Ph.D. Proposal Presentation by Ali P. Gordon
Friday, August 13, 2004

( Dr. David L. McDowell, Chair)

"Crack Initiation Modeling of a Directionally-Solidified Ni-base Superalloy"

Abstract

Directionally-solidified (DS) GTD-111 is a newly-developed Ni-base superalloy manufactured by GE Energy for second stage turbine applications. Its grains are 5 mm in width transverse to the DS axis. Service conditions in blades, which can generally exceed 600ºC, facilitate the onset of one or more microstructural damage mechanisms in the forms of fatigue, creep, and environmental corrosion. Because of the distinctive microstructure and chemical composition of the material, the manner in which these mechanisms interact to initiate cracks is complex. These physical mechanisms have been observed through preliminary experimental observation from creep and fatigue-tested samples of longitudinal (L) and transverse (T) specimens. A mechanistically-based crack initiation model correlating the evolution of the dominant mechanism to failure is sought. The model will accurately correlate crack initiation lives of both as-cast and pre-exposed material in static air at elevated temperature. The individual mechanisms corresponding to fatigue, creep, and environment terms that represent the physics of damage accumulation and failure observed in service-exposed materials will be incorporated into damage mechanism maps. The mathematical formulation will account for variations in temperature, orientation, cyclic frequency, stress ratio, sustained-load hold times and interactions resulting from complex load histories (i.e., high- and low-cycle fatigue, creep-fatigue and thermal-mechanical fatigue). Correlations in terms of environmental degradation (e.g. vacuum or oxidation/corrosion) will be included in the crack initiation formulation. Additional fatigue experiments, as well as numerical simulations, are necessary for the development of the formulation.

Dr. Tom Sanders (MSE) is involved in calculations of phase diagrams in precipitation kinetics in nickel-base super-alloys. He is currently working with DS GTD-111.

Dr. Stephen Antolovich (MSE) has helped to develop a fundamental understanding of deformation mechanisms during low cycle fatigue at elevated temperatures in Ni-base superalloys.

Dr. Erian Armanios (AE) has broad experience in damage tolerance of advanced materials and optimization techniques.