Revealing thickness dependence of hardness, strain rate sensitivity, and creep resistance of nano-crystalline magnesium/titanium multilayers by nanoindentation

The size effect on mechanical properties of hcp/hcp multilayer has been short of understanding hitherto. In this study, we prepared Magnesium/Titanium (Mg/Ti) multilayers with various individual layer thicknesses ( h = 50, 100, 150, 200 nm) by magnetron sputtering. Relying on nanoindentation, hardness and elastic modulus were little changed for the multilayers with h of 50 ∼ 150 nm, while they were evidently lower when h increased to 200 nm. The determined strain rate sensitivities were 0.029, 0.032, 0.035, and 0.062 for the samples with h = 50, 100, 150, 200 nm, respectively. According to evolution of grain size, it suggests that Hall-Petch law dominates the strengthen effect of this hcp/hcp multilayers, rather than blocking effect of interface and decrease of dislocation content by reducing individual layer thickness. On the other hand, creep resistance was gradually promoted as reducing h . It indicates that interface and dislocation content could be important for the time-dependent plastic deformation. The room-temperature creep mechanism was discussed based on the strain rate sensitivity of steady-state creep flow. It indicates that dislocation glide could be suppressed and grain boundary glide, dislocation climb, and even interface glade could be plausible when h decreased to 50 nm.