A Physically Based Model for High Temperature Deformation of Inconel 718Plus™

Abstract

The microstructural evolution of Inconel 718Plus during hot forming operations is modeled through a physically based model which includes the effects of precipitating particles. Inconel 718Plus has been a successful alloy since its introduction in 2003 owing to its moderate cost, good formability and weldability, and its higher maximum service temperature compared to its ancestor, Inconel 718. It is well known that the service performance and hot-flow characteristics of this alloy are strongly dependent on the microstructure, particularly the grain size. Thus, comprehension of the microstructural evolution and its modeling is an important task. In precipitation hardening superalloys and microalloyed steels, it is particularly more challenging to model the microstructural evolution in the processing windows where material softening and precipitation processes take place concurrently. The model presented in this work is based on dislocation density evolution which is considered as a result of the competition between dislocation generation and dynamic recovery at the early stages of deformation. In the hardening region, recovery through climb is described by the diffusion of vacancies and glide is assumed to be proportional to the strain rate in accordance with the models proposed by Bergstrom. Since the deformation is assumed to be controlled by glide and climb, the peak stress is modeled based on a modified hyperbolic-sine model which takes into account the temperature dependence of self-diffusion of Nickel and elastic modulus. It is known that under high temperature deformation conditions Inconel 718Plus may undergo dynamic precipitation. Second-phase particles in the material may impede the grain boundary motion and contribute to an increase in flow-stress due to Orowan looping. To account for the dynamic precipitation, the present model combines previously obtained experimental results and precipitation models to predict volume fraction and particle radius. For the peak stress modeling, the effect of precipitation is expressed as an extra stress term. The flow stress is calculated for the deformed and the recrystallized material separately and the total flow stress for the material is calculated using a law of mixtures considering the fraction of recrystallized material, while recrystallization is described as a nucleation-growth process via Avrami formalism. Cylindrical compression tests were employed to observe the hot flow behavior and validate the model. The predictions are compared with the experimental findings and good agreement is observed.