Suppressing atomic diffusion with the Schwarz crystal structure in supersaturated Al–Mg alloys

Abstract

High atomic diffusivity in metals enables substantial tuneability of their structure and properties by tailoring the diffusional processes, 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. 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. Developing advanced engineering alloys using the Schwarz crystal structure may lead to useful properties for high-temperature applications.

Freezing solute atoms in nanograined aluminum alloys via high-density vacancies

“…Such a catastrophic precipitation behavior greatly reduces the strengthening potential by artificial aging of NG alloys produced by SPD processing 16 . Another consequence of such decomposition of supersaturated solid solution is the significant reduction in strength at elevated temperature, due to the rapid recovery and grain coarsening 23 . The intractable low-temperature (generally below ~100 °C and even at room temperature) precipitation of stable precipitate phases becomes another challenge of thermal instability that seriously limits the practical usage of NG Al alloys and other NG alloys with supersaturated solid solution 16 at elevated temperatures, in parallel with the widely-concerned severe grain coarsening 24 .…”

“…Minimizing crystal defects is intuitively a strategy to slow down atomic diffusion and avoid unfavorable low-temperature precipitation in the NG alloys. This strategy was recently manifested in NG supersaturated Al-Mg alloys 23 , where an average grain size ( d ) of ~ 8 nm was achieved by HPT processing at 77 K. A Schwarz crystal structure with zero mean curvature constrained by twin boundaries was unprecedently produced 23 . Due to the ultrafine nano grain size, the vacancy concentration is quite low within the grains 23 , 25 .…”

“…This strategy was recently manifested in NG supersaturated Al-Mg alloys 23 , where an average grain size ( d ) of ~ 8 nm was achieved by HPT processing at 77 K. A Schwarz crystal structure with zero mean curvature constrained by twin boundaries was unprecedently produced 23 . Due to the ultrafine nano grain size, the vacancy concentration is quite low within the grains 23 , 25 . As a result, the diffusion-controlled Al 3 Mg 2 precipitation from supersaturated nanograins is completely suppressed at temperatures as high as 450 o C. In contrast, in the NG Al-Mg alloys with an average grain size of ~ 50 nm, intergranular Al 3 Mg 2 precipitation at low temperatures was evident, inspired by the excessive crystal defects 23 .…”

“…Due to the ultrafine nano grain size, the vacancy concentration is quite low within the grains 23 , 25 . As a result, the diffusion-controlled Al 3 Mg 2 precipitation from supersaturated nanograins is completely suppressed at temperatures as high as 450 o C. In contrast, in the NG Al-Mg alloys with an average grain size of ~ 50 nm, intergranular Al 3 Mg 2 precipitation at low temperatures was evident, inspired by the excessive crystal defects 23 .…”

Recent Progress on Regulating Strategies for the Strengthening and Toughening of High-Strength Aluminum Alloys

“…In addition, for practical production applications, there exists a relatively serious problem for any high-strength aluminum alloy: the strength and toughness of the materials cannot be well matched [2]; application has therefore been limited because of alloys' sensitivity to intergranular and stress corrosion. The best method to enhance the strength and toughness of materials is to develop a sequence of new materials and processes [3]. Many scholars have devoted themselves to improving the strength and toughness of HSAA, mainly by highlighting regulating strategies such as optimizing the alloy composition, improving the heat treatment process, and adopting special processing methods [4].…”