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

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 10,000 h 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 500 h 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 3000 h 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. 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.