Study On The Multi Deformation Process Of Re Containing Magnesium Alloy Staircase Cooling

Results have shown that plastic deformation can significantly improve the mechanical properties of magnesium alloy by grain refinement.But considering the weak formability and the difficulty for forming the large-scale complex workpieces by one step,the workpieces are often completed by multi-steps forming and the final microstructure develops from previous microstructure.The traditional multiple processing methods are usually under the constant temperature,but the Mg-13Gd-4Y-2Zn-0.5Zr heat resistant rare-earth magnesium alloy has the advantages of the traditional magnesium alloy and high temperature resistance,corrosion resistance.In addition,the addition of rare earth elements can improve the plasticity of alloy to expand the scope of application of magnesium alloy.Therefore,the changes in the microstructure and properties of the Mg-13Gd-4Y-2Zn-0.5Zr alloy during the multi-pass deformation of the staircase cooling are studied in this paper.In this paper,the microstructure evolution of Mg-13Gd-4Y-2Zn-0.5Zr rare earth heat resistant magnesium alloy during multi-step and multi-directional deformation process is studied.It mainly includes the evolution rule of grain size and long range ordered phase in different deformation and different deformation conditions;analysis of alloy hardness variation of alloy after deformation and the reasons for the changes;high temperature tensile test were carried on the specimens,analysis of the tensile stress-strain curve,peak stress,and elongation and its causes.Dynamic recrystallization occurred in the process of deformation and occurred in grain boundaries.Besides,the new grains processed the shape of “necklace”,therfore,it belongs to the typical continuous dynamic recrystallization and it had the effect of grain refinement.The rule of the change of the grain size after stepped cooling was: it increased after one passdeformation but decreased obviously after two passes,and then decreases slightly after three passes.After four passes,the deformation is basically the same as that of the three passes.There were three states of the LPSO phase in the alloy: the fine lamellar LPSO phase distributed in the grain;the skeletal eutectic LPSO phase at the grain boundary and the rod LPSO phase at the grain boundary.During the deformation process,the LPSO phase was all twisted or even broken.However,due to the decrease of deformation temperature,the change trend of LPSO phase in the four passes was smaller than that in the two pass and three passes.The hardness of Mg-13Gd-4Y-2Zn-0.5Zr rare-earth heat resistant magnesium alloy was higher than that of single pass deformation after multi-step deformation.And the hardness of alloy increased first and then decreased with the increase of deformation pass.The peak stress and elongation of alloy showed a trend of first increased and then decreased.The reason for this phenomenon was due to the reduction of deformation temperature during the process of step cooling deformation.As the deformation temperature stepped down,the dislocation movement in the alloy slowed down,which made the grain refinement and LPSO phase strengthening decrease.This is the same as the microstructure change rule during the multi-step deformation process.