Microstructure And Mechanical Properties Of Semi-solid Slurry Of Mg-Y-Zn-Al Alloy Prepared By Self-inoculation Method

As the most promising alloys for lightweight industrial products,magnesium alloys have attracted the attention of researchers.However,the disadvantages of magnesium alloys,such as poor plasticity at room temperature and low strength at high temperatures,have always limited its development and application in the industrial field.The way of precipitation strengthening can be promoted the strength of magnesium alloys effectively.Therefore,selecting and optimizing precipitation phases are particularly important.The long period stacking ordered structure（LPSO）phase can significantly improve the comprehensive performance of magnesium alloys,due to its excellent physical properties and thermal stability.Therefore,the development of high-performance and heat-resistant magnesium alloys containing LPSO phase is imperative.At present,most of the methods to obtain high-performance and heat-resistant Mg alloys contains alloying,heat treatment and hot deformation,which could improve the performance of the Mg allos by solution strengthening,grain refinement and precipitation strengthening.However,there would be higher energy consumption and higher production costs for these methods.Semi-solid metal processing technology will obtain semi-solid slurry with fine spherical structure,and will effectively avoid the defects of traditional casting.Therefore,this project intends to use the technology to produce a semi-solid slurry of Mg alloys containing LPSO phase.In this paper,a semi-solid slurry of Mg-Y-Zn-Al alloy containing LPSO phase was prepared by self-inoculation method.Optimization of the preparation process parameters（melting temperature,isothermal holding temperature and time）of the semi-solid Mg-Y-Zn-Al alloy slurry,and study on the influence these parameters on the microstructure and correspoinding evolution mechanism of semi-solid slurry would be discussed.In addition,the effect of hot-rolling on the microstructure and mechanical properties of the alloy prepared by quench of semi-solid slurry was also investigated.The evolution mechanism of LPSO phase and the contribution of different strengthening mechanisms to yield strength of alloy were quantitatively analyzed.The detailed experimental results are as follows:1.The factors that affect the microstructure of semi-solid slurry include melting temperature,isothermal holding temperture and time.The experimental results indicate that with the decrease of melting treatment temperature（690-670℃）,the morphology of primaryɑ-Mg slurry after quenching in water gradually changes from coarse dendrites to a rose-like structure,and becomes nearly spherical when the temperature dropped to 670℃.Isothermal holding temperature decreased from 610℃to 570℃,the eutectic phase in the semi-solid structure transformed from 18R-LPSO phase to Mg₈YZn phase.Also,the roundness of primaryɑ-Mg and average grain size of alloy both increased with the increase of isothermal holding time.The high quality of the semi-solid slurry microstructure was obtained after holding for 10～15 mins.The average grain sizes of the semi-solid slurry holding for 10 mins and 15 mins were about 101μm and 132μm,and the roundness is 1.24 and 1.23,respectively.The growth of the grain at the initial isothermal holding was mainly controlled by the Ostwald ripening mechanism,the grain growth is main reason for the coarsening of the grains after holding for 15 mins.The best processing parameters for preparing semi-solid Mg-Y-Zn-Al alloy slurry were 5%of the self-inoculations,5mm of cubic particles and 670℃of melt treatment temperature,590℃of holding temperature and10～15 mins of isothermal holding time.2.The Mg-Y-Zn-Al alloy（SIM15 alloy）prepared by quenching semi-solid slurry using the above self-inoculation process parameters was hot-rolled under different reductions（30%,45%and 60%）.The microstructure of SIM15 alloy was mainly composed ofɑ-Mg matrix,lamellar and blocky 18R-LPSO phase,fine stripe14H-LPSO phase and blocky Al₂Y phase.During the hot rolling process,partial of the lamellar 18R-LPSO phases distributed in the grain boundaries transformed into14H-LPSO phase in the grain interior.Also,the crushed 18R and 14H-LPSO phase could act as nucleation particles and promote dynamic recrystallization through the PSN nucleation mechanism.With the increase of hot rolling reduction,the yield strength,tensile strength and elongation of SIM15 alloy increased and the fracture mode gradually changed from mixed fracture to ductile fracture.Through the quantitative analysis on the contribution of different microstructures to the yield strength of the hot-rolled alloy,the increase of yiled strenght of the alloy main ariesd from grain refinement,load transfer strengthening of the 18R-LPSO phase and precipitation strengthening of the 14H-LPSO phase.