Fig. 1 a Digital photograph of the processed Al alloy rod, b surface profiles of the rolling surface and the turning surface, c surface profile measured in the transition area between the rolling and turning surface, d, e respective SEM images of rolling surface and turning surface. Turning and rolling refer to the turning surface before UECR and the rolling surface after UECR, respectively

Fig. 2 a Cross-sectional microhardness distribution of the Al alloy rods, b nanoindentation hardness for UECR and UR sample in different depths to surface, c-e inverse pole figure (IPF) maps calculated from EBSD data of the Al alloy rod in the original state and after UECR and UR, respectively. The treated surface is outlined by a red dashed line. The RD refers to the rolling direction, and the ND refers to the normal direction

Fig. 3 TEM observations of the Al alloy rod after UECR and UR: a-c for the UECR sample, a NL structures obtained from 10 μm deep to the surface, b NL structures obtained from 40 μm deep to the surface, c UFG structures obtained from 80 μm deep to the surface; d, e for the UR sample, d heavily deformed microstructure obtained from 10 μm deep to the surface, e UFG structures obtained from 40 μm deep to the surface. a2, b2, c2, d2, e2 are the respective magnified views of a1, b1, c1, d1, e1

Fig. 4 a, d Magnified IPF map of the top 30-μm-thick layer, in which the thin black lines indicate boundaries of misorientation between 2° and 15°, and the thick black lines indicate boundaries above 15°; b, e bright-field image, c, f corresponding dark-field image. The red arrows in a, d indicate the treated surface. a-c are for the UECR sample, and d-f are for the UR sample