The formation and evolution of Ni₂Cr precipitates in Ni–Cr model alloys as a function of stoichiometry characterized by synchrotron x-ray diffraction

Journal Article · 09 September 2022 · Materials Science and Engineering. A, Structural Materials: Properties, Microstructure and Processing

DOI: https://doi.org/10.1016/j.msea.2022.143930 · OSTI ID:2418294

Aerne, N.  ^[1]; Sprouster, D. J. ^[2]; Tucker, J. D.  ^[3]

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1. Oregon State Univ., Corvallis, OR (United States); OSTI
2. Stony Brook Univ., NY (United States)
3. Oregon State Univ., Corvallis, OR (United States)

Ni-based alloys containing significant amounts of Cr, may precipitate new phases during long-term service at elevated temperatures. In Ni–Cr model alloys, the formation of Ni₂Cr has been found to impact the material properties, including the strength and ductility after isothermal aging below the critical temperature of 590°C. In this work, we quantify the formation and evolution of long-range ordering in four Ni–Cr binary model alloys (Ni/Cr = 1.8, 2.0, 2.2, 2.4) after isothermal aging up to 10,000h at temperatures between 373 and 475°C. The alloys were characterized by hardness testing and synchrotron-based x-ray diffraction to quantify the impact of Ni₂Cr phase fraction and precipitate size on mechanical properties. After 500h of isothermal aging, the formation of Ni₂Cr was detected in all four alloys at 475°C. In the stochiometric alloy samples (Ni/Cr = 2.0), the formation of Ni₂Cr was found after 500 and 3000h aging at 418 and 373°C, respectively. We found that the matrix lattice contraction and Ni₂Cr phase fraction both saturate after early aging times. This is in stark contrast to the hardness and Ni₂Cr precipitate size that both continue to increase with increasing aging time. Our results highlight that changes in hardness correlate linearly with Ni₂Cr precipitate size rather than phase fraction. This important structure-property relationship can potentially help define Ni–Cr-based component lifetimes directly through an understanding of how Ni₂Cr formation impacts strength and ductility. Here, we find that a precipitation hardening model for critical resolved shear stress with weakly coupled dislocations shows good agreement with the material property changes quantified from experimental measurements.

• Research Organization: Brookhaven National Laboratory (BNL), Upton, NY (United States). National Synchrotron Light Source II (NSLS-II); Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States); Oregon State Univ., Corvallis, OR (United States)
• Sponsoring Organization: National Science Foundation (NSF); USDOE Office of Nuclear Energy (NE)
• Grant/Contract Number: AC07-05ID14517; NE0008423; SC0012704
• OSTI ID: 2418294
• Journal Information: Materials Science and Engineering. A, Structural Materials: Properties, Microstructure and Processing, Journal Name: Materials Science and Engineering. A, Structural Materials: Properties, Microstructure and Processing Journal Issue: C Vol. 856; ISSN 0921-5093
• Publisher: ElsevierCopyright Statement
• Country of Publication: United States
• Language: English