Microstructure-sensitive low-cycle fatigue in a nickel-base single crystal superalloy: Constitutive modeling and life assessment

Abstract

Mechanical properties of nickel-base single crystal superalloys undergo degradation during service due to microstructural changes like coarsening and rafting. The low-cycle fatigue performance along [001] crystal orientation in the coarsened and rafted DD6 alloys was studied by experiments and theoretical analysis. Fatigue tests across various microstructure states with different strain ratios at elevated temperatures revealed substantial life reduction. A novel tensorial representation for microstructure was proposed and demonstrated unique advantages over conventionally used scalar parameters. For the first time, the effects of coarsening and rafting were decoupled through the independent components of fabric tensors of $\gamma$-matrix and ${\gamma }^{\prime }$-precipitate phases. A microstructure-sensitive crystal plasticity-based constitutive model and a fatigue life prediction model were developed for mechanical behaviors along [001] crystal orientation using fabric tensors. The stress drop and life reduction in the coarsened and rafted alloys were well modeled. Both the accuracy and conservativeness of the life prediction results were significantly improved. The applications of fabric tensors in constitutive modeling and fatigue assessment show the potential to offer enhanced insights and predictive capabilities, enabling more comprehensive evaluations of structural integrity in various operational conditions and environments.