Study On Regulation And Mechanism Of Pulsed Current On Microstructure And Properties Of Cu-Ni-Mn Alloys

Cu-Ni-Mn alloy has been widely used in aerospace industry,petrochemical industry,shipbuilding industry and mining applications owing to its good physical,mechanical and corrosion resistance properties.Because the alloy has strong age hardening characteristics,aging treatment is usually used to increase the strength of the alloy in addition to cold deformation treatment.However,the precipitated phases produced during the aging treatment and the work hardening phenomenon reduce the plasticity and corrosion resistance of the alloy.At the same time,the precipitated phases also have grain boundaries preferential precipitation phenomenon,which aggregate at the grain boundaries and cause brittle fracture of the alloy along the grain boundaries.The coarse dendritic grains of the as-cast alloy also affect the comprehensive performance of the alloy.Aiming at the problems of the inability to have both the high strength and high plasticity of the Cu-Ni-Mn alloy,and the decline in the corrosion resistance of the alloy after aging treatment and cold deformation.In this paper,through the combination of hot deformation,cold deformation,aging and electropulsing treatment(EPT),the alloy with high strength and high plasticity was manufactured,and the corrosion resistance of the alloy was improved.At the same time,the microstructure evolution of Cu-Ni-Mn alloy after EPT was studied and analyzed by using SEM,EDS,EBSD,TEM and other characterization methods.Explore the transformation of the grain orientation,high and low angle grain boundaries of the deformation-aged Cu-Ni-Mn alloy after EPT,and reveal the alloy recrystallization behavior;Study the changes in the number of precipitated phases and distribution density,strain distribution,dislocation density and recrystallized grain size before and after EPT,and jointly clarify the influence of microstructure evolution on the properties of the alloy to obtain the manufacturing process of Cu-Ni-Mn alloy with excellent comprehensive performance;Experiments verify and combine the mechanism diagram and theoretical calculation formula to discuss and explain the influence mechanism of Joule heating effect and electronic wind effect on the evolution of alloy microstructure respectively.Research indicates:A larger amount of deformation can increase the dislocation density in the alloy,and effectively suppress the preferential precipitation of 0-MnNi precipitates at the grain boundary.The dislocation tangles and the concentrated distribution of ?-MnNi precipitates result in a higher electrical resistivity in this area,resulting in a higher Joule heating effect during EPT,which promotes the recovery and recrystallization process,and accelerates the dissolution rate of ?-MnNi precipitates in this region.After EPT,only low-density dislocation lines and a small amount of ?-MnNi precipitates distributed in the grain boundary can be observed in the microstructure of Cu-Ni-Mn alloy.At the same time,?-MnNi precipitates have a higher stress concentration during the deformation process,which causes the alloy to fracture preferentially from there.After EPT,the dislocation density in the microstructure of the alloy decreases,and the low angle grain boundary gradually transforms into the high angle grain boundary,and the alloy recovers and recrystallizes.In addition to the Joule heating effect,the effect of the electronic wind force on the alloy is also obvious.With the increase of the pulse current density,the effect of the electronic wind force is more significant.The electronic wind force can promote the movement of dislocations and open the dislocation tangles;it can also promote the recrystallization of the alloy,accelerate the recrystallization process,and promote the recovery of the alloy and a lesser degree of recrystallization at a lower temperature;At the same time,the electronic wind force can increase the dissolution power of ?-MnNi precipitates,and reduce the dissolution time of ?-MnNi precipitates.Under the combined action of the Joule heating effect and the electronic wind force of the pulse current,the changes in the number of ?-MnNi precipitates and distribution density,dislocation density and recrystallized grain size before and after EPT,and the generation of annealing twins are the main reasons for the changes in the mechanical properties and corrosion resistance of the alloy.The selection of appropriate deformation process and EPT process is the prerequisite for manufacturing Cu-Ni-Mn alloy with excellent comprehensive performance.Under low density and long time EPT(LDL-EPT),the comprehensive performance of the alloy with 80%deformation is better than that of the alloy with 96%deformation under the high density and short time EPT(HDS-EPT).After EPT,the tensile strength of the alloy is 950.5MPa,and the elongation is 11.5%,which is 30.1%higher than the tensile strength of the as-cast alloy treated by solution aging treatment with the same elongation.When the tensile strength is 800 MPa,the elongation can still reach 15%,which shows both high strength and plasticity,but poor corrosion resistance;After parameter adjustment,the alloy has higher strength,corrosion resistance and a certain degree of plasticity.Its tensile strength is 1102.3MPa,corrosion resistance is 2mm/year,and the elongation is 5%.