Mechanism of grain refining by equal channel-angular pressing technique has been studied by using copper single crystals and polycrystals. The microstructures of each specimen consisted of cell having a width of submicron after single pressing. The cell structure was formed in interior of coarse grains in polycrystals. The cell boundaries had low angles of misorientation. The misorientations of cell boundaries increased with repetitive pressing in polycrystals. It was concluded that cell structure was changed into the ultrafine grain by repetitive pressing in polycrystals. Ultrafine grains separated by boundaries having high angles of misorientation were not formed in single crystals after repetitive pressing. It was suggested that grain boundaries were helpful to transition into high angle of misorientation.
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