wt.%ZrB₂ is added,the as-cast alloy achieves the optimal microstructures and mechanical properties,the tensile strength of 225 Mpa and the elongation of 17.5%are obtained.?2?The use of the electromagnetic stirring technique resulted in further refinement of the grains of the as-cast Mg-Zn-Y-Mn alloy containing 0.0075 wt.%ZrB₂.When the electromagnetic stirring frequency was 25 Hz,the stirring voltage was 340 V,and the stirring time was 10 min,the alloy obtained the best microstructure and mechanical properties,and its tensile strength and elongation reached 230 MPa and 15%,respectively.?3?The solution treatment of Mg-Zn-Y-Mn alloy containing 0.0075 wt.%ZrB₂ was carried out at different time.The optimum process parameters were500?×40 h.After solid solution treatment and furnace cooling,the continuous reticulated W phase at the grain boundary occurred spheroidization and distributed uniformly in the matrix,the block 18R-LPSO phase was dissolved gradually while the lamellar 14H-LPSO phase was precipitated on the matrix.The transformation of the second phase has a remarkable optimization on the mechanical properties of the alloy,especially for the plasticity,which make the tensile strength of the alloy reach 235 MPa and the elongation reach 22%.?4?The dynamic recrystallization behavior of solid solution alloy after forward extrusion has been observed.The different extrusion rate and extrusion temperature have significant influence on the morphology of each phase and dynamic recrystallization in the alloy.The extrusion rate is constant,the higher the extrusion temperature,or the extrusion temperature is constant,the faster the extrusion rate,the more completely the DRX will be.When the extrusion temperature is 360?and the extrusion rate is 30 mm/min,the dynamic recrystallization size and volume fraction reach the minimum of 1.5?m and52.04%,respectively.And the alloy exhibits optimal mechanical properties with the tensile strength and elongation of 358 MPa and 23.9%,respectively.