Development of Ni₄₄Ti₃₅Zr₁₅Cu₆ Quaternary Shape Memory Alloy: Experimental and Density Functional Theory Studies

The effect of Zr and Cu additions on the phase stability, microstructure and martensitic transformation temperature of binary NiTi alloys has been investigated with a combined approach of experimental measurements and first principle density functional theory calculations. In this work, multilayer Ni/Ti/Zr/Cu films have been deposited using the magnetron sputtering on Si substrates at room temperature and subsequently annealed at 350 °C for complete inter-diffusion of multilayers to create amorphous phase followed by high temperature annealing at 600 °C for 5 min to achieve Ni₄₄Ti₃₅Zr₁₅Cu₆ shape memory alloys. The high temperature annealing is required for the formation of crystalline phases. The x-ray diffraction pattern of the film annealed at 600 °C indicated the presence of martensite phase. The surface and interface studies were performed focusing on morphology and interlayer diffusion over the nano-level structure at 600 °C. The elemental mapping showed near-uniform distribution of constituent elements after annealing. Atomic force microscopy results showed the increase in surface roughness at annealing temperature of 600 °C due to grain growth. The formation energy results indicate that the Zr and Cu additions to NiTi favor the formation of monoclinic B19’ phase. The differential scanning calorimetry results show that the martensitic transformation temperature increases with Zr and Cu additions. Nanoindentation studies for Ni₄₄Ti₃₅Zr₁₅Cu₆ films showed an increase in depth recovery for the crystalline film. A theoretical model has been used, to determine the hysteresis of shape memory alloys as predicted by the geometric nonlinear theory of martensite.