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

Abstract Nanostructured crystalline Al/amorphous AlN multilayer films with a wide layer thickness (h) range from ∼10 nm up to ∼200 nm 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 nm, while mainly terminated within the layer interior in the I regime of h > ∼20 nm. On the contrary, the coherent nanotwins were boosted when h > ∼20 nm 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 GPa 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.