Functionally Graded Shape-Memory and Pseudoelastic Response in Ni-Rich/Ti-Rich and Vice Versa NiTi Multilayer Thin Films Deposited on Si(111)

In this study, functionally graded NiTi multilayer thin films were created by radio frequency magnetrun sputtering and subsequent annealing. The chemical compositions of the multilayer thin films which determined by energy-dispersive x-ray spectroscopy are about Ni51Ti/Ni49Ti (numbers indicate at.%). The structures, surface morphology, and transformation temperatures of annealed thin films at 500°C for 1 h were studied using grazing incidence x-ray diffraction, atomic force microscopy, and differential scanning calorimetry (DSC), respectively. Moreover, a coupled nanoindentation/atomic force microscopy technique is employed to characterize both the superelastic and shape memory effects. Specifically, an analysis of recoverable energy through the stress-induced phase transformation and recoverable strain through de-twinning and the subsequent thermally induced phase transformation were performed. The DSC and x-ray diffraction results indicated the multilayer was composed of austenitic and martensitic thin films. The continuous stiffness measurement technique showed that the stiffness and elastic modulus of the multilayer change gradually through the thickness of the multilayer thin films. Also, the thin films exhibited a combined pseudoelastic behavior and shape-memory effect which produces a two-way shape-memory effect in both thin films.