Comprehensive analysis of ordering in CoCrNi and CrNi2 alloys

Abstract

Chemical Short-Range Order (CSRO) has attracted recent attention from many researchers, creating intense debates about its impact on material properties. The challenges lie in confirming and quantifying CSRO, as its detection proves exceptionally demanding, contributing to conflicting data in the literature regarding its true effects on mechanical properties. Our work uses high-precision calorimetric data to unambiguously prove the existence and, coupled with atomistic simulations, quantify the type of CSRO. This methodology allows us to propose a mechanism for its formation and destruction based on the heat evolution during thermal analysis and facilitates a precise identification of local ordering in CoCrNi alloys. Samples of CoCrNi (Co₃₃Cr₃₃Ni₃₃) and CrNi₂ (Cr₃₃Ni₆₆) alloys are fabricated in varying ordered states, extensively characterized via synchrotron X-ray diffraction, X-ray absorption spectroscopy, and transmission electron microscopy. Samples with considerably different ordered states are submitted to tensile tests with in-situ synchrotron X-ray diffraction. We demonstrate, despite inducing varied CSRO levels in CoCrNi, no significant alterations in overall mechanical behavior emerge. However, the CrNi₂ alloy, which undergoes long-range ordering, experiences significant shifts in yield strength, ultimate tensile stress and ductility.

Fig. 1. Microstructural characterization by SXRD and TEM in both alloys under different heat treatment conditions.

SXRD of a CrNi₂ and b CoCrNi under different thermal treatment conditions. Additional SXRD peaks are detected in CrNi₂ after aging for 170 and 240 h (see the inset in a, which is plotted on a log scale), indicating the presence of the long-range ordered oI6 phase. Only disordered FCC peaks are detected in CoCrNi. The triangles mark the 2nd harmonic of the main FCC peaks; they appear due to a secondary, low-intensity radiation with twice the energy of the main radiation. TEM c BF and d DF images (from circled reflections in e and f SAED patterns from the CrNi₂ alloy aged for 240 h. Additional spots from the long-range ordered oI6 phase (red circles) are seen amidst the primary FCC spots. g BF-TEM image of a grain boundary of the CoCrNi sample showing the absence of coarser particles. h and i SAED of the CoCrNi alloy after aging for 240 h. No additional spots were detected beyond the disordered FCC phase. To mitigate dynamic scattering effects, all patterns were acquired using precession angles of 0.5°.

Fig. 2. Specific heat variations with temperature for CrNi₂ and CoCrNi after different heat treatments.

For CrNi₂ a recrystallized and followed by aging at 748 K for b 1 h, c 10 h, d 100 h, e 170 h, and f 240 h. For CoCrNi g recrystallized and followed by aging at 748 K for h 1 h, i 10 h, j 100 h, k 170 h, and l 240 h. The solid lines are for the first heating cycle and the dashed lines correspond to the second heating. The exothermic (EXO) direction is marked by an arrow.

Fig. 3. Schematic of the heat evolved during DSC testing (heating cycles).

Below around 748 K, the kinetics are too slow to change the CSRO in the experimental timescale. Above 748 K, CSRO can be either created (purple shade) or destroyed (red shade), as indicated by exothermic and endothermic reactions, respectively.

Fig. 4. Monte Carlo atomistic simulations employing a machine learning potential developed to capture CSRO in CrCoNi.

a Direct comparison of ΔH_CSRO between simulations and experiments. The $\Delta H_{\mathrm{CSRO}}$ can be understood as $H_{\mathrm{CSRO}}^T-H_{\mathrm{CSRO}}^{748\mathrm{K}}$, or in other words, as the difference in enthalpy between the sample at a temperature T and the equilibrium state at 748 K. b Enthalpic contribution stemming from CSRO as a function of temperature (blue) and Cr–Cr Warren–Cowley parameter ${\alpha }_{\mathrm{Cr}-\mathrm{Cr}}^{m=1}$ (green) for first neighbors. At temperatures above 748 K, the figure shows the enthalpy is higher (or more positive), meaning that there is heat absorption by the system to destroy CSRO. Good agreement between the experimental and simulated values is seen only above 873 K because only at this temperature do the kinetics of the system allow the system to reach equilibrium within the experimental timescale. Below this temperature, the experimentally measured ΔH_CSRO is expected to be lower than simulations because experiments do not achieve the equilibrium CSRO due to slow kinetics, while simulations always predict equilibrium values. A 4.3% error was considered for the experimental data as explained in the “Methods” section, also further details on the calculation of the experimental curve are given in Supplementary Information Fig. 6.

Fig. 5. Analysis of the mechanical properties of CrNi₂ and CoCrNi alloys under different heat treatment conditions.

a Hardness of CrNi₂ and CoCrNi under different heat treatment conditions. As the aging time increases in the binary alloy, there is an increase in hardness. In the ternary alloy, the hardness remains unchanged for all heat treatment conditions. The scale bar was created using the standard deviation. b Tensile tests of CrNi₂ and CoCrNi recrystallized and aged for 240 h. In the binary alloy, an increase in ultimate tensile strength (${\sigma }_{\mathrm{UTS}}$) and yield strength (${\sigma }_{0.2}$) is observed after aging for 240 h. The measured grain size for each sample is shown. However, regarding strain to failure (ε_f), no significant changes are observed. As for the ternary alloy, the values of ${\sigma }_{\mathrm{UTS}}$, ${\sigma }_{0.2}$, and ε_f remains consistent in both recrystallized and aged conditions for 240 h. c and d Mechanical properties of the CrNi₂ (c) and CoCrNi (d) for different aging times. The scale bar was created using the standard deviation for both figures (c) and (d). e Results of dislocation density analysis and tensile curves for CoCrNi alloy, using the modified Williamson–Hall method for 8 FCC Peaks (111, 200, 220, 311, 222, 400, 331, 420) in the transverse direction integration (Azimuthal range 0 ± 20°) after subtracting the instrumental broadening. The dislocation density (ρ) is determined by the slope, as a function of strain. As shown in the figure, both samples have similar capacities to accommodate dislocations at different strain levels. All error bars correspond to one standard deviation of the measured quantity.

Fig. 6. Pseudo-binary phase diagram between CrNi₂ and CoCrNi endpoints, charting the dissolution temperature of the ordered M₂Cr phase as a function of Co content, where M is Co or Ni.

The diagram, calculated to match experimental evidence, indicates a dissolution temperature for the equiatomic alloy that is below 673 K.