Tensile Properties And Strengthening Mechanisms Of UFG 1050 Aluminum Alloy After Annealing With External Fields

In this work,the hardness test,electron back scattering diffraction pattern(EBSD),transmission electron microscopy(TEM)were used to investigate the recovery and recrystallization of as-cast and as-rolled 1050 aluminum alloy after cryoECAP and annealed at 150-400? without and with high magnetic field for 1h,and the effects of magnetic annealing on the recovery and recrystallization were achieved.The tensile test,TEM and positron annihilation lifetime spectroscopy(PALS)were used to investigate the effects of annealing with external fields(magnetic field and DC current)on the uniform elongation and the "yield point" phenomenon.The mechanism of the annealing-induced strengthening and the effects of annealing with external fields on this mechanism were investigated by the combination of the yield strength,strain hardening rate,dislocation density and the concentration of the vacancy cluster.The strengthening contributions of samples annealed at 150-400? without and with DC current for 1h were calculated according to the dislocation strengthening and boundary strengthening.This work is significantly meaningful to understand the strengthening mechanism of the ultrafine grained(UFG)materials,and can supply new annealing methods for improving the properties of UFG materials.When annealed at 150-250? without magnetic field for 1h,with increasing temperature,the hardness decreases slightly,the grain size increases slightly from 0.85?m to 1.9?m,and the dislocation density decreases,indicating the occurrence of the recovery.When annealed at 300-4000?,the hardness decreases significantly,the grain size increases sharply(7.8-22.4?m),indicating that the recrystallization process is basically completed.With the application of high magnetic field,the grain growth rate and the dislocation movement are enhanced at 150-250?;but the abnormal grain growth is suppressed and a more homogeneous grain size distribution is produced at 300-400?.The process of the recovery and the recrystallization in the as-rolled sample after cryoECAP is faster.Due to the submicron order grain size of cryoECAPed UFG 1050 aluminum alloy,low strain hardening ability and limited uniform elongation(2.3%)behaviors are exhibited.The fraction of HABs which acts as the more effective dislocation sinks in samples annealed without and with high magnetic field increases from 63.8%and 61.7%to 70.8%and 66.2%,respectively,with annealing temperature increasing from 90? to 210?.Therefore,the strain hardening ability is reduced and the uniform elongation decreases from 1.55%and 1.6%to 0.55%and 0.6%,respectively.When samples annealed with high magnetic field,the lower fraction of HABs leads to the higher uniform elongation.Moreover,the yield point phenomenon occurs in all samples annealed at 120-210? without and with high magnetic field due to the lack of mobile dislocations during tensile deformation.The "yield point"phenomenon becomes more obvious with increasing annealing temperature.An annealing-induced strengthening is observed in the UFG 1050 aluminum alloy annealed at 90-180? without and with high magnetic field for 4h.The yield strength reaches a peak value of 164MPa for samples annealed at 150? without high magnetic field,which is 14.7%higher than that of cryoECAPed sample.According to the PALS analysis,the dislocation density decreases and the concentration of the vacancy cluster increases with the annealing temperature increases from 90? to 150?.Moreover,the storage energy is consumed quickly during the recovery,and the more equilibrium structures of HABs form.These equilibrium structures of HABs and vacancy clusters can act as effective barriers to dislocation motion during tensile deformation,thus leading to higher yield strength.The lower yield strength in the samples annealed at 90-150? with high magnetic field might be attributed to the less stable structures of HABs and the lower concentration of the vacancy clusters.The annealing induced strengthening is observed in the UFG 1050 aluminum alloy annealed at 90-210? without DC current for 3h.The yield stress reaches a peak value of 167.5MPa for samples annealed at 150?.The yield strength of samples annealed with DC current is lower than that of samples annealed without DC current at 15 0-210?,which may result from the lower concentration of the vacancy cluster and the larger grain size.Moreover,compared with high magnetic field,the application of DC current is more effective to enhance the recovery and the recrystallization,and can produce more vacancy clusters due to the enhanced vacancy movement,and finally shifts the peak annealing temperature to a lower value(120?).The yield strength of UFG 1050 aluminum alloy comes from the dislocation strengthening and grain boundary strengthening contributions.The annealing induced strengthening occurs when samples are annealed at 150-200? without and with DC current for 1h.For these samples,there assist the third strength contribution-the equilibrium structures of HABs and vacancy clusters pinning dislocation during tensile test.When annealing at 250?,the vacancy clusters disappear during the recovery,the grain size increases(1.9-2.3?m),the area of HABs decreases,thus the above third strength contribution is significantly reduced and the strength contribution mainly comes from the dislocation strengthening and grain boundary strengthening.When annealing at 300-400?,the yield strengths of the samples annealed without and with DC current rapidly decrease by 65-80%compared with that of the cryoECAPed sample due to the disappearance of dislocation strengthening and the significantly increased average grain sizes(6.4-22.4?m).With the application of DC current,when samples are annealed at 150-200?,the higher fractions of boundaries with angle of 3°-7° will enhance the energy of the samples,and results in a less stable structure of HABs and the lower yield strength.Mover,the enhanced vacancy movement accelerates the annihilation of the vacancy cluster,which also reduces the yield strength.When samples are annealed at 250?,the yield strength is lower due to the lower dislocation density and the larger grain size.When samples are annealed at 300-400?,the smaller average grain size results in the higher yield strength.