Hardness-thermal stability synergy in nanograined Ni and Ni alloys: Superposition of nanotwin and low-energy columnar boundary

Refining grains into nanoscale can significantly strengthen and harden metallic materials; however, nanograined metals generally exhibit low thermal stability, hindering their practical applications. In this work, we exploit the superposition of the contribution of nanotwins, low-angle grain boundaries, and microalloying to tailor superior combinations of high hardness and good thermal stability in Ni and Ni alloys. For the nanotwinned Ni having a twin thickness of ∼2.9 nm and grain size of 28 nm, it exhibits a hardness over 8.0 GPa and an onset coarsening temperature of 623 K, both of which are well above those of nanograined Ni. Re/Mo microalloying can further improve the onset coarsening temperature to 773 K without comprising hardness. Our analyses reveal that high hardness is achieved via strengthening offered by extremely fine nanotwins. Meanwhile, the superior thermal stability is mainly ascribed to the low driving force for grain growth induced by the low-angle columnar boundary architecture and to the additional pinning effect on the migration of twin/columnar boundaries provided by minor Re/Mo solutes. The present work not only reveals a family of nanotwinned metals possessing the combination of ultra-high hardness and high thermal stability but also provides a strategy for tailoring properties of metallic materials by pairing low-angle grain boundaries and twin boundaries.