Because of the nature of interatomic bonding, atomic diffusivity is notably higher in metals relative to that in ceramics and compounds with covalent or ionic bonds. This feature enables substantial tuneability of structures at various length scales by tailoring the diffusion controlled processes during the synthesis and subsequent treatments, resulting in a broad spectrum of properties and performance in metallic materials, but this causes their customized properties to be unstable at elevated temperatures. Eliminating diffusive interfaces by fabricating single crystals or heavily alloying helps to address this issue but does not inhibit atomic diffusion at high homologous temperatures. Such instability becomes a major bottleneck in the development of metallic materials, greatly limiting their technological applications at high temperatures.
Recently, we discovered that the Schwarz crystal structure was effective at suppressing atomic diffusion in a supersaturated aluminum–magnesium alloy with extremely fine grains. By forming these stable structures, diffusion-controlled intermetallic precipitation from the nanosized grains and their coarsening were inhibited up to the equilibrium melting temperature, around which the apparent across-boundary diffusivity was reduced by about seven orders of magnitude. The melting temperature of the nanograined alloy is unprecedentedly elevated by about 69 K. This phenomenon stems from the fact that the minimal interface structure with zero mean curvature not only possess extremely high thermal stability, but also changes the vibration mode of interface atoms, thereby inhibiting the diffusion of atoms.
The Schwarz crystal seems to provide a robust barrier for arresting the diffusion of atoms in metals and substitutional alloys, boosting stability up to melting temperatures. Developing advanced engineering alloys using the Schwarz crystal structure may lead to useful properties for high-temperature applications.
This research was published in Science, 373 (2021) 683.
Fig. 1. (A-C) Structure characterization of the Schwarz crystal Al-Mg sample; (D-E) Stability of lattice constant and grain size.