Research Projects

Ni-based superalloys (NBSAs) have revolutionized the mechanical durability of the components used in high temperature (HT) application, particularly in gas turbine engines. At operating conditions, creep and thermo-mechanical fatigue (TMF) are the primary deformation mechanisms which are largely resisted by the ordered γ' and γ'' precipitates in disordered γ-matrix. Ironically, high strength, hardness, strain hardening and complex chemistry of NBSAs also poses several manufacturing challenges such as excessive tool wear, low material removal rates, difficulty in casting and refractory elements segregation at HT forming. Additive manufacturing (AM) has emerged as an alternate advanced manufacturing route for NBSA components where layer by layer deposition not only allows extreme intricacies in components but also minimizes the material waste and need for assembly by producing complex monolithic parts. Despite being widely accepted HT materials, extra-ordinary demands of aerospace industries (weight reduction for improved fuel efficiency and reduced C footprints) are pushing the limits of current materials technology related to NBSAs. In addition, AM introduces high variation in the resulting microstructure leading to a greater uncertainty in mechanical performance of AM NBSAs parts. This presents a big challenge in seamless adoption of AM parts as safety-critical components used in aerospace. These industrial expectations can be fulfilled by enhancing the upper limit of service temperature and improved thermo-mechanical reliability of AM NBSAs which are the broad objectives of this project. Achieving these objectives warrants sound understanding of interactions between AM, emerging microstructure, and resulting mechanical performance. In particular, effect of unique microstructural features of 3D printed NBSAs (Columnar grains, residual stress, porosity) on their HT creep and TMF behavior still remains an open challenge. Sustainable solutions to these challenges entail correlating enormous number of microstructural, loading, and processing parameters and desired mechanical properties which is prohibitively expensive or even impossible through experiments alone. Therefore, employing computational materials design approach becomes inevitable for faster deployment and efficient usage of HT AM components. Physics based reliable and efficient materials models is the key to improved numerical predictions beyond experimental observations. Here, we aim to develop a micro-mechanical (two scale) non-local crystal plasticity (CP) model for a polycrystalline NBSA, Inconel 718 (IN718). Experimentally observed phase-specific (γ'', γ', γ) temperature dependent deformation mechanisms (HT: vacancy diffusion driven dislocation climb, cross-slip, grain boundary sliding, precipitation coarsening; RT: precipitate shearing, grain boundary strengthening) will be added to CP model developed by PI. Model will be implemented as a constitutive module in DAMASK, an open source CP simulation toolkit. Experimental inputs to model will be passed via extensive thermo-mechanical testing and pre- and post-deformation characterization of post-heat treated (using innovative hybrid strategies) AM IN718 printed using optimized LPBF and DED process parameters. Selected CP model parameters will be calibrated against temperature dependent macro (flow stress) and micro (local strain mapping obtained via digital image correlation) experimental observations. Predictive accuracy of the model will be validated against independent set of experimental observations of thermo-mechanical loading. Validated model will be used to simulate high-temperature creep-fatigue interactions and anisotropic damage initiation for different realistic representative microstructures generated via DREAM3D. The outcome of virtual tests will pave the pathways for optimal microstructural design of AM NBSAs, e.g., tailoring γ' and γ'' shape/size/distribution for improved creep and TMF resistance.