Fig. 1. EBSD images showing the hetero-structure after recrystallization annealing in CrCoNi, 316L, and 316LN, respectively. (a–c) Inverse pole figure (IPF) images of the three alloys. All are equi-axed grains of complete recrystallization. (d–f) Grain boundary (GB) images of the three alloys. Ʃ3: twin boundary. LAGB: low-angle GB of misorientation ranging from 5° to 15°. HAGB: high-angle GB of misorientation ≥15°. (g) and (h) Distribution of both grain size and grain boundary orientation in the three alloys. Note the size distribution of bi-modal in both CrCoNi and 316L and multi-modal in 316LN, respectively. ${{\rho }_{\text{t}}}$: density of annealing twins.

Fig. 2. Tensile properties of CrCoNi, 316L, and 316LN. (a–c) Tensile engineering stress-strain curves (${{\sigma }_{\text{e}}}-{{\varepsilon }_{\text{e}}}$) in CrCoNi, 316L, and 316LN, respectively, at 298 K (a), 77 K (b), and 4.2 K (c). Red line: CrCoNi, blue line: 316L, green line: 316LN. Circle: yield strength, Square: ultimate tensile strength (UTS). Note the presence of serrated flow at 4.2 K (c). (d–f) Strain hardening rate ($\partial \sigma /\partial \varepsilon $) normalized by flow stress (${{\sigma }_{\text{f}}}$) vs true strain (${{\varepsilon }_{\text{t}}}$) curves at three temperatures in the three alloys. Square: uniform elongation. (g) Yield strength (${{\sigma }_{\text{y}}}$) (upper panel) and uniform elongation (${{\varepsilon }_{\text{u}}}$) (lower panel) vs testing temperatures ranging from 373 K to 4.2 K in CrCoNi, 316L, and 316LN, respectively. Note the rise at first and drop later in ${{\varepsilon }_{\text{u}}}$ in both 316L and 316LN, in sharp contrast to the monotonic rise in CrCoNi. (h) (${{\sigma }_{\text{y}}}$, ${{\varepsilon }_{\text{u}}}$) balance in the three alloys. Arrows show the trend with decreasing temperature.

Fig. 3. Full-size Charpy V-notch impact properties in CrCoNi, 316L, and 316LN. (a) and (b) Load-deflection (L-D) curves (left y-axis, solid lines) and absorbed energy-deflection (Av-D) curves (dashed lines by integrating the L-D curves, right y-axis) at 298 K and 77 K, respectively. PGY and PM are yield load and peak load, respectively. PU indicates the onset of unstable fracture propagation.AK: Charpy impact energy (number as indicated by arrows towards right y-axis). AI: crack initiation absorbed energy (blue area under L-D curve before PM). AP: crack propagation absorbed energy (yellow area under L-D curve after PM). (c) Balance of AI and AP in the three alloys with temperatures ranging from 373 K to 4.2 K. Solid and half-open symbols represent AI and AP at 77 K and 298 K. (d) Change of slope with applied temperatures after PU in the three alloys.

Fig. 4. Fracture toughness through compact-tension C(T) specimen testing in 316L and 316LN. An increasing fracture resistance in terms of the J-integral vs crack extension ∆a (i.e., resistance curve, R curve) in 316L (a) and 316LN (b) at 298 K and 77 K, respectively. According to ASTM standard [33], a power law, $J={{\text{C}}_{1}}{{\left( \text{ }\!\!\Delta\!\!\text{ }a \right)}^{{{\text{C}}_{2}}}}$, is adopted to fit the data, see solid lines. Dashed lines are construction lines ($J=M{{\sigma }_{\text{Y}}}\text{ }\!\!\Delta\!\!\text{ }a$, $M=4$ for high work-hardening materials, effective yield strength ${{\sigma }_{\text{Y}}}=\left( {{\sigma }_{\text{y}}}+UTS \right)/2$ with 0.2 mm offset. See inserted tables for the values of ${{J}_{\text{C}}}$ and ${{K}_{\text{C}}}$ at 298 K and 77 K, respectively.

Fig. 5. HV distribution of Charpy V-notch impact (CVN) and compact-tension (C(T)) specimens at 298 K and 77 K. (a) and (b) HV profiles ahead of the crack tip and along the cracking path in 316L and 316LN, respectively. White arrow: crack tip. White line: crack propagation path. HV distribution along the red arrow is plotted in Fig. S4, showing the gradient change of HV across the plastic zone. (c) ΔHV (HV increment at crack tip) and ${{r}_{\text{c}}}$ (the hardening zone size) for CVN and C(T) specimens at 298 K and 77 K in 316L (left column) and 316LN (right column), respectively. All scale bars are the same of 3 mm.

Fig. 6. XRD spectra before and after tensile and impact testing at 298 K, 77 K, and 4.2 K, respectively: (a) CrCoNi, (b) 316L, (c) 316LN.

Fig. 7. Microstructure after tensile deformation at varying temperatures in 316L and 316LN. (a–c) and (d–f) EBSD images in 316L and 316LN, respectively, at 298 K, 77 K, and 4.2 K. Inserts in (a) and (d): grain boundary (GB) map. Red: Ʃ3 twin boundary. Green: LAGB. Blue: HAGB. Arrows: twins. Insets in (b), (c), (e) and (f): phase map. Red area: martensite (${\alpha }'$). Blue area: austenite ($\gamma $). (g–i) Bright-field TEM images in 316L at 298 K, 77 K, and 4.2 K, respectively. (g) Twin plates in $\gamma $. Inset: the selected area electron diffraction (SAED) pattern with the [110] zone axis (z.a.). (h) ${\alpha }'$ in $\gamma $. Inset: the SAED pattern of ${\alpha }'$ with the [$\bar{1}11$] z.a. (i) The evidence of both ${\alpha }'$ and ε in $\gamma $. Insets: the SAED patterns of ${\alpha }'$ and ε, respectively, with the [$\bar{1}11$] z.a. and [$2\bar{1}\bar{1}0$] z.a.

Fig. 8. Microstructural evolution near crack tip after impact testing at 298 K and 77 K in 316L. (a) EBSD image quality (IQ) map showing the boundary features at the tip of the main crack (C1) at 298 K (upper panel). Red: Ʃ3 twin boundary. Green: LAGB. Blue: HAGB. Note that there is no phase transformation, see phase image (lower panel). (b) EBSD image at the tip of secondary crack (C2) (upper panel). Note the presence of deformation twins of high density. Lower panel: phase image. Red arrow: twin boundaries. (c) Close-up view of the dashed area in (b). Blue arrows: shear bands of two orientations. (d) Phase map at the tip of the main crack (C1) at 77 K. Note the production of a large amount of ${\alpha }'$ (red area) in SBs. Also, note the formation of micro-voids, one is shown by the arrow. (e) IQ image with GB (upper panel) and phase image (lower panel) at the tip of secondary crack (C2). (f) Close-up view of the dashed box in (d), showing the formation of micro-void inside the first SB (black arrows), along with the production of plenty of second SBs (yellow arrows). Note ${\alpha }'$ in secondary SBs via phase transformation upon impacting especially at cryogenic temperatures.

Fig. 9. Microstructural evolution near the crack tip after impact testing at 298 K and 77 K in 316LN. (a) EBSD IQ (upper panel) and phase image (lower panel) map at the tip of main crack (C1) at 298 K. Note the prevalent shear bands (blue arrows), along with deformation twins (red arrows). Also, note there is no phase transformation inside SBs (arrows in lower panel). (b) EBSD IQ image overlapping with GB image (upper panel), along with phase image (insert) at the tip of secondary crack (C2). Note the high density of slender SBs and twins. (c) EBSD IQ image overlapping with GB image near the path of crack propagation at 77 K. Note the presence of SBs and twins of high density. (d) IQ image at the tip of secondary crack (C2). Note only few ${\alpha }'$ inside SBs as shown in phase maps of two insets.

Fig. 10. Microstructural evolution near the crack tip after fracture toughness testing at 298 K and 77 K in 316L. (a) EBSD IPF image showing the propagating path of crack (C) at 298 K. (b) Close-up view of the crack tip. Note the production of SBs and twins (red arrows) of high density, along with micro-voids (black arrow), as shown in IQ image (upper panel). Only a few ${\alpha }'$ is observed, see phase map (lower panel). (c) Close-up view of the white box in (b), clearly showing the SBs (blue arrows) and twins. (d) IPF map along the propagation path of crack (C) at 77 K. (e) Close-up IQ and phase map of the crack tip. The ${\alpha }'$ formation via phase transformation is evident, see phase image (lower panel), inside both SBs and grains. (f) ${\alpha }'$ formation along cracking path inside both SBs (yellow arrows) and grains.

Fig. 11. Microstructural evolution near the crack tip after fracture toughness testing at 298 K and 77 K in 316LN. (a) EBSD IPF image showing the propagating path of crack (C) at 298 K. (b) Close-up view of the dashed box in (a). Note the SBs of high density (upper panel). No ${\alpha }'$ is observed, see phase image in the lower panel. (c) Close-up view of the white box in (b), clearly showing the LAGBs of high density, along with twins. (d) IPF map along the propagation path of crack (C) at 77 K. See a large void nearby (black arrow). (e) Close-up IQ map (upper panel) and phase map (lower panel) of the crack tip. (f) Close-up view along crack paths. The ${\alpha }'$ formation via phase transformation is evident, see phase image inside both SBs (yellow arrows) and grains.

Fig. 12. Fractography after impact testing in 316L and 316LN. (a) Fracture surface of 316L at 298 K. Only initiation (I) and stable crack propagation (P) area are visible. (b) Close-up image in P area. (c) Fracture surface of 316L at 77 K. Besides I and P, large unstable crack propagation (U) and shear-lip (S) areas are also visible at 77 K. (d) Close-up image in P area. (e) and (f) Fracture surface and close-up image in P area in 316LN at 298 K, respectively. (g) and (h) Fracture surface and close-up image in P area in 316LN at 77 K, respectively.

Fig. 13. Fractography after fracture toughness testing in 316L and 316LN. (a) The transition zone from the pre-crack to a pronounced stretch-zone of 316L at 298 K. Insert is a comprehensive outline of the fracture surface. Pre-crack boundary and stretch-zone boundary are marked by the dashed line on the left shaded area of (a). (b) Fracture surface in crack growth region in 316L at 298 K. (c) Transition zone from the pre-crack to a pronounced stretch-zone of 316L at 77 K. (d) Fracture surface at crack growth region of 316L at 77 K. (e) and (f) Transition zone and fracture surface in crack growth region in 316LN at 298 K, respectively. (g) and (h) Transition zone and fracture surface in crack growth region in 316LN at 77 K, respectively.

Fig. 14. Strength-toughness balance. (a-1) and (a-2) AK and KC vs temperature (T), respectively. (b-1) and (b-2) (AK, UTS) balance and (KC, ${{\sigma }_{\text{y}}}$) balance. AK: Charpy impact energy. KC: fracture toughness. ${{\sigma }_{\text{y}}}$: yield strength. UTS: ultimate tensile strength. Dotted arrows show the trend with decreasing temperature from 298 K to 77 K.