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

Grain boundaries significantly influence the nucleation and growth of precipitates in supersaturated solid so
lutions. 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-48 Cr (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.