A microstructure-based fatigue model for additively manufactured Ti-6Al-4V, including the role of prior ? boundaries

Microstructure-based models of additive manufactured (AM) Ti-6Al-4V should faithfully repre-sent the unique microstructural features of these materials to provide a more thorough under-standing of their role in the mechanical performance of the material. For AM Ti-6Al-4V, the prior fi boundaries are likely sites of microscopic plastic strain localization, often leading to fatigue crack initiation. Within the context of crystal plasticity finite element methods, there is an existing gap in the current literature for creating synthetic microstructures of Ti-6Al-4V that capture both the prior fi boundaries and alpha laths. This work focuses on generating such statistically equivalent microstructures, where the prior fi boundaries and alpha laths are explicitly modeled. The framework can generate synthetic microstructures that consider one prior fi grain or multiple fi grains (and thus prior fi grain boundaries) and their associated alpha laths forming a Widmansta center dot tten microstructure, which are used as inputs for fatigue simulations. Within the fatigue model, the critical accumulated plastic strain energy density (APSED) is a metric used to predict the location and number of cycles of crack initiation. From a statistical average perspective, the presence of prior fi grain boundaries was detrimental to the fatigue performance. The effectiveness of the prior fi grain boundary in impeding slip, and therefore localizing APSED, is dependent on the loading conditions, prior fi orientations, and alpha variants present in neighboring prior fi grains. A higher percentage of failures occurred at the prior fi grain boundary in the case of lower applied stress amplitudes, where the applied loads were below the percolation limit of the material and microplasticity was localized to a few locations relative to the microstructure. Additionally, for the case of crack initiation associated with the alpha variants within a given prior fi grain, a com-bination of soft-hard alpha lath orientations resulted in sites of APSED localization and predicted fatigue crack initiation. The framework generated in this paper to create representative synthetic microstructures of the Widmansta center dot tten microstructures associated with AM Ti-6Al-4V and the associated fatigue modeling enables trade-off studies between microstructural features and fa-tigue performance.