Near-fully dense nanocrystalline hafnium via pressureless two-step sintering

Grain refinement in hafnium (Hf), a typical refractory metal , encounters significant challenges due to its inherent high sintering temperature requirements and uneven sintered capillary force that lead to rapid grain coarsening and inhomogeneity. Herein, we present a novel approach of the pressureless two-step sintering to prepare near-fully dense nanocrystalline Hf with an average grain size of around 40 nm, which represents the smallest grain and highest density for nanocrystalline pure Hf to the date. Such achievements not only broaden the applicability of pressureless two-step sintering but also offer insights to produce large size of finer-grained refractory metals and their alloys. • Pressureless two-step sintering was used to prepare near-fully dense nanocrystalline Hf. • D e l e t e t h i s f o r m u l a The nanocrystalline Hf sample shows a hardness of 15.69 GPa, which is about 6.7 times that of coarse-grained Hf. • The FCC Hf originates from a stress-induced phase transformation, triggered by grain boundary migration.