Fig. 1 Schematic illustrations of the roller and sample a; the deep rolling process b

Fig. 2 Cross-sectional microstructures of the pure nickel treated by the DR with the indent depth of a 100 μm, b 200 μm, c 300 μm, d 400 μm, respectively

Fig. 3 TEM bright field images of the microstructures on top surface of a specimen-1, b specimen-2, c specimen-3, d specimen-4

Fig. 4 TEM images at different depths from the top surface of the specimen-4: a at ~?300 μm depth, showing that dislocation cells were formed; b at ~?200 μm depth, showing that subboundaries were formed to separate cells; c at ~?80 μm depth, showing that subgrains were formed; d at ~?30 μm depth, showing that roughly equiaxed ultra-fined grains were formed

Fig. 5 Schematic illustration of the grain refinement process during the DR treatment

Fig. 6 Relationship between surface roughness and indent depth

Fig. 7 Variation in the microhardness with the depth form surface for the four specimens

Fig. 8 Residual stress distribution along the depth from the surface for the four specimens

Fig. 9 Cross-sectional microstructures of specimen-4 annealed at 300 °C a, 400 °C b, 500 °C c, 600 °C d, 700 °C e for 2 h, respectively

Fig. 10 TEM images of specimen-4 at the depth of about 30 μm annealed at 300 °C a, 400 °C b, 500 °C c, d, 600 °C e, 700 °C f for 2 h, respectively