Journal
ACTA MATERIALIA
Volume 200, Issue -, Pages 351-365Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.actamat.2020.09.021
Keywords
Medium-entropy alloy; Fatigue-crack propagation; Crack closure; Cryogenic temperatures; Grain size effects
Funding
- U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division [DE-ACO2-05CH11231, KC13]
- U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division at the Oak Ridge National Laboratory
- Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy [DE-ACO2-05-CH11231]
- German Research Foundation (DFG) [LA 3607/1-1]
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CrCoNi-based high-entropy alloys have demonstrated outstanding mechanical properties, particularly at cryogenic temperatures. Here we investigate the fatigue-crack propagation properties of the equiatomic, single-phase, face -centered cubic, medium-entropy alloy (MEA), CrCoNi, that displays exceptional strength, ductility and toughness, all of which are enhanced at cryogenic temperatures. Fatigue-crack growth is examined, at a load ratio of 0.1 over a wide range of growth rates, from similar to 10(-11) to >10(-7) m/cycle, at room (293 K) and cryogenic (198 K, 77 K) temperatures for two grain sizes (similar to 7 and 68 mu m), with emphasis on near-threshold behavior. We find that the Delta K-th fatigue thresholds are increased with decreasing temperature and increasing grain size: from 5.7 MPa root m at 293 K to 8 MPa root m at 77 K in the fine-grained alloy, and from 9.4 MPa root m at 293 K to 13.7 MPa root m at 77 K in the coarse grained alloy. Mechanistically, transgranular cracking at 293 K transitions to a mixture of intergranular and transgranular at cryogenic temperatures, where the increased propensity of nano-twins appears to inhibit growth rates by deflecting the crack path. However, the main factor affecting near-threshold behavior is roughness-induced crack closure from interference between the crack flanks, which is enhanced by the rougher fracture surfaces at low temperatures, particularly in the coarser-grained microstructure. Fatigue-crack propagation behavior in CrCoNi is comparable to nickel-based superalloys but is superior to that of the high-entropy CrMnFeCoNi (Cantor) alloy and many high-strength steels, making the CrCoNi alloy an excellent candidate material for safety-critical applications, particularly involving low temperatures. Published by Elsevier Ltd on behalf of Acta Materialia Inc.
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