Heterophase interface-mediated formation of nanotwins and 9R phase in aluminum: Underlying mechanisms and strengthening effect

Abstract

Abstract Nanostructured crystalline Al/amorphous AlN multilayer films with a wide layer thickness (h) range from ∼10\u202fnm up to ∼200\u202fnm were prepared by using magnetron sputtering. Nanotwins and 9R phase were substantially observed in the Al layers, showing a strong thickness dependence. The 9R phase predominantly penetrated through the Al layer in the II regime of h ≤ ∼20\u202fnm, while mainly terminated within the layer interior in the I regime of h > ∼20\u202fnm. On the contrary, the coherent nanotwins were boosted when h > ∼20\u202fnm and the percentage of twinned Al grains was greatly increased. The formation mechanisms of 9R phase and coherent nanotwins were discussed in terms of the interfacial chemistry/physics modulated by the amorphous AlN layers, which displayed gradient characteristics and hence was sensitive to the layer thickness. A significant thickness dependence of hardness was also evident that the hardness monotonically increased with reducing h in the I regime, while reached a peak value and hold almost unchanged in the II regime. The hardness in the II regime is about 1\u202fGPa greater than the predictions from an interfacial barrier crossing model. This discrepancy is mainly contributed by the layer-penetrating 9R phase rather than the nanotwins. This study provides a new perspective on fabricating nanotwinned Al by utilizing heterophase interfaces.