Grain refinement is the most universal and effective method of strengthening metallic materials, which is known as the “Hall-Petch” relationship. However, when grain size is refined to sub-micro regime (Ultrafine Grain, UFG) or even nano regime (Nano Grain, NG), the plasticity of metallic materials becomes poor. Massive studies indicate that the low strain hardening ability resulted from the enhanced dynamic recovery and lack of dislocation accumulation in fine grains is the main reason for low ductility in UFG/NG metals. To resolve this “strength-ductility” conflict, different strategies have been taken, like bimodal/multimodal structure, nanotwins, gradient structure and intragranular nano dispersoids. Among them, the introduction of nano dispersoids into the fine grains attracted lots of attention due to its wide applicability and great success in simultaneously increasing the strength and ductility of the UFG/NG metal. In addition to the enhanced mechanical performance, the introduced second-phase particle may also bring some extraordinary functional properties into the metallic material. In this paper, a brief view of the strategies to improve ductility of the UFG/NG metals and the relevant toughening mechanisms are revealed. Special attentions are paid to the utilization of intragranular nano dispersoids in Aluminum alloys.
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