Modeling the Effects of Damage and Microstructural Evolution on the Creep Behavior of Engineering Alloys

A quantitative representation of the creep behavior of materials is required to determine the operating lives of high temperature plant. Although the creep performance of such materials is normally governed by the development of microstructural features that can either be associated with the normal aging phenomena or by the development of damage in the material, most previous analyses of creep data have been empirical. It has been implicitly assumed that similar forms of creep curves can be adequately represented by a single generic equation. However, it is clear that different materials are subject to different combinations of structural change during their creep lives (e.g., cavitation/cracking, particle coarsening, phase changes, dislocation accumulation) all of which can influence the creep performance. An empirical representation can always be made to fit an available database, but effective extrapolation to longer lives and more complex loading conditions requires that the differing mechanisms be integrated in the creep equations. This paper will explore the implications of the evolution of microstructure and damage on the creep performance of a range of materials and will consider the potential of a microstructure-based state-variable (or damage-mechanics) approach for improved design life prediction of new plant and remaining life assessment of geriatric plant.