Slow precipitate coarsening kinetics in extremely fine nanograined Ni-Cr alloys

Grain boundaries significantly influence the nucleation and growth of precipitates in supersaturated solid solutions. However, due to the inherent instability of grain boundaries in nanograined metals, the precipitation process usually occurs concurrently with grain growth during thermal annealing, making it difficult to reveal the grain size effect on the precipitate coarsening behavior. In this study, nanograined Ni-48Cr (at.%) supersaturated alloys with grain sizes ranging from about 85 nm to 10 nm were fabricated by plastic deformation, in which the relaxed grain boundaries led to the extremely fine nanograined structure having good thermal stability. During the high-temperature annealing process, nanoscale Cr particles were precipitated from the nanograined matrix, forming a nanograined dual-phase structure. The growth kinetics of the Cr precipitates in the extremely fine nanograined structures are different from the larger grain size samples, showing a trend that the smaller the as-deformed grain size, the slower the precipitate coarsening rate. Detailed microstructural characterization and kinetic analysis indicate that the extremely fine nanograined structure may change the rate-limiting process for precipitate coarsening from lattice diffusion to grain boundary diffusion as grain size is refined to about a few tens nanometers, which leads to a slow coarsening rate. The nanograined dual-phase Ni-Cr alloy of the annealed 10 nm sample shows the highest strength among these samples as well as excellent thermal stability, which provides guidance for the design of high-performance alloys.