Effects Of The Heat Treatment And The Loading Conditions On Recrystallization Behavior Of Magnesium Alloys Containing Diffenert Configuration Of {10-12} Twins

As the lightest structural metallic materials,magnesium(Mg)alloys have wide application prospect in electronic communication,aerospace,automobile manufacturing and other industries.However,their application on a large scale has been limited because of their poor ductility and insufficient absolute strength.As an effective means of softening and grain refinement,recrystallization has great significance for reglating the textures strength and grain size of metallic materials,and then improve the plastic formability of metallic materials.For magnesium alloys which are prone to twinning deformation,the relationship between twin configuration and recrystallization behavior has attracted much attention.In recent years,it has been shown that twin type and twin variants orientation can influence the recrystallization behavior of magnesium alloys,but the relationship between them is not clear.{10-12} twins is the most common type in the deformation of magnesium alloys,and there are still differences in the study of recrystallization behavior of magnesium alloys containing different configuration of{10-12} twins.In addition,most of the existing heat treatment methods of magnesium alloys are isothermal annealing.The effect of induction heating quenching which heating fast and inputing high density energy into materials on the recrystallization behavior of magnesium alloys is still to be studied.In this paper,the compression and compression-extrusion direction tests of different strain paths(cumulative compressive strains are all 1%)were carried out on an extrusion Mg-0.087Mn(wt%)alloy rod along the extrusion direction(ED)in the present study,generating slip and twinning-detwinning behavior in different degrees,and obtaining the microstructure containing different {10-12} twin configuration.Then,the samples under different loading conditions were subjected to isothermal annealing and induction heating quenching,and the microstructure evolution during heat treatment was characterized by quasi-in-situ optical microscope and quasi-in-situ EBSD technique.The related characteristic parameters of grain and twin in microstructure were analyzed statistically and mechanical properties were tested.The effects of loading conditions and heat treatment on recrystallization behavior and mechanical properties of magnesium alloys containing different {10-12} twin configuration were investigated.The main conclusions can be drawn:(1)compared to single loading,reciprocating loading maintaining the same cumulative strain can increase the twin volume fraction,twin incidence and dislocation density near the twin boundary and grain boundary in the alloy,while reducing the average size of twins.Compared with sample only compressed 1%,the twin volume fraction increased by 1.4% and the twin incidence increased by 21.7%.The average twin size decreased by 3.5?m.(2)During isothermal annealing and induction heating quenching,detwinning occurred in magnesium alloy.During isothermal annealing,the speed and degree of detwinning are correlated with loading conditions and twin size.The longer total strain path,the stronger the thermal activation migration ability of the twin boundary;the smaller the twin size,the greater the tendency to be swallowed by the matrix,and the more prone to detwinning.During the process of induction heating quenching,the speed and degree of denaturation are closely related to the loading conditions,quenching temperature and twin size.The longer the total strain path,the higher the temperature,and the smaller the twin,the more prone to detwinning.(3)During isothermal annealing and induction heating quenching,recrystallization nucleation occurred at the beginning of isothermal annealing and induction heating.The nucleation sites were mainly near grain boundaries.It is shown that when the cumulative strain is 1%,decreasing the twin size and increasing the dislocation density near the twin boundary can not make {10-12} twin becoming an effective recrystallization nucleation site by twinning-detwinning reciprocating loading.(4)During isothermal annealing and induction heating quenching,the grain growth is accompanied by the detwinning.During the isothermal annealing,the degree of grain growth increases significantly with the total strain path and annealing temperature.Samples with a total strain path of 1% and 7%(first compressed 4% and then reversed stretched 3%)show only weak grain growth after 120 min annealing at 350?;samples with a total strain path of 11%(first compressed 6% and then reversed stretched 5%)show weak grain growth after 120 min annealing at 300?,and the grain size can reach110-250?m after 120 min annealing at 350?.During the quenching process,the grain growth degree of samples with total strain path of 1% and 7% increases with the quenching temperature.The grain size of rapidly-growth grain in sample with the total strain path of 7% at each temperature can reach to: 22-23?m(275?),50-85?m(300?),60-95?m(325?),90-200?m(350?).The grain size of samples with a total strain of11% increases and then decreases with the temperature of quenching.The grain size of rapidly-growth grain in sample with the total strain path of 11% at each temperature can reach to: 50-90?m(275?),86-160?m(300?),70-110?m(325?),60-105?m(350?).(5)Compared with isothermal annealing,induction heating quenching improves the thermal-activated migration of twin and grain boundaries.The sample which first compressed by 6% and then reversed stretched by 5%,the twin volume fraction decreased by 3.7% after annealed for 15 min at 300?;no abnormal grain growth occurred until annealed for 120 min at 300?.Whiel the twin volume fraction was reduced by 4.3% at a lower 275? subjected to quenching one time(heating for 15s);and rapid-growth grains appeared after the tourth times(totally heating for 15s)during the quenching process.(6)Loading conditions and heat treatment can effect grain size.Then affects the tensile strength and elongation of the alloy.During the isothermal annealing and induction heating quenching,the tensile strength and elongation of magnesium alloys decrease with the increase of grain size of rapidly-growth grains in the microstructure.The grain size of samples with a total strain of 11% annealed at 300? for 2h is 21.8?m,the tensile strength and elongation is 220 MPa and 12% respectively;After annealing at350? for 2h,the size of rapidly-growth grains can reach to 110-250?m,the tensile strength and elongation are 154 MPa and 7.3%.The size of rapidly-growth grains are about 60-105?m after quenching at 350?,and the mean grain size is 83?m,the tensile strength and elongation are 173 MPa and 8.5%.