Tailoring the microstructure of a Cu-0.7Cr-0.07Zr alloy submitted to ECAP at cryogenic temperature for improved thermal stability

Abstract

Severe Plastic Deformation (SPD) routes, such as Equal Channel Angular Pressing (ECAP), are capable of producing ultrafine-grained (UFG) materials with outstanding mechanical properties. Although grain refinement and the increase in mechanical processing could be enhanced through ECAP processing at cryogenic temperatures, this route presents the drawback of microstructural instability, which is responsible for reversing the gain in mechanical properties and microstructure refinement when the material is stored at room temperature or submitted to relatively high temperatures. Aiming to design severely deformed and stable microstructures, this study investigated pure copper (Cu) and a commercial ITER-grade Cu-0.7Cr-0.07Zr alloy submitted to ECAP at 298 and 123 K followed by annealing heat treatments. ECAP processing introduced a high density of dislocations and vacancy concentration in the materials assessed. Pure Cu showed poor microstructural stability when long-term stored at room temperature after ECAP processing at cryogenic temperatures, and when submitted to annealing post-ECAP processing. In contrast, the Cu-0.7Cr-0.07Zr alloy submitted to cryo-ECAP showed outstanding thermal stability even when submitted to annealing at 673 K for 120 min. For this alloy, the fine Cr-rich precipitates were responsible for preventing static recovery and preserving a grain size of ~6.6 μm and an area fraction of recrystallized grains of 6.2%. Finally, it was possible to pave the way for the design of refined and stable microstructures after cryo-SPD processing.