Creep and creep modelling of a multimodal nickel-base superalloy

The creep properties of different gamma'-Ni3Al distributions in the Nil15 nickel superalloy produced by heat treatment have been examined. At the stresses and temperatures employed it is shown that particle bypass cannot occur by cutting or bowing and so presumably occurs by a climb-glide motion. The creep strength of Nil15 is very poor at 800 degrees C and 360 MPa compared to 700 degrees C because the increase in coarsening rapidly removes the bimodal structure. The Dyson creep model is a microstructure-based climb-glide bypass model for unimodal distributions that links microstructural evolution (e.g. evolution of the particle dispersion) to the creep rate. The creep tests are interpreted with the aid of the model and appear to suggest that the fine gamma', when present, strongly influences the dislocation motion. A quasi-bimodal model is developed to account for bimodal distributions and the predictions compared to experiment. The model predicts a number of the observed experimental trends, and it's shortcomings are identified in order to identify avenues for future improvement. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.