Atomic-scale investigation of Mo distribution and its effect on phase stability of a nanocrystalline CrCoNi-based medium-entropy alloy

"Suzuki segregation" of Mo to planar defects was proposed to contribute to secondary hardening after post-deformation annealing in the MP35N alloy, a CrCoNi-based single-phase superalloy. However, a direct proof of this proposal is challenging and reported observations are controversial. Here, we prepared nanocrystalline MP35N thin films by co-deposition on a microtip array of pre-sharpened Si tips. This enabled a direct investigation of the Mo distribution in a nanoscale structure with many grain boundaries (GBs) after annealing by atom probe tomography. We observed that no Mo but Cr segregates to GBs between 360 and 460 °C. Additionally, we found that MP35N is thermally more stable than a (Mo-free) Cr-Co-Ni medium-entropy alloy under the same conditions. Factors that influence the kinetics of phase decomposition and thermal stability, including grain size, Mo addition, and the loss of Cr due to oxidation at GBs were addressed.