Microstructure based creep constitutive model for precipitation strengthened alloys: theory and application

A kinetic creep equation for precipitation strengthened alloys has been derived using the climb/glide particle bypass micromechanism. It is applicable only when stresses lie within the range delineated by the dislocation network strength and the athermal yield strength - the lower of particle shear strength or Orowan strength. In this 'dispersion controlled' creep regime, strain rate is a hyperbolic sinh function of stress and interparticle spacing, making the state of dispersion an important microstructural feature. Existing equations have been used to quantify the evolution of dislocation and particulate microstructures and also stress redistribution between the two phases. The resulting constitutive equation set has been used to predict proof stresses, minimum creep rates and lifetimes for Nimonic 90. Comparison to industry data over a 600-900 degrees C temperature range concludes that proof stresses are creep controlled once temperatures are above 700-725 degrees C. Minimum creep rates and lifetimes over a similar temperature range are well predicted between 600 and 700 degrees C but become progressively less accurate at higher temperatures/lower stresses, particularly lifetimes.