a nanocrystalline metal
An ordinary piece of metal is not one crystal but a mosaic of many small crystal grains, each a patch of orderly atoms, meeting its neighbours along mismatched seams called grain boundaries — like a tiled floor whose tiles are laid at random angles, with grout lines between them. In a normal metal the grains are large (microns to millimetres), so the grout — the grain-boundary region — is a tiny fraction of the whole. A nanocrystalline metal is the same idea shrunk hard: the grains are only nanometres across (below about 100 nm), so the grout lines are everywhere.
That changes the accounting completely. The grain-boundary region is a shell perhaps 0.5 to 1 nm thick around every grain; when grains are microns wide it holds a negligible share of the atoms, but when grains are only 10 nm across a substantial fraction of all atoms — often tens of a percent — live in or right beside a boundary, in disordered, loosely-packed surroundings rather than a neat lattice. A nanocrystalline metal is thus a material where the boundaries stop being thin seams and become a major structural component in their own right.
The classic reason to want tiny grains is strength. Grain boundaries block the gliding dislocations that let metals deform, so finer grains mean a stronger metal, captured by the Hall-Petch relation: the yield strength rises as the grain size d falls, in proportion to 1 over the square root of d. But here honesty matters: this strengthening does not continue forever. Below roughly 10 to 20 nm the trend flattens and can even reverse (an inverse Hall-Petch effect), because so much of the material is now grain boundary that deformation switches to boundary sliding. The Hall-Petch law is a rule with a floor, not a promise without limit.
Take copper with ordinary 50-micron grains and make a version with 20 nm grains, and the nanocrystalline copper can be several times stronger, just as Hall-Petch predicts. But push the grains down toward 5 nm and it stops getting stronger and may soften — so much of the metal is now grain boundary that the grains slide past one another instead of the interior deforming.
Hall-Petch: strength rises as 1/sqrt(grain size) — until, near 10 nm, the trend flattens or reverses.
A nanocrystalline metal is still crystalline — do not confuse it with a metallic glass, which has no grains at all. Its defining feature is the huge grain-boundary fraction, and the honest limit is that Hall-Petch strengthening saturates and can reverse at the smallest grain sizes.