Temperature and Orientation Dependence of Fracture Behavior of Directionally Solidified Duplex-Phase Crystals Composed of Ni&lt;sub&gt;3&lt;/sub&gt;X-Type Intermetallic Compounds

A micromechanical model has been developed for predicting the time-dependent crack growth threshold and its variability by considering oxide formation or cavity formation ahead of an elastic crack subjected to a sustained load at a stress intensity factor, K, at elevated temperatures in air. It is demonstrated that stress relaxation associated with a volume-expansion process such as the formation of creep cavities or oxides with a positive transformation strain can induce residual stresses at the tip of the elastic crack. The near-tip residual stresses must be overcome by the external load, thereby instigating a growth threshold, K th , for the onset of time-dependent crack growth. This micromechanical framework provides the basis for developing appropriate predictive models for the time-dependent crack growth thresholds associated with several damage processes, including (1) oxidation-assisted intergranular crack growth, (2) K-controlled creep crack growth along an intergranular path, and (3) stress corrosion cracking. The micromechanical threshold models have been utilized to predict the time-dependent crack growth thresholds of a variety of Ni-base superalloys. The material parameters that contribute to the variability of the time-dependent crack growth thresholds have been identified and related to variations of mixed oxides or creep cavities formed near the crack tip. A size scale effect is also predicted for the transformation toughening phenomenon, which is largest at or below K th but diminishes at increasing K levels above the threshold. Finally, the micromechanical models are utilized to identify means for suppressing time-dependent crack growth in Ni-base alloys.

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Temperature and Orientation Dependence of Fracture Behavior of Directionally Solidified Duplex-Phase Crystals Composed of Ni<sub>3</sub>X-Type Intermetallic Compounds

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References 11 publications

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