Pt Schwarz crystals stabilized by minimal-surface grain boundaries and twins at the grain size limit

Schwarz crystals with uniformly distributed twins enclosed by general grain boundaries (GBs) resembling triply
minimal-surfaces were observed in high stacking fault energy metal Pt which is believed not readily to twin. By
refining the grains of pure Pt from 140
μ
m to several nanometers via plastic deformation technique, deformation
twins were found to emerge numerously and the structural morphologies of Schwarz crystals can be identified at
grain sizes below 4 nm. Both experiments and atomistic simulations suggest that Pt Schwarz crystals are ther
mally stable at temperatures approaching the thermal equilibrium melting point. The measured room temper
ature microhardness as a function of grain size, on the other hand, may increase monotonically up to 12.9 GPa,
revealing a sustained Hall-Petch relationship. The observed outstanding mechanical stabilities can be attributed
to that, regardless of the critical twin size and the stacking fault energy of the material, the plastic deformation at
the grain size limit is solely limited by twinning and both the nucleation and bowing-out of twinning partial
dislocations from GB sources are notoriously difficult according to simulations and theory.