Effects of Grain Size and Crystallographic Orientation on Fatigue Cracking at Twin Boundaries in CrCoNi Medium-Entropy Alloy

doi:10.11900/0412.1961.2024.00211

Acta Metall Sin

2026, Vol. 62

Issue (7): 1246-1256 DOI: 10.11900/0412.1961.2024.00211

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Effects of Grain Size and Crystallographic Orientation on Fatigue Cracking at Twin Boundaries in CrCoNi Medium-Entropy Alloy

WANG Yujie¹, LI Linlin¹(

), ZHANG Zhenjun²^,³(

), ZHANG Zhefeng²^,³

1 State Key Laboratory of Digital Steel, Northeastern University, Shenyang 110819, China
2 Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
3 School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China

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WANG Yujie, LI Linlin, ZHANG Zhenjun, ZHANG Zhefeng. Effects of Grain Size and Crystallographic Orientation on Fatigue Cracking at Twin Boundaries in CrCoNi Medium-Entropy Alloy. Acta Metall Sin, 2026, 62(7): 1246-1256.

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Abstract

The CrCoNi medium-entropy alloy is known to demonstrate superior comprehensive mechanical properties relative to most fcc multiple-principal-elemental alloys. To ensure the long-term stability and safety of this alloy in practical applications, its fatigue characteristics and damage mechanisms must be thoroughly explored. Grain refinement is a crucial method for strengthening this alloy, and the effect of grain size on twin-boundary (TB) fatigue cracking must be clarified to reduce the intergranular fatigue cracks and improve fatigue performance. By conducting SEM measurements, the fatigue cracking behaviors at TBs in a CrCoNi medium-entropy alloy with two different grain sizes were systematically studied employing the slipping morphology method under low-cycle fatigue tests. The surface roughness around the TBs of the fatigued samples was characterized via laser confocal microscopy and white-light interference microscopy to quantify variations in the surface damage levels. Irrespective of grain size, the transition from slip-band cracking to TB cracking was observed to vary with the increasing difference in the Schmid factors (DSF) between the matrix and twin. Further, the propensity for TB cracking was facilitated by the escalation of DSF. Moreover, the magnitude of the required DSF for TB cracking was influenced by the grain size. The requirement of DSF for TB cracking decreased with increasing grain size. With the increase in grain size, even the minimal DSF is expected to result in the significant pilling up of dislocations near the TBs, thereby worsening the damage and rendering these boundaries favorable sites for fatigue cracking.

Key words: CrCoNi medium-entropy alloy fatigue cracking twin boundary grain size slip band

Received: 24 June 2024

ZTFLH:

TG144

Fund: National Natural Science Foundation of China(52371101);Liaoning Revitalization Talents Pro-gram(XLYC2203105)

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	WANG Yujie
	LI Linlin
	ZHANG Zhenjun
	ZHANG Zhefeng

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https://www.ams.org.cn/EN/10.11900/0412.1961.2024.00211 OR https://www.ams.org.cn/EN/Y2026/V62/I7/1246

Fig.1 Schematic of fatigue specimen (unit: mm)

Fig.2 Inverse pole figures (IPFs) and twin boundaries maps (a, b) and the corresponding grain size distributions (c, d) of the CrCoNi medium-entropy alloy (MEA) specimens annealed at 900 ^oC (a, c) and 1100 ^oC (b, d) for 1 h (The twin boundaries were shown by red lines in Figs.2a and b)

Table 1 Mechanical properties of CrCoNi MEA specimens with different grain sizes at room temperature

Fig.3 Grain boundary (GB) cracking and slip band (SB) cracking morphologies for CrCoNi MEA specimens fatigued under a total strain amplitude of 0.4% (a, d) GB cracking in fine-grained (FG) (a) and coarse-grained (CG) (d) specimens, respectively (b, c) SB cracking in FG specimen under low (b) and high (c) magnifications (e, f) SB cracking in CG specimen under low (e) and high (f) magnifications

Fig.4 Surface deformation features in FG (a) and CG (b) CrCoNi MEA specimens fatigued at a total strain amplitude of 0.4%; and height fluctuations of slip bands for FG (c) and CG (d) specimens along the arrows in Figs.4a and b, res-pectively

Fig.5 Low (a, c) and high (b, d) magnified post-fatigue surface morphologies for CrCoNi MEA specimens at a total strain amplitude of 0.4% (TB—twin boundary) (a, b) TB cracking in FG specimen (c, d) TB cracking in CG specimen

Fig.6 Surface deformation features in FG (a) and CG (b) CrCoNi MEA specimens fatigued at a total strain amplitude of 0.4%; and height (h) fluctuations of slip bands for FG (c) and CG (d) specimens along the arrows in Figs.6a and b, respectively

Fig.7 Bright-field TEM images of post-fatigued CoCrNi MEA specimens tested with a total strain amplitude of 0.4% (a, b) dislocation arrays composing planar slip bands in CG specimen (c, d) stacking faults (SFs) and dislocation arrays composing planar slip bands in FG specimen

Fig.8 Mechanism schematics of the slip morphology method for determining the crystallographic orientation
(a) crystallographic relationship of twinning in fcc metals (Ω_M—Schmid factor of slip systems in the matrix, Ω_T—Schmid factor of slip systems in the twin, b₁—Burgers vector of dislocations in the matrix, b₂—Burgers vector of dislocations in the twin)
(b) a typical surface morphology showing slip bands interacting with a twin boundary (RD—rolling direction)

Table 2 Statistical data of the TB cracking/SB cracking vs grain size and crystallographic orientation

Fig.9 Influences of crystallographic orientation and grain size on the fatigue cracking tendency along TBs in the CrCoNi MEA

Fig.10 Schematics of TB fatigue cracking under dislocation piling up in FG (a) and CG (b) specimens

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