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 degrees 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 degrees 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 degrees 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 degrees 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.