Ultrahard and super-stable pure aluminum with Schwarz crystal structure

Refining grains to sub-10 nm size is extremely difficult in pure aluminum due to its high stacking fault energy and low melting point. Herein, we experimentally demonstrate that Schwarz crystal (SC) structure with minimal interfaces constrained by coherent twin boundaries was induced in extremely fine grains (6 nm) of pure aluminum (99.995%) under extremely high shear strain via cryogenic high-pressure torsion. The SC structure exhibits an unprecedented hardness of 2.51 GPa and keeps stable up to 928 K (about 0.99 Tm).