J. Mater. Sci. Technol. ›› 2025, Vol. 207: 160-176.DOI: 10.1016/j.jmst.2024.04.033

• Research article • Previous Articles     Next Articles

Unveiling the roles of initial phase constituents and phase metastability in hydrogen embrittlement of TRIP‐assisted VCrCoFeNi medium‐entropy alloys

Sang Yoon Songa, Dae Cheol Yanga, Han-Jin Kimb, Sang-In Leec, Hyeon-Seok Dod, Byeong-Joo Leed, Alireza Zargarane, Seok Su Sohna,*   

  1. aDepartment of Materials Science and Engineering Korea University, 02841 Seoul, Republic of Korea;
    bGreen Energy Materials Research Team Hyundai Motor Company Uiwang R&D Center, 16082 Uiwang, Republic of Korea;
    cInstitue of Advanced Technology Development Hyundai Motor Group, 13529 Seongnam, Republic of Korea;
    dDepartment of Materials Science and Engineering Pohang University of Science and Technology, 37673 Pohang, Republic of Korea;
    eGraduate Institute of Ferrous & Energy Materials Technology, Pohang University of Science and Technology, 37673 Pohang, Republic of Korea
  • Received:2024-02-19 Revised:2024-04-04 Accepted:2024-04-18 Published:2025-02-01 Online:2024-05-09
  • Contact: *E-mail address: sssohn@korea.ac.kr (S.S. Sohn)

Abstract: Medium-entropy alloys (MEAs) that exhibit transformation-induced plasticity (TRIP) from face-centered cubic (FCC) to body-centered cubic (BCC) are considered promising for liquid hydrogen environments due to their remarkable cryogenic strength. Nonetheless, studies on hydrogen embrittlement (HE) in BCC-TRIP MEAs have not been conducted, although the TRIP effect and consequent BCC martensite usually deteriorate HE susceptibility. In these alloys, initial as-quenched martensite alters hydrogen diffusion and trap behavior, and deformation-induced martensitic transformation (DIMT) provides preferred crack propagation sites, which critically affects HE susceptibility. Therefore, this study aims to investigate the HE behavior of BCC-TRIP MEAs by designing four V10Cr10Co30Fe50-xNix (x = 0, 1, 2, and 3 at%) MEAs, adjusting both the initial phase constituent and phase metastability. A decreased Ni content leads to a reduced fraction and mechanical stability of FCC, which in turn increases HE susceptibility, as determined through electrochemical hydrogen pre-charging and slow-strain rate tests The permeation test and thermal desorption analysis reveal that the hydrogen diffusivity and content are affected by initial BCC fraction, interconnectivity of BCC, and refined FCC. As these initial phase constituents differ between the alloys with FCC- and BCC- dominant initial phase, microstructural factors affecting HE are unveiled discretely among these alloy groups by correlation of hydrogen-induced crack behavior with hydrogen diffusion and trap behavior. In alloys with an FCC-dominant initial phase, the initial BCC fraction and DIMT initiation rate emerge as critical factors, rather than the extent of DIMT. For BCC-dominant alloys, the primary contributor is an increase in the initial BCC fraction, rather than the extent or rate of DIMT. The unraveled roles of microstructural factors provide insights into designing HE-resistant BCC-TRIP MEAs.

Key words: Medium-entropy alloys (MEAs), Transformation-induced plasticity (TRIP), Hydrogen embrittlement, Hydrogen-induced crack, Hydrogen diffusion and trapping