Microstructure And Room Temperature Tensile Behavior Of Directionally Solidified Mg-Gd-Y-Er Alloy

Poor plastic forming ability at room temperature is one of the fundamental reasons limiting the development of magnesium alloys in the field of engineering materials.Directional solidification is a technique that adopts compulsory means during the solidification process of an alloy to solidify and grow in accordance with the required crystal orientation to obtain a columnar crystal structure with a certain crystal orientation.The columnar crystal structure has similar geometric characteristics to twin crystals,and the constraints at the grain boundaries during deformation can be reduced from 5 to 3 for equiaxed crystals.The columnar grain structure with reduced grain boundary constraints will have profound significance for improving the grain boundary strain coordination of HCP structure magnesium alloys with only two independent slip systems at room temperature.In this article,directional solidification technology was used to prepare Mg-4.00Gd-0.30Y-0.01Er alloy by adjusting the alloy composition and solidification process parameters(the drop-down rate).The elongation(?)of the prepared alloy exceeded 40%.Taking this alloy as the research object,the effects of drop-down rate on the morphology and growth orientation of the experimental alloy were studied by means of OM,SEM,XRD and EBSD.The experimental alloys were tensile tested at room temperature and high temperature and the relationship between the crystal growth orientation and mechanical properties was discussed.The results show that:With the increase of the drop-down rate,the directional solidification structure of the alloy changed from cell dendritic to columnar dendrite,and the primary arm spacing gradually decreased from 156?m with a drop-down rate of 1mm/min to 62?m at 11mm/min.The experimental alloy columnar crystal orientation changes from the(1012)crystal plane to the(1013)crystal plane,and then changes to the(1120)crystal plane as the drop-down rate increases.The tensile strength(?b)of the experimental alloy increased with the increase of the drop-down rate,from 96MPa at 1mm/min to 140MPa at 11mm/min;the elongation(?)showed a downward trend,which decreased from 42.19%at 3mm/min To 6.32%of 11mm/min;the energy of deformation(DE)first increases and then decreases with the increase of the drop-down rate,from 31.77J/m3 at 1mm/min to 36.57J/m3 at 5mm/min,and then decreases to 11.12J/m3 at 11mm/min;the trength-ductility balance tends to increase first and then decrease,and the optimum is 4256.76MPa·%at 5mm/min.The tensile deformation mechanism of the experimental alloy is dominated by<a>slip of the basal plane and the coordination of tensile twins and compressive twins.The growth orientation of columnar crystals is highly consistent,and alloys with base surface<a>slip in "soft orientation" by adjacent show the better plasticity;alloys with base surface<a>slip in "hard orientation" by interphase or adjacent perform plasticity worse plasticity.