The accuracy of prediction made by finite element modeling of machining processes significantly depends on the constitutive model used to describe the material behaviour at typical values of strain, strain rate and temperature encountered in cutting. In this paper a new constitutive relationship is established by modifying the Zerilli-Armstrong model based on the concept of dislocation mechanics. The efficacy of the proposed model has been demonstrated by implementing it as a constitutive law for Inconel 718, a nickel based super alloy. The constitutive data generated by using the distributed primary zone deformation model is utilised for calculating the material constants. The efficacy of the proposed model is validated by using a three-pronged evaluation process. The first evaluation procedure involved comparing flow stress predictions made by the proposed model with data generated by the distributed primary zone deformation model. The second evaluation process involved finite element simulations of the orthogonal machining process with the proposed material model as the constitutive relation. A user material subroutine is used for implementing the proposed constitutive relation into a finite element model of orthogonal machining process through ABAQUS/Explicit platform. Good agreement has been achieved between model predictions and experimental data in simulating chip formation with attendant cutting forces. In the third evaluation process, proposed constitutive law is further validated by comparing its flow stress predictions with the split Hopkinson pressure bar test data available in literature. © 2017 The Society of Manufacturing Engineers