Experimental Identification and Desorption Quantification of Hydrogen Trapping Sites in Samples of Wrought IN718 and Heat Treated Additively Manufactured IN718

RIP2025-00117: BACKGROUND

The manufacturing process significantly influences IN718's microstructure. Wrought IN718 typically features a uniform distribution of γ' and γ" phases, a refined grain structure, and NbC precipitates. Controlled processing and heat treatments minimize detrimental phases like Laves and δ, preventing crack initiation and mechanical degradation. Conversely, additively manufactured (AM) IN718 exhibits a heterogeneous microstructure due to rapid solidification and thermal gradients, with columnar dendritic grains and Nb/Mo segregation often forming Laves phases. Post-processing heat treatments homogenize the microstructure, dissolve Laves phases, and precipitate γ' and γ" phases.

Hydrogen embrittlement (HE) occurs as hydrogen diffuses into the lattice, accumulating at defects such as dislocations, grain boundaries, and precipitates, weakening the material under stress. Understanding hydrogen’s interaction with these features (uptake, diffusion, or trapping) is crucial for developing HE-resistant materials. Hydrogen trapping plays a central role in embrittlement, requiring accurate modeling for prediction and mitigation. Thermal desorption spectroscopy (TDS) is essential for studying hydrogen behavior, revealing how it escapes trap sites, diffuses, and recombines, all strongly influenced by the material's microstructure.

ABSTRACT

The goal of this study is to bridge the gap between hydrogen desorption thermodynamic data from TDS and the microstructural features in wrought IN718 and heat-treated (AM IN718 + HT) samples that influence hydrogen desorption behavior. We hypothesize that desorption models, which provide parameters such as trap binding energies, trap densities, and trapping/de-trapping rates, must be complemented by detailed microstructural characterization to elucidate the observed desorption behavior. To test this hypothesis, we employed a suite of advanced materials characterization techniques, including Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), X-ray Diffraction (XRD), and Electron Backscatter Diffraction (EBSD). Preliminary TDS and isoTDS results suggest that microstructural features such as dislocations, grain boundaries, and second-phase particles significantly impact hydrogen trapping and desorption behavior. Our analysis of thermographs for IN718, AM IN718, and AM IN718 + HT samples identified key differences in activation peak energies, desorption energies, and effective diffusivities, which were further analyzed using empirical methods like the Kissinger method and skew analysis. These findings guided the subsequent characterization phases, helping us correlate microstructural features with hydrogen trapping mechanisms.

STATEMENT OF IMPACT

This study advances the understanding of the interplay between microstructure and hydrogen trapping, enabling improved prediction and control of desorption processes in Ni-based superalloys. By integrating thermodynamic modeling with microstructural insights, this work provides a comprehensive framework for addressing hydrogen embrittlement challenges in high-performance materials.