Microstructure-Based MultiStage Fatigue Modeling of NiTi Alloy Fabricated via Direct Energy Deposition (DED)

A microstructure-based multistage fatigue (MSF) model was employed to study the process-structure-property relations for cyclic damage and fatigue life of NiTi alloy fabricated via an additive manufacturing (AM) technique. Various defect characteristics (i.e., level of porosity, pore size, and their spacing) and microstructural features (i.e., grain size, mean grain orientation, and misorientation angles), dictated by the manufacturing and post-manufacturing heat treatment processes, were used to predict the fatigue life of AM and wrought NiTi specimens. The specimens fabricated via AM underwent two different heat treatment conditions (i.e., aging followed by air cooling and annealing followed by water quenching). Using the process-dependent parameters, the MSF model could capture the differences in fatigue behavior of each condition. The predicted lower and upper bounds of fatigue life based on the range observed for microstructural features and defect characteristics were able to account for the scatter observed in experimental fatigue data.