One of the limiting factors in gas turbine design is the allowable metal temperatures and loads in critical components. Specially designed superalloys are used when the conditions are most severe. One of these superalloys is Inconel 718. To be able to design components for higher temperature and higher loads, an accurate understanding and computational model of the material is needed. In this thesis the deformation mechanisms of Inconel 718 have been investigated and a theoretical basis for modelling in a large deformation context has been established. Finally a viscoplastic nonlinear kinematic hardening material model with an Armstrong-Frederick backstress evolution law has been implemented as a first step in describing the constitutive behaviour of the material Inconel 718.