Fig. 1. Additive manufacturing and cyclic deep cryogenic treatment (CDCT) of bulk CoCrFeMnNi HEA samples: (a) schematic illustration of the LMD experimental setup; (b, c) SEM image and the particle size distribution of the raw CoCrFeMnNi HEA powders; (d) schematic diagram of CDCT process; (e) extraction schemes of dog-bone tensile samples from the top, middle and bottom locations of the LMD-built samples along the BD.

Fig. 2. Comparison of residual stress distribution in LMD-built CoCrFeMnNi treated with cyclic and single-step DCT: (a, b) residual stress at the center of LMD-built blocks after (a) CDCT for different cycles and (b) single-step DCT for different cryogenic durations [15] (error bars indicate ranges of five repeated measurements in each condition); (c) schematic diagram of 11 × 11 matrix for the residual stress measurement; (d-i) residual stress distributions over middle BD × SD plane of the LMD-built blocks after different DCT conditions: (d) as-built, (e, g, i) CDCT for 2, 4 and 10 cycles, and (f, h) single-step DCT for 24 and 48 h ((e) and (f) have same cryogenic duration of 24 h, and (g) and (h) have same cryogenic duration of 48 h, common stress scale applies to all stress distributions shown); (j) average residual stress value at each height along the BD of the LMD-built and CDCT-processed HEA samples.

Fig. 3. Comparison of tensile behavior of LMD-built CoCrFeMnNi treated with cyclic and single-step DCT: (a) engineering stress-strain curves of CDCT samples of different cryogenic cycles extracted from the middle; (b) engineering stress-strain curves of single-step DCT samples of different cryogenic durations extracted from the middle [15]; (c) engineering stress-strain curves of samples extracted from the top; (d) engineering stress-strain curves of samples extracted from the bottom.

Fig. 4. XRD patterns of LMD-built CoCrFeMnNi treated with cyclic and single-step DCT: (a) XRD patterns of cyclic and single-step DCT samples; (b) right shift of (200) peaks of the LMD-built and cyclic and single-step DCT-processed samples, corresponding to the compressive residual stress data in Fig. 2(a, b). Data for DCT24h, DCT48h and DCT120h from Ref. [15].

Fig. 5. TEM microstructures of CDCT samples: BF-TEM images of (a) the LMD-built and CDCT-processed CoCrFeMnNi HEA samples with (b) one, (c) two, (d) four and (e, f) ten cryogenic soaking cycles. Insets show the corresponding SAED patterns taken from regions highlighted by red dashed circles.

Fig. 6. TEM microstructures of the as-built, DCT24h, and DCT-2 samples: BF-TEM images showing the typical microstructures at the top and bottom regions of the (a, d) as-built, (b, e) DCT24h, and (c, f) DCT-2 samples, respectively.

Fig. 7. HR-TEM analysis of the DCT-4 sample: (a) typical BF-TEM image showing the various crystalline defects; (b) schematic of a defect microstructure; (c, d) HR-TEM images indicating the phase transformation from FCC to HCP; (e-g) HR-TEM images of typical microstructures of nanotwins and stacking faults.

Fig. 8. HR-TEM analysis of the DCT-1 sample: representative nanotwins and stacking faults with (b) the enlarged HR-TEM image highlighted by the white box in (a).

Fig. 9. HR-TEM analysis of the DCT-10 sample: (a) BF-TEM with SAED pattern shown in the inset taken from the region highlighted by the white dashed box; (b) HR-TEM image of the DCT-10 sample showing the representative nanograin microstructure.

Fig. 10. TEM deformation microstructures of DCT-4 state at different tensile strains: (a1-d1) BF-TEM and corresponding (a2-d2) HR-TEM micrographs marked by white dashed boxes exhibiting twin evolution with increasing tensile strain at room temperature; (a3-d3) BF-TEM micrographs showing the evolution of dislocation structure with increasing tensile strain. Nanotwins and stacking faults are labeled as NT (yellow arrows) and SF (red arrows), respectively.

Fig. 11. FEM simulation of residual stress after single-step and cyclic DCT: stress vs soaking time in (a) the single-step DCT48h, and (b) the cyclic DCT-4 process. Stress values at their peak or end of the process cycles are indicated, with TRS and CRS denoting tensile and compressive residual stress, respectively. Temperature variation over time is also shown (green curves).