Size dependent strengthening in high strength nanotwinned Al/Ti multilayers

Mechanical behavior of metallic multilayers has been intensively investigated. Here we report on the study of magnetron-sputtered highly textured Al/Ti multilayer films with various individual layer thicknesses (h = 1–90 nm). The hardness of Al/Ti multilayers increases monotonically with decreasing layer thickness without softening and exceeds 7 GPa, making it one of the strongest light-weight multilayer systems reported to date. High-resolution transmission electron microscopy and X-ray diffraction pole figure analyses confirm the formation of high-density nanotwins and 9R phases in Al layers. The density of nanotwins and stacking faults scales inversely with individual layer thickness. In addition, there is an HCP-to-FCC phase transformation of Ti when h ≤ 4.5 nm. The high strength of Al/Ti multilayers primarily originates from incoherent layer interfaces, high-density twin boundaries, as well as stacking faults.