The microstructure and mechanical behavior of Mg/Ti multilayers as a function of individual layer thickness

We have used magnetron sputtering to deposit magnesium and titanium layers alternately onto a single-crystal silicon substrate with equal individual layer thickness (h, from 2.5 to 200 nm) to form multilayers. We have investigated the mechanical behavior of the multilayers and its dependence on h. Transmission electron microscopy and X-ray diffraction analyses suggest that the multilayers exhibit strong texture with respect to Mg (0 0 0 2) and Ti (0 0 0 2) with an epitaxial growth pattern. Two primary orientation relationships between Ti and Mg have been identified, depending on h. Instrumented nanoindentation and microcompression have been used to examine the hardness/strength and the strain rate sensitivity of the multilayers. Based on nanoindentation, we have found that the strength of these multilayers generally increases as h is decreased. The microcompression measured strength is remarkably higher than that derived from indentation. The Hall-Petch law can be used to interpret the increase in strength at relatively large h (>50 nm), while the confined layer slip model provides a better explanation for the relationship between strength and h at smaller h. We have also attempted to present an in-depth discussion about the applicability of relevant strengthening mechanisms on these hexagonal close-packed/hexagonal close-packed multilayers.