Microstructure Evolution Of Mg-Zn-Y-xGd Alloys During Isothermal Heat Treatment

Semi-solid processing technology is an efficient and energy-saving processing technology,which can improve alloy properties by reducing casting defects,macrosegregation and gas inclusions.Based on Mg-10Zn-4Y alloy,Mg-10Zn-4Y-xGd alloy(x=0,3.5,7,10)were prepared by adding different content of Gd element.The Mg-10Zn-4Y-xGd alloys were isothermally heat treated,and the semi-solid slurry was obtained,upon which the compression test was carried out.The effects of isothermal temperature and time on the microstructural evolution and properties of Mg-10Zn-4Y-xGd alloy during semi-solid isothermal heat treatment were investigated to reveal the semi-solid microstructural evolution of Mg-10Zn-4Y-xGd alloy and enrich the semi-solid theoretical basis of Mg-RE alloy system.The results are as follows:For as-cast Mg-10Zn-4Y-xGd alloys,when the content of Gd element is less than or equal to 3.5wt.%,the structure of as-cast Mg-10Zn-4Y alloy mainly consists of?-Mg,W phase(Mg3RE2Zn3)and a small amount of I phase(Mg3REZn6).With the increase in the content of Gd element,the I phase in as-cast Mg-10Zn-4Y alloy disappears,and the microstructure of the alloy mainly consists of?-Mg and W phase.With the increase of Gd content,the microstructure of as-cast Mg-10Zn-4Y alloy is refined,therefore,the yield strength increases.However,the increase of Gd leads to the decrease of I phase and the increase of W phase area fraction,which leads to the decrease of fracture strength of Mg-10Zn-4Y alloy.Comprehensive analysis shows that the optimum content of Gd element is 7wt.%,that is,Mg-10Zn-4Y-7Gd alloy has better microstructures and mechanical properties than the other alloys.During isothermal heat treatment process,it is found that when the addition of Gd element is less than or equal to 3.5wt.%,with the increase of isothermal holding time,the primary?-Mg grains coarsen and grow up,and some of them are spheroidized.However,due to the coalescence of?-Mg grains,irregular large-sized grains appear in the microstructure.When the addition of Gd element is more than 3.5wt.%,with the increase of isothermal time,the grain size of?-Mg in the microstructures tends to spheroidize while coarsening and growing,and the roundness is good.During isothermal heat treatment,with the prolongation of holding time,a large number of fine eutectic Mg grains appear in lamellar W phase,which leads to a sharp decrease in the average grain size of the alloy.The results show that the semi-solid structure of Mg-10Zn-4Y-7Gd alloy during isothermal heat treatment is more rounded,and the non-dendritic structure can be obtained after holding at 570~oC for 30 min of which the average grain size is 39.8?m.Through the analysis of isothermal heat treatment of Mg-10Zn-4Y-7Gd alloy at 580~oC,it is found that with the prolongation of isothermal time,a large number of eutectic Mg grains would appear in the lamellar W phase,and these eutectic Mg grains would form new a-Mg grains through the coalescence of adjacent grains or self-coarsening,or merge with the adjacent primary?-Mg grains and form grains of large size.By means of electron probe microscope and three-dimensional CT analyses,it is found that the mechanism of Ostwald ripening and coalescence dominate the evolution of semi-solid structure during isothermal heat treatment.