Phase stability of γ'-Ni₂ Cr and α-Cr in the Ni-Cr binary

Journal Article · Sat Jul 07 00:00:00 EDT 2018 · Acta Materialia

DOI:https://doi.org/10.1016/j.actamat.2018.07.015· OSTI ID:1460656

Miller, Cody ^[1]; Field, Robert ^[2]; Kaufman, Michael ^[2]

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1. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
2. Colorado School of Mines, Golden, CO (United States)

For this study, the stability of the γ'-Ni₂Cr long-range order (LRO) phase present on the Ni-Cr binary phase diagram was examined in a Ni-55Cr (wt. %) alloy. It is shown that the γ'-Ni₂Cr phase is metastable, and that the two-phase γ-Ni + α-Cr phase field extends to room temperature. The as-cast alloy consisted of a typical dendritic microstructure, with Cr-rich primary α-Cr dendrites and a Cr-lean, γ-Ni interdendritic. Annealing the as-cast alloy at 500°C for 1000 h (γ'-Ni₂Cr + α-Cr phase field) resulted in pockets of α-Cr and γ-Ni + γ'-Ni₂Cr at the dendritic/interdendritic interface, with little change to the interdendritic γ-Ni microstructure. Annealing at 900°C for 4 h (γ-Ni + α-Cr phase field) resulted in the interdendritic precipitation of α-Cr, both as a continuous and discontinuous precipitate. Annealing at 900°C for 4 h followed by 500°C for 1000 h resulted in the interdendritic precipitation of α-Cr at 900°C, but no precipitation of γ'-Ni₂Cr during subsequent annealing at 500°C. This behavior suggests that the γ'-Ni₂Cr phase is not the equilibrium phase, but rather a metastable transition phase. Furthermore, it is speculated that this phase in more complex alloys (i.e. Alloy 690) is also metastable, and its metastability should be considered in applications involving long-term, high temperature exposures.

• Research Organization: Los Alamos National Laboratory (LANL); Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA); USDOE Office of Nuclear Energy (NE), International Nuclear Energy Policy and Cooperation (NE-6). International Nuclear Energy Research Initiative (INERI)
• Grant/Contract Number: AC52-06NA25396
• OSTI ID: 1460656
• Alternate ID(s): OSTI ID: 1702948; OSTI ID: 22744774; OSTI ID: 1473820
• Report Number(s): LA-UR--18-22732; LA-UR--18-26540; LA-UR-18-22732
• Journal Information: Acta Materialia, Journal Name: Acta Materialia Journal Issue: C Vol. 157; ISSN 1359-6454
• Publisher: Elsevier
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
Alloy 690
Long-range order
Metastable
Ni-CrNi2Cr