Crystallographic nature of cell boundaries developed during cyclic deformation of a near-1¯11-oriented copper single crystal

Given the strong correlation between fatigue crack initiation and dislocation cell structures, elucidating the crystallographic nature of cell boundaries is fundamental to understanding fatigue mechanisms. In this study, three-dimensional electron backscatter diffraction combined with focused ion beam slicing was employed to characterize the formation planes of cell boundaries in a fatigued copper single crystal oriented near the $\left(\overline{\text{1}}\text{11}\right)$ direction. Beyond the (111) boundaries representative of mature cell structures, (110), (011), and (121) boundaries were identified as geometrically necessary segments initiated by cross-slip, which forms at the early stage of cell structures. The (110) and (011) boundaries include dislocation networks formed by Shockley partials dissociated from the perfect dislocations on different potentially activated slip planes. Subsequently, the (121) boundaries emerge after the cross-slip of screw dislocations and pile-up on the cross-slip plane. Cell bands preferentially form along the (121) plane because of the symmetrically arranged alternating {110} boundaries. The rotation axis between adjacent cells exhibits a strong dependence on misorientation angle, and the rotation axes are scattered around the [111] and [121] directions when the misorientation is small (<1.0°). The [121] axis originates from the vector sum of [110] and [011], which corresponds to twist boundaries on the (110) and (011) planes. The (111) twist boundaries form and gradually replace the other geometrically necessary boundaries with an increase in the misorientation to accommodate increasing plastic incompatibility. This study uncovers the crystallographic nature of fatigue-induced cell structures and offers insights into enhancing metal performance through dislocation control.