Study On Grain Boundary Character Distribution In High Purity Copper After Deformation And Annealing

The main deficiency of traditional grain boundary engineering with three parameters of misorientations to characterize the grain boundary character distribution is that it is impossible to determine the structural characteristics of grain boundaries with low ∑ values.For example,it can not distinguish the coherent and non-coherent ∑3 grain boundaries,and can not give the important information such as the proportion and distribution of the coherent ∑3 grain boundaries(coherent twin boundaries),which play a key role in improving the corrosion performance of grain boundaries.In this paper,grain boundary matching(GBIC)is used as the grain boundary structure parameter to study the evolution of grain boundary distribution during recrystallization and grain growth of high-purity copper after deformation.Then the GBIC characteristics and evolution of grain boundaries with low ∑ values such as ∑3 and ∑9 were studied,and the evolution mechanism was discussed.At the same time,the grain boundary corrosion resistance of materials was verified by corrosion experiments,and the effect of deformation and annealing on effective thickness layer was studied,which provided an important basis for the systematic in-depth study and further promotion of grain boundary engineering of middle and low level fault energy face-centered cubic metal materials.The main results of this paper are as follows :Homogenized high-purity copper samples with average grain size of about 7 μm and no texture were obtained by repeated multi-direction forging at room temperature and recrystallization annealing at 600℃.During the process of grain growth at 650℃,the proportion of ∑3 grain boundaries increases from 30.4% to 51.7%,and the proportion of ∑3 grain boundaries in ∑3 grain boundaries with {1 1 1}/{1 1 1} GBIC characteristics increases from64.8% to 83.6%.At the same time,the proportion of ∑9 grain boundary increases very little,and the GBIC of ∑9 grain boundary is both {1 1 1}/{1 1 5} and {0 0 1}/{4 4 7}.The results show that during grain growth,the non-coherent ∑3 grain boundaries transform to coherent ∑3 grain boundaries,and the migration of large Angle grain boundaries leads to the extension of coherent∑3 grain boundaries,which is the main reason that the proportion of coherent ∑3 grain boundaries increases with grain growth.However,∑9 grain boundaries are mainly formed during the homogenization process and are less affected by grain growth.After small deformation(5%,hickness reduction)cold rolling,the proportion of ∑3 grain boundary increases from 48.9% to 58.5% during recrystallization and grain growth during annealing at 650℃.The proportion of ∑3 grain boundaries with {1 1 1}/{1 1 1} GBIC characteristics increases from 65.8% to 87.2%.Although the ratio of ∑9 grain boundaries did not change obviously,the GBIC of ∑9 grain boundaries gradually changed from the coexistence of{1 1 1}/{1 1 5} and {0 0 1}/{4 4 7} to the do minant one of {1 1 1}/{1 1 5},and {0 0 1}/{4 4 7}gradually disappeared.It means that the formation of ∑3 and ∑3 grain boundaries and the transformation of ∑9 grain boundaries from higher energy {0 0 1}/{4 4 7} to lower energy {1 11}/{1 1 5} are more favorable after small deformation rolling and annealing at 650℃ than when treated directly at 650℃.This may be related to the deformation induced grain boundary migration induced by annealing at 650℃ after small deformation rolling.The results of corrosion experiments show that the high purity copper samples with the highest proportion of ∑3 grain boundaries and the high proportion of ∑9 grain boundaries with{1 1 1}/{1 1 5} GBIC characteristics have better grain boundary corrosion performance.Therefore,the grain boundary corrosion resistance can be improved by small deformation cold rolling followed by high temperature annealing to make the grains grow up.Repeated cyclic treatment of small deformation(5%)cold rolling with annealing at 650℃and medium deformation(30%)hot rolling at 410℃ and annealing at high temperature are beneficial to the increase of effective thickness layer,and the former is better.In the grain boundary characteristic distribution of the latter,the proportion of general grain boundaries is still higher,and the average size of grain clusters formed by ∑ 3,∑ 9 and ∑ 27 grain boundaries is significantly smaller than that of the former.