Prolonged high-temperature exposure: Tailoring nanocrystalline Cu–Ta alloys against grain growth

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In this work, various alloy compositions of immiscible copper-tantalum (Cu–Ta) are systematically studied to understand the interplay between cluster stability, precipitation hardening, and the overall stability of the matrix's average grain size. An alloy composition of Cu-3at.%Ta is found to exhibit dramatic improvements relative to other neighboring Ta contents (both higher and lower) only after exposures of more than 300 h at 800 °C. An extremely low steady-state growth rate, i.e., 3.1 nm per 100 h exposure, speaks to the extreme kinetics which allow this composition to retain its high mechanical strength. The key attribute responsible for the NC Cu-3at.%Ta alloy exhibiting such behavior is a high cluster density tailored through the Ta solute content to achieve the best combination of stability without sacrificing strength by limiting Orowan coarsening of the clusters compared to other Ta concentrations. In general, the growth kinetics of this Cu-3at.%Ta alloy are so sluggish that it places it among the most thermally resistant alloys ever produced. The work demonstrates that, if designed properly, bulk nanocrystalline alloys can withstand the prolonged high-temperature exposure required for high-temperature applications, while grain growth is stagnated or halted.