Title: Effects of cryogenic temperature and grain size on fatigue-crack propagation in the medium-entropy CrCoNi alloy

Abstract

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 ~10^–11 to >10^–7 m/cycle, at room (293 K) and cryogenic (198 K, 77 K) temperatures for two grain sizes (~7 and 68 µm), with emphasis on near-threshold behavior. We find that the ΔK_th fatigue thresholds are increased with decreasing temperature and increasing grain size: from 5.7 MPa√m at 293 K to 8 MPa√m at 77 K in the fine-grained alloy, and from 9.4 MPa√m at 293 K to 13.7 MPa√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,more » particularly in the coarser-grained microstructure. Here, 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.« less

Authors:

Rackwitz, Julian  ^[1];

• Search DOE PAGES for author "Rackwitz, Julian"
• Search DOE PAGES for ORCID "0000-0002-1768-9284"

Yu, Qin  ^[2];

• Search DOE PAGES for author "Yu, Qin"
• Search DOE PAGES for ORCID "0000-0002-5099-0327"

Yang, Yang ^[2]; Laplanche, Guillaume  ^[3];

• Search DOE PAGES for author "Laplanche, Guillaume"
• Search DOE PAGES for ORCID "0000-0001-9559-0928"

George, Easo P.  ^[4];

• Search DOE PAGES for author "George, Easo P."
• Search DOE PAGES for ORCID "0000-0001-9898-9694"

Minor, Andrew M. ^[5]; Ritchie, Robert O.  ^[5]

• Search DOE PAGES for author "Ritchie, Robert O."
• Search DOE PAGES for ORCID "0000-0002-0501-6998"

---

+ Show Author Affiliations

1. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States); Tech. Univ. of Berlin (Germany)
2. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
3. Ruhr Univ., Bochum (Germany)
4. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States); Univ. of Tennessee, Knoxville, TN (United States)
5. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States); Univ. of California, Berkeley, CA (United States)

Publication Date:

Fri Sep 11 00:00:00 EDT 2020

Research Org.:

Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES). Materials Sciences & Engineering Division; German Research Foundation (DFG); USDOE

OSTI Identifier:

1860598

Alternate Identifier(s):

OSTI ID: 1811094; OSTI ID: 1839249

Grant/Contract Number:  

AC05-00OR22725; AC02-05CH11231

Resource Type:

Accepted Manuscript

Journal Name:

Acta Materialia

Additional Journal Information:

Journal Volume: 200; Journal Issue: N.A.; Journal ID: ISSN 1359-6454

Publisher:

Elsevier

Country of Publication:

United States

Language:

English

Subject:

36 MATERIALS SCIENCE; medium-entropy alloy; fatigue-crack propagation; crack closure; cryogenic temperatures; grain size effects