Fig. 1. The relaxed 3D simulation cells for the nanocrystalline samples (a) with pure fcc structure; (b) with pure hcp structure. (c)-(f) The relaxed 3D simulation cells for the nanocrystalline fcc samples with numerous embedded hcp nano-lamellae.

Fig. 2. Deformation-induced hcp nano-lamellae and nanotwins in the CoCrNi MEA tested under cryogenic temperature and high strain rate. (a) Bright-field TEM image showing hcp nano-lamellae; (b) HREM image showing hcp nano-lamellae and DTs; (c) HREM image showing the stacking sequences for a typical hcp nano-lamella in the fcc matrix; (d)-(f) HREM images showing the structures with hcp nano-lamellae, and the hcp phase fractions are 52 %, 71 %, 93 % in the selected areas, respectively.

Fig. 3. Deformation-induced DTs in the CoCrNi MEA tested under room temperature and high strain rate. (a) Bright-field TEM image showing DTs; (b) Close-up views for the rectangular area in (a) showing secondary nanotwins; (c) HREM image taken with the <110> zone axis, showing orientation relationship of fcc matrix and twins; (d) HREM image showing the dislocations at the tip of TBs.

Fig. 4. Vickers microhardness of samples prior to and after dynamic deformation under both cryogenic and room temperatures. (a) Optical images of indentation arrays. (b) Vickers hardness comparison.

Fig. 5. (a) The simulated stress-strain curves for the nanocrystalline samples with the same interspacing (7.34 nm) and the different hcp layer widths (the curves for pure fcc and pure hcp samples are also included). (b) The effect of hcp layer width on the average flow stress.

Fig. 6. Snapshots for the pure fcc sample at various applied tensile strains: (a) 4 %; (b) 7 %: (c) 10 %. Snapshots for the pure hcp sample at various applied tensile strains: (d) 4 %; (e) 7 %: (f) 10 %.

Fig. 7. The detailed atomistic deformation mechanisms for the pure fcc sample at applied tensile strains of 10 % showing (a) deformation-induced nanotwins; (b) hcp phase transformation; (c, d) The detailed atomistic deformation mechanisms for the pure hcp sample at applied tensile strains of 10 % showing basal SFs, reverse transformation (from hcp phase to fcc phase) and < c + a > edge dislocations (1/6[$2\bar{2}03$]).

Fig. 8. (a) The detailed atomistic deformation mechanisms for the pure hcp sample at applied tensile strains of 10 % showing {$10\bar{1}1$} twins. Experimental TEM observations for the DTs in the hcp cobalt sample after SMAT: (b) The {$10\bar{1}1$} twins; (c) The secondary {$10\bar{1}1$} twins generated inside the primary {$10\bar{1}1$} twins. (d) Schematic of formation sequences and orientations of the secondary {$10\bar{1}1$} twins inside the primary {$10\bar{1}1$} twins.

Fig. 9. The effect of hcp layer width on the strengthening.

Fig. 10. Snapshots at applied tensile strains of 4 % (a, c, e) and 7 % (b, d, f) for the nanocrystalline samples (a, b) with numerous embedded SFs; (c, d) with numerous embedded six hcp atom layers; (e, f) with numerous embedded ten hcp atom layers. The interspacing is kept the same (7.34 nm) for these three samples.

Fig. 11. The detailed atomistic deformation mechanisms for the nanocrystalline samples (a1-a3) with numerous embedded SFs; (b1-b3) with numerous embedded six hcp atom layers; (c1-c3) with numerous embedded ten hcp atom layers at the applied tensile strain of 4 %. The fcc atoms are not shown in this figure for the more clarity.

Fig. 12. (a) The simulated stress-strain curves for the 3D nanocrystalline samples with the same hcp layer width (six hcp atom layers) and the different interspacings. (b) The effect of interspacing on the average flow stress. (c) The effect of interspacing on the strengthening.

Fig. 13. Snapshots at applied tensile strains of 4 % (a, c) and 7 % (b, d) for the nanocrystalline samples: (a-b) The interspacing is 2.45 nm; (b) The interspacing is 5.50 nm. The width of hcp nano-lamellae is fixed (six hcp atom layers) for these two samples.

Fig. 14. The detailed atomistic deformation mechanisms for the nanocrystalline samples: (a1-a3) The interspacing is 2.45 nm; (b1-b3) The interspacing is 5.50 nm. (a1)(b1) are snapshots at the applied tensile strain of 4 %; while (a2)(a3)(b2)(b3) are snapshots at the applied tensile strain of 7 %. The fcc atoms are not shown in this figure for the more clarity.