Isotropic and kinematic hardening as functions of gamma prime precipitates in a nickel-based superalloy

Abstract

A microstructure-explicit constitutive model has been developed for the P/M nickel-based alloy, IN100. In this model, the isotropic and kinematic stress components have been derived as a function of the size and volume fraction of γ′ precipitates. The hardening contributions of these precipitates consider dislocation/precipitate interactions which have been identified in terms of particle shearing and/or Orowan by-passing mechanisms. The critical shearing/looping particle size has been determined from the knowledge of the yield stress of heat treated specimens having different γ′ sizes. This critical size is used to calculate the relative contribution of particles in a distribution based on their size being cut or looped by mobile dislocations. The back stress has been examined through two terms, the first being a short range variable arising from solid solution and the second is a long range variable related to particle by-passing. The material parameters necessary to fully define the isotropic and kinematic hardening components are determined through a series of strain controlled low cycle fatigue tests carried out at 923K (650°C) on smooth specimens of the IN100 alloy varying in their secondary and tertiary γ′ precipitate sizes while maintaining constant volume fractions. Numerical simulations of the fully reversed strain-controlled loading, at a strain rate of 10−5s−1, and strain ranges of ±0.006, ±0.007 and ±0.008mm/mm, were carried out and the stress vs. strain, as well as, the isotropic, kinematic and viscous stress components are compared with the corresponding experimental results.