Theoretical Study of Ni Rich NiTi-Oxide Interface

JUAN J; SANDOVAL, M.; ORAZI, V.; BECHTHOLD, P.; GONZÁLEZ, E.A.; JASEN, P.V.; A. HERNÁNDEZ-LAGUNA; C. SAINZ-DÍAZ

Santiago

Congreso; 44th International Congress of Theoretical Chemists of Latin Expression; 2018

Nitinol, a nearly equiatomic nickel?titanium shape memory alloy, has wide applications in cardiovascular stents, micro-actuators, and high damping devices. Nitinol exhibits two unique mechanical behaviors: thermal shape memory and superelasticity. This material is widely used in medical applications due to its good biocompatibility. Nitinol has extremely good biocompatibility, due to the formation of a passive titanium-oxide layer (TiO2 ). This corroborates basic thermodynamics data that specify that the free energy of formation of TiO 2 is favored over other nickel or titanium oxides. This oxide layer serves for two purposes: increasing the stability of the surface layers by protecting the bulk material from corrosion and creating a physical and chemical barrier against Ni oxidation and modifying the oxidation pathways of Ni. However, the presence of Ni in metallic or oxidized form on its surface or its release from the alloy core is a serious problem because of its allergenic properties and toxicity to human tissue. For this reason, we performed a theoretical study of Ni segregation for a Ni rich NiTi-oxide interface with and without vacancy type defects. Our results show that Ni diffusion is strongly dependent of the presence of vacancies and the nature of an intermediary oxidation layer. We also analyzed changes in the electronic structure and bonding at the interface.