Superior low-cycle fatigue resistance of CoCrNi moderate compositional complex alloys: An energy-based fair assessment

doi:10.1016/j.metadv.2026.06.005

Abstract

Abstract:

Moderate compositional complex alloys (MCCAs), as a potential new type of metallic structural material, have not undergone detailed investigation regarding their low-cycle fatigue (LCF) damage behaviors and cyclic deformation mechanisms over a wide range of strain amplitudes. In this study, the tensile and cyclic loading deformation behavior of a series of non-equiatomic CoCrNi MCCAs with varying stacking fault energy (SFE) were systematically investigated. It was found that the tensile properties exhibited a simultaneous strengthening and toughening trend. Under cyclic loading conditions, lowering the SFE also favored improved LCF properties. The damage mode gradually shifts from grain boundary (GB) cracking to slip band (SB) cracking as the strain amplitude increases. The microstructure evolved from simple dislocation structures to complex patterns such as dislocation walls, accompanied by the emergence of twins and stacking faults. Additionally, to objectively evaluate the LCF performance, a fatigue life prediction model, 𝑊_s=𝑊₀·𝑁_f^−1/𝛽, based on hysteresis energy was employed. This model revealed that the LCF performance of the MCCAs is controlled by their ability to accommodate defects (reflected by the intrinsic fatigue toughness 𝑊₀) and the rate of converting mechanical work into fatigue damage (reflected by the damage transition exponent β). We demonstrated that the current non-equiatomic MCCAs exhibit one of the highest LCF properties among various common metallic materials available. Finally, based on the hysteresis energy model, metallic materials with higher static toughness and lower cyclic hardening exponents are considered to exhibit better LCF resistance.

Key words: Moderate compositional complex alloys, Low-cycle fatigue, Hysteresis energy, Fatigue life, Twinning, Planar slip

Z.Q. Wang, R. Liu, X.G. Wang, Z.J. Zhang, Z.F. Zhang. Superior low-cycle fatigue resistance of CoCrNi moderate compositional complex alloys: An energy-based fair assessment[J]. Metals Advances, 2026, 48: 20-33.

Figures/Tables 16

Fig. 1. X-ray diffraction (XRD) patterns of Ni32, Ni40, Ni48 MCCAs after annealing treatment.

Fig. 2. Fully recrystallized microstructures and twin boundaries of the three MCCAs after annealing: (a-c) Ni32, (d-f) Ni40, and (g-i) Ni48. (a, d, g) EBSD inverse pole figure maps; (b, e, h) band contrast images showing high-density twin boundaries (red lines) and high-angle grain boundaries (black lines); (c, f, i) corresponding quantitative statistical results, indicating similar grain sizes and twin boundary fractions across the three alloys.

Fig. 3. Mechanical properties of three MCCAs. (a) Tensile engineering stress-strain curves. (b) Dependence of yield strength and tensile strength on uniform elongation showing synchronous increase in ductility with the decrease of Ni content. (c) Corresponding true stress versus true strain and work-hardening rate curves.

Fig. 4. Cyclic stress response (CSR) curves of three MCCAs at different strain amplitudes: (a) Ni32, (b) Ni40, (c) Ni48, respectively.

Fig. 5. Comparison of cyclic stress response (CSR) curves of different components at the same strain amplitude: (a) 1%, (b) 0.75%, (c) 0.5%, (d) 0.25%, respectively.

Fig. 6. (a) Comparison of LCF properties of three MCCAs. (b) Cyclic stress-strain hysteresis loops at max stress cycles. (c) Cyclic hardening and (d) cyclic softening abilities of three MCCAs.

Fig. 7. TEM images displaying the deformation mechanism of three MCCAs under uniaxial loading, the transformation from single-level twin to multi-level twin. (a, b) Ni32. (d, e) Ni40. (g, h) Ni48. (c, f, i) Quantitative statistics of twin width and distance.

Fig. 8. SEM images of fracture (crack propagation region) features of three MCCAs after cyclic loading. (a, d, g) Ni32, Δε/2=0.25%, 𝑁f=222081. (b, e, h) Ni40, Δε/2=0.75%, 𝑁f=4239. (c, f, i) Ni48, Δε/2=1.0%, 𝑁f=1757.

Fig. 9. Surface deformation morphologies and fatigue cracking behaviors of three MCCAs after cyclic loading. (a) Ni32, Δε/2=1.0%, (b) Ni48, Δε/2=1.0%, (c) Ni40, Δε/2=0.75%, (d) Ni40, Δε/2=0.5%, (e) Ni32, Δε/2=0.25%, (f) Ni48, Δε/2=0.25%, the quantitative statistics about (g) the slip band density, (h) the crack density and (i) the ratio of GB/SB crack at various strain amplitudes.

Fig. 10. Bright field TEM images of deformation microstructure after cyclic loading of three MCCAs showing the transformation from twinning to plane slip and cross-slip. High-density twins at strain amplitude of 1.0% in (a) Ni32 and (b) Ni48. (c, d) Mixed deformation mechanism with twinning and planar slip at strain amplitude of 0.75% in Ni32. (e, f) Cross-slip and planar slip in Ni40. High-density dislocations in (111) plane at strain amplitudes of 0.5% and 0.25% in (g) Ni32, (h) Ni40 and (i) Ni48.

Fig. 11. TEM images show the evolution of deformation microstructure of three MCCAs from twinning to multiple slip to planar slip at different strain amplitudes.

Table 1. Fatigue lives of the three MCCAs at different strain amplitudes.

Total strain amplitude Δε/2 (%)	Fatigue life, 𝑁_f
Total strain amplitude Δε/2 (%)	Ni32	Ni40	Ni48
1%	4018	2426	1757
0.75%	6799	4240	3684
0.5%	17137	14778	11358
0.25%	222079	124888	127952

Fig. 12. Fatigue life prediction models and corresponding parameters: (a) the hysteresis energy model, (b) the Basquin model, (c) the Coffin-Manson model.

Table 2. Comparison of different model parameters.

Empty Cell	$ \sigma_{\mathrm{f}}^{\prime} $	b	$ \varepsilon_{\mathrm{f}}^{\prime} $	c	𝑊₀	β
Ni32	828	−0.0658	1.4027	−0.5138	1567	1.64
Ni40	925	−0.0768	1.2736	−0.5476	2129	1.5
Ni48	908	−0.0745	2.1301	−0.5045	1292	1.61

Fig. 13. Comparison of damage transition exponent β versus intrinsic fatigue toughness 𝑊0 with other common metallic materials in the literature available. The current MCCAs show the highest fatigue life in comparison with the other common metallic materials.

Fig. 14. Illustrations for improving LCF properties: (a) improving intrinsic fatigue toughness 𝑊0, (b) improving damage transition exponent β.

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