A comprehensive study of the microstructure and mechanical behavior of HCP-based multilayered nanofilms
Metallic multilayer films with the individual layer thickness h decreased to submicron or even less have attracted a great deal of attention over the past decades because of their impressive mechanical properties, especially the high strength. Most experimental and analytical efforts in this research field have been endeavored to the multilayer films composed of body-centered cubic (BCC) or face-centered cubic (FCC) metals. Consider the unique deformation mechanisms of hexagonal close-packed (HCP) metals, the microstructure and mechanical behavior of HCP-based multilayered nanofilms are of particular interest in this work while the typical HCP-based multilayer system, magnesium (Mg)/titanium (Ti), is primarily investigated.Mg and Ti layers, with equal individual layer thickness ranging from 2.5 nm to 200 nm, were alternately deposited onto a single crystal silicon substrate via magnetron sputtering to form multilayered Mg/Ti nanofilms. The as-deposited films exhibit a strong texture along Mg (0002) and Ti (0002) with preference for an epitaxial growth pattern, as characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Two primary orientation relationships between Mg and Ti have been identified, depending on the length scale of h. Both instrumented nanoindentation and microcompression experiments were performed to study the effects of individual layer thickness on the hardness/strength and strain rate sensitivity in Mg/Ti multilayers. The strength of Mg/Ti multilayered nanofilms was found to be generally increasing as the individual layer thickness is decreased, eventually reaching the peak value of ∼1.56 GPa at the smallest h. The dislocation pile-up based Hall-Petch law can be used to interpret the increase in strength at relatively large layer thickness (>50 nm), while the confined layer slip model provides a better explanation for the relationship between strength and layer thickness at smaller layer thickness. The flow strength measured from microcompression is much higher than the nanoindentation derived value when h decreases to several nanometers, which can be explained by the vanishing Schmid factor under uni-axial compression. As a critical criterion for industrial applications, the thermal stability of Mg/Ti multilayered nanofilms was also evaluated by examining the microstructure and hardness after annealing. The multilayers with h ≥5 nm possess excellent capabilities in maintaining the lamellar morphology and high strength up to 200 °C. However, the h =2.5 nm Mg/Ti multilayered nanofilms following the annealing at 200 °C for 2 h exhibit a drop in hardness to ∼3.3 GPa, which is intimately related to the morphological changes at elevated temperatures. Preliminary experimental results on Mg/Zr multilayer nanofilms show trend in mechanical properties similar to those of the Mg/Ti nanofilms as a function of individual layer thickness. Further investigation is needed to come up with a more complete understanding of this system.