Sensitivity analyses and inverse design of alloy 718 for robust high temperature properties

Alloy 718 (UNS N07718) is a widely utilized superalloy in extreme-temperature applications for its superior high-temperature properties. While it is one of the few commercially printable superalloys, additively manufactured alloy 718 exhibits a yield strength variability of almost 400 MPa. This significant variability originates from microstructural variations due to potential variability in composition, process parameters, and heat treatments. However, the sensitivity of microstructural features (mainly phase distributions) to these origins remains unexplored. Here, we combined CALPHAD-based ICME techniques to simulate microstructural features in as-built and fully-aged equilibrium conditions at 650°C of LPBF-manufactured alloy 718. The alloy's strengthening phases γ’ and γ’’, exhibit substantial variation (~18 and 14 mole %, respectively), despite the elemental concentrations being adjusted within standard ranges. Surrogate models were developed to conduct global sensitivity analysis using Sobol’ indices to identify the key elements contributing to overall phase variabilities. From our studies, Al emerged as the most influential element in the alloy composition, with a 1.29 mole % variation accounting for 86.24% of γ’ phase variability. Remarkably, γ’’- despite being Nb-based phase, showed significantly higher sensitivity to Al % than Nb %, causing 85.55% of the phase’s variability. We show that these variabilities in phase fractions are also highly sensitive to temperature changes. Using Bayesian optimization, we then defined the ranges for the critical elements to achieve target high temperature strength below threshold (30%) variability. This study provides insightful guidelines to delineate reliable design pathways and specifications for certification of alloys with robust properties.