Abstract: Dispersion-strengthened copper alloys can achieve ultra-high strength, but usually at the expense of ductility. In this study, a strategy for overcoming strength-ductility tradeoff of Cu alloys is realized through the introduction of bimodal grains structures. Cu-Ta alloys with only 0.5 at.% Ta content were successfully prepared by mechanical alloying combined with spark plasm sintering. The samples prepared by one-step and two-step ball milling methods are named as Cu-Ta (Ⅰ) and Cu-Ta (Ⅱ), respectively. The microstructural characterizations revealed that ultra-fine equiaxed grains with uniformly dispersed Ta precipitates were obtained in the Cu-Ta alloys. High strength of 377 MPa for yield strength together with elongation of ∼8% was obtained in Cu-Ta (Ⅰ). Bimodal grains structures composed of fine-grain zones and coarse-grain zones were successfully introduced into Cu-Ta (Ⅱ) by a two-step ball milling approach, and both yield strength (463 MPa) and elongation (∼15%) were significantly synergistic enhanced. The hardness values of both Cu-Ta (Ⅰ) and Cu-Ta (Ⅱ) were almost kept nearly constant with the increase of annealing time, and the softening temperatures of Cu-Ta (Ⅰ) and Cu-Ta (Ⅱ) are 1018 and 1013 °C, reaching 93.9% and 93.5% T_m of pure Cu (1083 °C), respectively. It reveals that the Cu-0.5 at.% Ta alloys exhibit excellent thermal stability and exceptional softening resistance. Ta nanoclusters with semi-coherent structures play an essential role in enhancing the strength and microstructural stability of alloys. Bimodal structures are beneficial to the activation of back stress strengthening and the initiation and propagation of microcracks, thus obtaining the extraordinary combination of strength and elongation. This study provides a new way to fabricate dispersion-strengthened Cu alloys with high strength, high elongation, excellent thermal stability and softening resistance, which have potential application value in the field of the future fusion reactor.

Key words: Cu-Ta alloy, Mechanical alloying, Ta precipitates, Bimodal structures, Mechanical property