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 similar to 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.