| The Mg-Nd-Zn-Zr system magnesium alloy belongs to high strength and heat resistant magnesium alloy,and its strength and toughness can be impro ved by extrusion,and the grain size can be refined so that it has great potential for fine grain superplasticity.In this paper,the microstructure stability of extruded plates of Mg-Nd-Zn-Zr alloy was investigated by heat treatment.The structure states required for the subsequent superplasticity tests were also obtained.Subsequently,superplasticity tensile experiments were performed in order to investigate the effect of experimental conditions and internal organization on the superplasticity properties of the Mg-Nd-Zn-Zr alloy.Finally,the evolution of the organization and voids of the extruded Mg-Nd-Zn-Zr alloy after superplastic deformation and its fracture mechanism were analyzed and discussed.By solid solution treatment of extruded plates of Mg-Nd-Zn-Zr alloy with an initial grain size of 2.78μm at 470℃～530℃,it has better microstructure stability below 500℃due to the pinning effect of the second phases on the grain boundary.Subsequently,at 200℃,the alloy after solution treatment（500℃×8h）is aged for1～16h respectively.With the extension of the aging time,the grain size of the alloy does not change,but the phase content s gradually increase.After comprehensive consideration,the solid solution state（500℃×8h）and the aging state（500℃×8h+200℃×8h）are selected for subsequent superplastic tensile experiments.The tensile tests are carried out at 350℃～500℃with initial strain rate of10-2s-1-5×10-4s-1.As the temperature increases and the initial strain rate decreases,the tensile strength of the extruded Mg-Nd-Zn-Zr alloy gradually decreases.At 450℃-5×10-4s-1,the maximum elongation is 1015.50%.According to the obtained experimental data,the superplastic characteristic parameters of Mg-Nd-Zn-Zr alloy are calculated,and the superplastic deformation mechanism diagram（σ-T relationship diagram）is established.When the temperature is 400℃-450℃,the main deformation mechanism is grain boundary slip（GBS）.Diffusion creep becomes dominant at 350℃-400℃,and dislocation creep becomes dominant at 45 0℃-500℃.At the same time,under different experimental conditions,other adjustme nt mechanisms are also accompanied.Using the orthogonal test method,the superplastic tensile tests of the solid solution state and the aged state Mg-Nd-Zn-Zr alloy are carried out at 400℃and5×10-3s-1.Although the average grain size of both are 15.60μm,their elongation generally reaches more than 300%.By comparing the superplastic tensile test data in the three states,it is found that the increase of the average grain s ize and the increase of the phase contents will increase the tensile strength of the alloy while decreasing the elongation.In addition,the increase in grain size will also change the superplastic characteristic parameters.The microstructure and fracture morphology of the extruded Mg-Nd-Zn-Zr alloy after being stretched to fracture are observed by OM and SEM.At the section shrinkageψ≈95%,the average grain size and axial ratio gradually increase with increasing temperature and decreasing initial strain rate.Observing the microstructure of different positions on the same sample,it is found that deformation will accelerate the diffusion rate of atoms,resulting in the phenomenon of deformation-induced grain growth（DIGG）.And when the stress concentration inside the alloy is too large,it will form cavities at the second phase or trigonal grain boundaries,which will relieve the stres s concentration through the nucleation,growth and connection of cavities and eventually cause the fracture of the material.The fracture mode is mainly ductile fracture,and the fracture contains a large number of tough nests. |
PDF Full Text Request