Thermally stable nanostructured Al-Mg alloy with relaxed grain boundaries

Grain refinement into the nanoscale greatly hardens metals and alloys but reduces the stability. By means of cryogenic plastic deformation at very high strain rates, a high proportion of relaxed grain boundaries (GBs) were generated in an Al-5Mg alloy through emitting partial dislocations from GBs when grain sizes are below a critical value. The nanostructured Al-5Mg alloy with an average grain size of 31 nm exhibited a high hardness of 2.67 GPa and a grain coarsening temperature of about 95 K higher than that of submicro-grains. In addition, Mg depletion rather than segregation at some relaxed GBs was observed, that is in account with considerable compressive strains in the vicinity of the relaxed Σ9 GΒ. The relaxed GBs with Mg depletion are effective in stabilizing the nanostructured Al-Mg alloys. It offers a novel approach to developing stable nanostructured alloys.