Effect Of TiB₂ And Electromagnetic-stirring On Microstructure And Properties Of Mg-Zn-Y-Mn Alloys With Long Period Stacking Ordered Structures

Magnesium that has the characteristic of light weight and good ductility,is one of the commonly used non-ferrous metals.However,the disadvantage of lowly absolute strength restricts the wide application of magnesium alloys.The magnesium-based alloys,which can achieve the requirements of engineering structural materials properties,energy saving and environmental protection,are becoming the hot topic in the study of high-strength alloys.Rare earth elements are beneficial to the improvement of comprehensive mechanical properties in magnesium alloys because their can purify,activate and refine magnesium alloy.The Mg-Zn-Y alloys have inspired a new wave of research due to the special microstructures and ideal mechanical properties.Three different structures of X phase(Mg₁₂YZn),W phase?Mg₃Zn₃Y₂?and I phase?Mg₃YZn₆?are appeared in terms of the Zn/Y atomic ratios.It is easy to generate X phases with long period stacking ordered?LPSO?structures when the Zn/Y atomic ratio is less than 1.The long-period stacking ordered?LPSO?phase enhances the mechanical properties of Mg alloys at the room temperature and elevated temperature.Therefore,Mg alloys with long period stacking ordered structures have rapidly become the research focus of magnesium alloy.The Mg-Zn-Y-Mn alloys with TiB₂ addition were prepared by conventional permanent mould casting method in this study.The influence of electromagnetic stirring process on the microstructure and properties of Mg-Zn-Y-Mn alloy containing TiB₂ was studied systematically.After heat treatment,the alloys were carried out by direct extrusion to obtain better mechanical properties.The as-cast,heat-treated and as-extruded alloys were characterized and tested to investigate the influence of TiB₂ addition and electromagnetic-stirring process on their microstructure and mechanical properties.The main results were described as follows:?1?The volume fraction of long-period stacking ordered?LPSO?structure was significantly increased with TiB₂ doping to the Mg₉₄Zn_2.5Y_2.5Mn₁ alloy.When the doping amount of TiB₂ was 0.006 wt%,the uniform and equiaxed dendrites appeared and the volume fraction of the LPSO phase was 69.5%higher than that of the TiB₂-free alloy.In addition,this alloy showed optimal mechanical performance with ultimate tensile strength and elongation of240MPa and 13.5%,respectively.?2?The grain of were effectively refined and the formation of LPSO phase was further promoted by optimizing the electromagnetic stirring process in the Mg₉₄Zn_2.5Y_2.5Mn₁ alloy with 0.006 wt.%TiB₂ addition.The growth pattern of LPSO and W phase transformed from symbiotic growth to divorced growth by adjusting the technological parameters of electromagnetic stirring,such as voltage,frequency and temperature.The maximum ultimate tensile strength and elongation of the alloy with electromagnetic stirring reached 265MPa and 16.5%,respectively.?3?The optimum conditions of solution treatment for Mg-Zn-Y-Mn alloys with Ti B₂ addition was 500?for 40h.The spheroidization of W phase and dissolution of 18R LPSO phase were identified in as-heated alloys with water quenching.Comparatively,the 14H LPSO phase was precipitated in as-heated alloys with furnace cooling.Besides,the size of W phase was refined and the precipitation of 14H LPSO phase was promoted by electromagnetic stirring in as-heated alloy.The maximum ultimate tensile strength and elongation of the alloy with electromagnetic stirring were 260MPa and 20.2%,respectively.?4?The samples for extrusion experiments were prepared by solid-solution?water-cooling?treatment of Mg₉₄Zn_2.5Y_2.5Mn₁ alloy with TiB₂ addition.The14H LPSO phase and the nanoscale particle phase were precipitated during the process of deformation,which preceded the dynamic recrystallization behavior.?5?The volume fraction and average grain size of dynamic recrystallization is sensitive to deformation rates.The volume fraction and average grain size of dynamic recrystallization were 1.22?m and 67.46%in electromagnetic-stirring alloys,when the extrusion ram speed was 20 mm/min.Continuous dynamic recrystallization?CDRXed?and discontinuous dynamic recrystallization?DDRXed?grains of as-extruded alloys were located at the original grain boundaries and inside the 14H LPSO phase,respectively.?6?Electromagnetic stirring process can significantly refine the grain size and decrease the volume fraction of the W phase in as-extruded alloys.The dynamic recrystallized grain size and volume fraction were 1.13?m and 63.61%,respectively,which lower than alloys without electromagnetic stirring.The tensile strength and elongation reached to 420MPa and 15.7%,which indicated that the mechanical properties of as-extruded alloys were enhanced by electromagnetic stirring.