Microstructures And Mechanical Properties Of Directionally Solidified Mg-xGd-0.5Y Alloy

In this paper,the columnar polycrystalline Mg-xGd-0.5Y(x=3.0,4.5,6.0,7.5,9.0,wt%)alloys were prepared by directional solidification,and the tensile tests were carried out at room temperature.XRD,EDS and EBSD were used to study the effects of Gd content and directional solidification process parameters(pull-up rate)on the directional solidification structure,crystal growth direction and tensile deformation behavior of the experimental alloys.The results showed that:1.The crystalline morphology of directionally solidified Mg-3.0Gd-0.5Y alloy is cellular dendrite with a primary arm spacing of 231μm and the growth direction of cellular dendrite is<1120>at 50m/s pull-up rate,which is consistent with the direction of heat flow.The crystal morphology of Mg-6.0Gd-0.5Y alloy is cellular dendrite,and its crystal growth direction is<1120>and<2021>.The primary arm spacing decreases with the increase of lift rate.The crystal morphology of Mg-9.0Gd-0.5Y alloy is columnar dendrite.With the increase of Gd content,the direction of crystal growth gradually deviates from the direction of heat flow,and the crystal morphology gradually transits from cellular dendrite to columnar dendrite.The yield strength(σs),tensile strength(σb)and elongation after fracture of directionally solidified Mg-3.0Gd-0.5Y alloy are 51MPa,72MPa and 14.88%,respectively.With the increase of Gd content,the elongation after breaking first increased and then decreased.The maximum elongation after breaking was 32.56%at 6.0%Gd and 5.44%at 9.0%Gd.The strength of the alloy increases remarkably with the increase of the pull-up rate.When the drawing rate is 50 um/s,Mg-6.0Gd-0.5Y alloy σb is 77MPa and σb is 56MPa.When the pull-up rate increased to 200 ums,σb and σs were 106MPa and 87MPa respectively,which were 66%and 48%higher than those of 50um/s.2.The deformation mechanism of the experimental alloy is mainly base slip<a>accompanied by twinning and strain-compatible deformation at grain boundaries.The twinning mode in the grain at 90° between the C-axis of the Mg cell and the external load is{1011} compressed twins with a matrix orientation difference of 56±5°<1120>and a difference of 38±5° from the substrate orientation<1120>{1011}-{1012} double crystal.The intragranular twinning mode at 45° between the C-axis of the Mg cell and the external load is a {1012} tensile twin with a matrix orientation difference of 86±5°<1120>.The proportions of small-angle grain boundaries,compressed(double)twins and tensile twins in Mg-3.0Gd-0.5Y alloy were 72.58%,11.4%and 16.02%,respectively.The proportions of small-angle grain boundaries,compressed(double)twins and tensile twins in Mg-4.5Gd-0.5Y alloy were 80.54%,5.83%and 13.63%,respectively.The proportions of small-angle grain boundaries,compressed(double)twins and tensile twins in Mg-6.0Gd-0.5Y alloy are 88.30%,0%and 11.70%,respectively.That is to say,in Mg-6.0Gd-0.5Y alloy,the proportion of<a>slip on the base surface is larger,the angle between the orientation C axis of more grains and the external load is between 30° and 45°,and the compatible strain between grain boundaries is better.The compatible strain between grain boundaries is better,and the compatible strain is in the mode of tensile twinning,and the room temperature plasticity is more obvious.3.The grain orientation of directionally solidified Mg-3.0Gd-0.5Y alloy is mainly concentrated in<1210>(the angle between cell C axis and tensile stress direction is close to 90°).In the process of uniaxial deformation at room temperature(tensile stress direction of crystal growth),due to the excessive stress required for most grain base slip initiation,the coordinated deformation of base slip can not be initiated,which hinders the deformation of adjacent grains.The deformation process can only be coordinated by compression twins,and the ductility is poor.The elongation after fracture is only 14.88%.The grain orientation of directionally solidified Mg-6.0Gd-0.5Y alloy is mainly distributed between<1210>and<0001>(the angle between cell C axis and tensile stress direction is close to 45°).During uniaxial deformation at room temperature,the grains initiate base slip and have good compatibility with each other.The main deformation mechanism is slip and twinning,with excellent plasticity and post-fracture elongation up to 32.56%.In summary,the cylindrical polycrystalline Mg-6.0Gd-0.5Y alloy with more uniform growth orientation prepared at 50 um/s pull-up rate obtained excellent room temperature plasticity of 32.56%after fracture under tensile load(tensile stress crystal growth direction).The directionally solidified magnesium alloys have excellent room temperature plasticity only by accurately adjusting and controlling the growth direction of crystals,changing the crystal orientation with the best performance but not preferential growth into preferential growth,and reducing the occurrence of heterocrystals deviating from preferential orientation.