| The grain boundary character distributions (GBCD) of cold-rolled and annealed pure copper were investigated by means of electron back scatter diffraction (EBSD) techniques. The single-section trace analysis method, five-parameter method and twin filtering method based on EBSD were employed to distinguish statistically the incoherent and coherent E3 boundaries in the processed samples. The mechanism of GBCD evolution was proposed based on the in-situ EBSD observation. The main results are summarized as below.(1) The processing varieties of cold-rolling and annealing have significant influence on GBCD. Special boundaries such as∑3,∑9 and∑27 were largely populated in the specimen of cold-rolled with low strain (10%, thickness reduction) followed by high temperature annealing, the fraction of total special boundaries reached 76%. The connectivity of general high angle boundaries (HABs) networks was interrupted substantially by special boundaries, and the GBCD are well optimized. As the strain of cold-rolling increased (20%-40%), the fraction of total special boundaries was decreased drastically when the sample were annealed, and the connectivity of general high angle boundaries (HABs) network was not interrupted significantly, implying the GBCD are not optimized. Further analysis shows that compared to the intermediate strain, low levels strain (10%) have little stored energy of deformation, which could not facilitate recrystallization extensively during annealing, but the strain induced boundary migration (SIBM) might be active and it was probably the root of GBCD optimization.(2) Crystalline orientations have minor effects on the GBCDs of the samples processed by two-step cold rolling and annealing method. It was found that the samples of single crystal but with different orientations appeared to posses the same GBCDs after treatment of two-step cold rolling and annealing, which means a processing of cold rolling with large strain (larger than 75%) plus recrystallization annealing and then followed by cold rolling with low strain (lower than 10%) plus high temperature short-term annealing. Special boundaries such as∑3,∑9 and∑27 were largely populated in all specimens with the fraction of total special boundaries between 70%-80%, the ratio of (∑9+∑27)/∑3 around 20% and the size of clusters of grains with∑3n (n=1,2,3) orientation relationships about 200 mocron, indicating the GBCD in these samples are well optimized. Further analysis shows that the deformation microstructures of the single crystal sample with different crystalline orientations tended to identical after cold rolling with large strain and it lead to the nearly same recrystallized microstructures. So crystalline orientations are not the main factors on GBCD evolution during processing of the two-step cold rolling and annealing.(3) The initial microstructures have great influence on the GBCDs of the samples in the processing of cold rolling and annealing. The specimen with finer initial grain size (12 microns) appeared to be largely populated by the so-called special boundaries such as∑3" (n=1,2,3) after cold rolling and annealing, the fraction of∑3n boundaries reached 75.7% and the averaged size of clusters of grains with∑3n (n=1,2,3) orientation relationships (∑3n CG) were as large as 200 microns in this specimen. However, the GBCDs of specimens with other initial microstructures such as those of deformed, partially recrystallized and recrystallized but with coarsened grains were not optimized in satisfaction after the same treatment of cold rolling and annealing. In-situ EBSD observation, twin-filtering and five parameter method (FPM) analyses indicated triple-junctions are the preferred sites for the nucleation of incoherent∑3 boundaries which play a important role in enhancing the fraction of special boundaries. It accounts for primarily the desired GBCD results as obtained in the specimen with finer initial grain size. |
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