The Woodruff School

SUMMARY

The understanding of the effects of aged microstructures on the thermomechanical fatigue (TMF) properties of nickel-base (Ni-base) superalloys remains unclear. Few experimental results are currently available in this area, and of the limited results available, some promote aged microstructures as beneficial, while others as detrimental. The importance of these aged structures arises from the fact that when components used in the hot sections of gas turbine engines remain in service for extended periods of time, the local temperature and stress provides the catalyst for the evolution of the microstructure. An experimental assessment of a negative misfit directionally solidified (DS) Ni-base superalloy was undertaken to characterize the aging kinetics and understand the influence of the TMF cycle temperature extremum, temperature-load phasing, mean strain, creep-fatigue, orientation effects, and microstructure on TMF fatigue crack initiation. To determine the effects of aging on the TMF response, the as-heat treated alloy was artificially aged to three unique microstructures identified in the aging kinetics study. The experiments revealed that not all aged microstructures are detrimental to the fatigue life behavior. Specifically, when the gamma prime precipitates age in a manner to align themselves parallel to the axis of the applied stress, an increase in the fatigue life over that of the as-heat treated microstructure is observed for out-of-phase TMF with dwells. To extend the experimental understanding of the aged microstructures into service component design and life analysis, a temperature-dependent crystal viscoplasticity (CVP) constitutive model is developed to capture the sensitivity of the aged microstructure through embedding additional variables associated with the current state of the gamma prime particles. As a result of the adaptations, the CVP model has the ability to describe the long-term aging effects of directional coarsening relevant to the analysis industrial gas turbine hot section components.

SUBJECT:	Ph.D. Dissertation Defense

BY:	Michael Kirka

TIME:	Thursday, February 27, 2014, 2:00 p.m.

PLACE:	MARC Building, 201

TITLE:	Thermomechanical Behavior of a Directionally Solidified Nickel-Base Superalloy in the Aged State

COMMITTEE:	Dr. Richard Neu, Chair (ME/MSE) Dr. David McDowell (ME/MSE) Dr. Thomas Sanders (ME) Dr. Surya Kalidindi (ME/MSE) Dr. Rosario Gerhardt (MSE) Dr. Stephen Antolovich (ME/MSE)

SUMMARY The understanding of the effects of aged microstructures on the thermomechanical fatigue (TMF) properties of nickel-base (Ni-base) superalloys remains unclear. Few experimental results are currently available in this area, and of the limited results available, some promote aged microstructures as beneficial, while others as detrimental. The importance of these aged structures arises from the fact that when components used in the hot sections of gas turbine engines remain in service for extended periods of time, the local temperature and stress provides the catalyst for the evolution of the microstructure. An experimental assessment of a negative misfit directionally solidified (DS) Ni-base superalloy was undertaken to characterize the aging kinetics and understand the influence of the TMF cycle temperature extremum, temperature-load phasing, mean strain, creep-fatigue, orientation effects, and microstructure on TMF fatigue crack initiation. To determine the effects of aging on the TMF response, the as-heat treated alloy was artificially aged to three unique microstructures identified in the aging kinetics study. The experiments revealed that not all aged microstructures are detrimental to the fatigue life behavior. Specifically, when the gamma prime precipitates age in a manner to align themselves parallel to the axis of the applied stress, an increase in the fatigue life over that of the as-heat treated microstructure is observed for out-of-phase TMF with dwells. To extend the experimental understanding of the aged microstructures into service component design and life analysis, a temperature-dependent crystal viscoplasticity (CVP) constitutive model is developed to capture the sensitivity of the aged microstructure through embedding additional variables associated with the current state of the gamma prime particles. As a result of the adaptations, the CVP model has the ability to describe the long-term aging effects of directional coarsening relevant to the analysis industrial gas turbine hot section components.