Effect Of Extrusion Process On The Formation Of Bimodal Grains And Mechanical Properties Of Mg-Gd-Y-Nd-Zr Alloy

Mg-Gd-Y alloy has broad application prospects in the aerospace field due to its high strength,but its application range is severely limited due to the poor plasticity of the alloy at room temperature.Studies have shown that the formation of bimodal grains composed of recrystallized fine grains and deformed coarse grains can synergistically improve the strength and ductility of magnesium alloys.In this paper,aiming at the preparation of high-strength and high-plastic magnesium alloys,the effects of extrusion speed,mold temperature and ingot temperature on the formation of bimodal grains and mechanical properties of Mg-11Gd-3Y-0.5Nd-Zr alloy were systematically studied by means of optical microscopy,scanning electron microscope,electron backscatter diffraction,hardness test and uniaxial tensile testing.The following conclusions are obtained:(1)The influence of extrusion process on the as-extruded microstructure was studied,and the extrusion process range for forming bimodal grains was obtained.With the increase of extrusion temperature and extrusion speed,the microstructure of the extruded rod changed from a partially recrystallized structure to a fully recrystallized structure,and the recrystallized grain size continued to increase.In order to form bimodal distribution grains,the temperature of the ingot should be controlled at 420 ℃～500 ℃,the temperature of the mold should be controlled at 320 ℃～360 ℃,and the extrusion speed should be 1mm/s.The bimodal distribution grains composed of elongated deformed grains and equiaxed recrystallized grains can be obtained after extrusion by the process within this range.With the increase of ingot temperature,mold temperature and extrusion speed,the coarse-grained area fraction of the alloy decreased continuously,and finally transformed into fully recrystallized grains.(2)An extrusion process with good comprehensive mechanical properties was obtained.When the ingot temperature is 500 °C and the extrusion speed is 1 mm/s,excellent mechanical properties can be obtained by extrusion at 320 °C and 360 °C mold temperature.When the mold temperature is 320 ℃,the tensile strength,yield strength and elongation after fracture of the extrusion-T5 state are 450 MPa,379 MPa and 8.9 %,respectively;When the mold temperature is 360 ℃,the tensile strength,yield strength and elongation after fracture of the extrusion-T5 state are 456 MPa,333 MPa and 8.9 %,respectively.(3)The mechanism of bimodal grains to improve the strength and ductility of alloys was revealed.The excellent strength of the alloy is mainly attributed to the grain refinement of the dynamically recrystallized grains,the dislocation strengthening of the deformed grains and the aging strengthening caused by the aging precipitates.The excellent plasticity of the alloy is mainly attributed to the weak texture of recrystallized grains and the coordination of deformation between coarse and fine grains.In the early stage of tensile deformation,plastic deformation mainly occurs in the fine recrystallized grains,and the high dislocation density strengthens the elongated coarse grains.As the deformation progresses,the non-basal slip system in the elongated coarse grains initiates and coordinates the later deformation of the alloy,enabling the alloy to obtain high plasticity while maintaining high strength.(4)The influence of bimodal grains on the fracture characteristics of alloys was studied.The fracture modes of alloys with bimodal grains are all ductile fractures.There are a large number of dimples and tearing edges on the fracture surface,and cleavage planes are rarely observed.In the alloys with complete recrystallization,the fracture surface can be observed more cleavage planes,resulting in lower alloy plasticity.