Cold rolling behaviour of Cu with highly oriented nanotwins: the importance of local shear strain

Abstract

To clarify the deformation mechanism of nanotwinned materials under a complex stress state and large strain level, the microstructure evolution of pure Cu samples with high density of preferentially oriented nanotwins cold rolled to a strain up to 65% was systematically characterized by means of electron backscatter diffraction and transmission electron microscopy in this study. Heterogeneous deformation behavior was observed in nanotwinned Cu at different rolling strains. At a rolling strain of 15%, uniform deformation carried by the interactions between dislocations and twin boundaries dominate. At the same time, a small amount of strain localization in the direction ±45° respective to the rolling direction takes place, carried by detwinning and shear bands. As the rolling strain increased to 50%, detwinning becomes a dominant deformation mechanism and produces a large amount of coarse twin/matrix lamellae. A unique laminated structure with high angle lamellar boundaries (larger than 50° misorientation angle) and an average lamellar thickness of about 150 nm evolves from the coarse twin/matrix lamellae. As the rolling strain is increased up to 65%, an extensive laminated structures embedded with a few preserved twin blocks are prevalently formed. It is found that the local shear strain plays a critical role in the microstructure evolution of the nanotwinned Cu under cold rolling. When the local shear strains are so small as to be negligible, uniform deformation is sustained by interactions of dislocation/twin boundaries and the nanoscale twins survive. As the local shear strain increases up to 1, detwinning prevails and results in coarse twin/matrix lamellae. When the local shear strain is larger than 1, formation of shear bands composed of dislocation cells, sub-grains and fragments of twin lamellae instead of the detwinning process takes place, finally evolving into a laminated structure.