In-situ Study On The Deformation Behavior And Mechanism Of Microstructure Evolution During Annealing Of Triple Junctions Of FCC Metals

Triple junctions(TJs),the basic microstructural element,play an important role in microstructural evolution,as they have a significant influence on grain boundary migration and grain growth.They are also key factors in the study of microstructure design and performance control for metallic materials during deformation and recrystallization.In this thesis,off-line in-situ electron backscatter diffraction(EBSD)was used to characterize the deformation(cold rolled with 5%～35%thickness reduction)and annealing behavior at TJs in high purity aluminum and high purity nickel.Based on the theories of micro-plastics and recrystallization,combining experimental TEM observations on dislocation structures of TJs in high purity copper,the deformation and annealing mechanism of TJs of face-centered cubic(FCC)metals are investigated,which will provide potential guide for microstructure design and property control in FCC metals.The strain near TJ and grain boundary is larger than that in grain interior in the pure aluminum and nickel samples cold rolled with different thickness reduction.The point to original point misorientation near TJ is significantly related to the grain orientation,which is larger in soft grain than that in hard one,indicating that soft grain acquire larger strain than hard one at TJ during cold rolling.In high-purity aluminum samples with 15%reduction in thickness,the slipping lines pass through the low angle boundaries,but not through the high angle boundaries.This observation keeps the same in the vicinity of TJs.As the reduction in thickness reaches 17%,grain boundary affect zones were formed in the vicinity of grain boundaries,which are more obvious near TJs.During annealing,both the grain boundaries and TJs in high purity aluminum migrate and leave ghost lines at the original position.The high-angle grain boundaries present the greatest migration rate,the low-angle grain boundaries are in the middle,while the TJs have the least one.This observation indicated that TJs have considerate drag effects on grain boundary migration.The grain boundary migrates from grain with low Schmid factor to that with high Schmid factor,which means that boundary migrates from the hard grain to soft one.Grain boundary becomes more curved after migration.During further annealing,grain boundary and triple junction continue to migrate and the largest migration distance is observed in the sample with the reduction in thickness of 10%.For the samples with the reduction in thickness beyond 17%,the grain boundary migration is hindered by the grain boundary affected zone.The recrystallization nuclei are developed at some TJs in high purity aluminum and nickel during annealing.All these nuclei show Z3 orientation relationship with the parent grains,and the corresponding Z3 boundaries are found to be incoherent.The nucleation rate in nickel is higher than that in aluminum.High-angle grain boundaries migrate from hard grains to soft ones leading to the formation of nuclei at the TJs in aluminum;while the nuclei are formed in-situ at TJs composed of at least two high angle boundaries in nickel.The newly-formed nuclei are swept by other fast-growing grains during further annealing.Those fast growing grains are mostly pre-existing grains prior to deformation,and they grow via a mechanism of so-called strain induced boundary migration(SIBM).Accompanying with SIBM,high fraction 13 boundaries and large-sized Z3n(n=1,2,3...)grain clusters are introduced in nickel due to low stacking fault energy.However,this phenomenon is not found in high purity aluminum,it is due to the difference in stacking fault energy.The formation of these nuclei at the early stage of annealing is just a release of the strain concentration at TJs.The dislocation structure near TJs with small to medium deformation in the samples of high pure copper is observed and analyzed by TEM.The effects of deformation and grain orientation on the dislocation structure near TJs are characterized.The results show that the dislocation density at TJs increases with the deformation.The dislocation lines are introduced in the slightly cold rolled copper while the dislocation networks are formed as the deformation increases.The dislocation density in soft grain is larger than that in hard one at the same deformation.The dislocation at TJs in copper is determined to be(1/2)<0 1 1>perfect dislocation.The further analysis based on the theories of micro-plastics and recrystallization point out that whether in high pure aluminum with high fault energy or in high pure nickel with low stacking fault energy,the plastic deformation at TJ is accomplished by the sliding of(1/2)<0 1 1>perfect dislocation.In the meanwhile,strain difference between soft and hard grain at TJs is resulted from the balance of the continuous stress distribution and the dislocation-sliding interruption by grain boundary.Hence,the grain boundaries migrate from the hard grains to the soft ones during subsequent annealing due to the higher stored deformation energy in the soft grains.’The migrating grain boundaries interact with(1/2)<0 1 1>perfect dislocations,leading to the atomic rearrangement and the formation of recrystallization nucleus of Σ3 relationship with the hard grain.The mobility of Σ3 grain boundaries is so low that the nuclei developed at the TJs are all swept by other grain boundaries with higher mobility or consumed by other rapid growing grains.