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

Nanostructured crystalline Al/amorphous AIN multilayer films with a wide layer thickness (h) range from similar to 10 nm up to similar 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 <= similar to 20 nm, while mainly terminated within the layer interior in the I regime of h > similar to 20 nm. On the contrary, the coherent nanotwins were boosted when h > similar to 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 AIN 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. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.