Alloy 718 is a precipitation hardened Ni-base superalloy with wide application in hydrogen containing atmospheres such as sour gas environments when tapping oilfields. It has been suggested [1–3] that the delta phase, forming primarily at gamma-grain boundaries promotes hydrogen embrittlement (HE) by initializing micro-cracks. However, the real mechanism of the delta phase in promoting HE in alloy 718 is not yet understood.
A difficulty in explaining the mechanisms of HE concerns the spatially and temporally resolved detection of hydrogen. Visualization of hydrogen is essential for revealing its local distribution with respect to its possible preference to trap at certain phases or lattice defects. However, the very low detection sensitivity, high mobility and very low concentration of hydrogen and the limited availability of tools for spatially resolving detection render this field challenging.
In order to study the role of delta phase in HE of alloy 718, we investigated the H distribution and desorption behavior in an electrochemically H-charged binary Ni-Nb model alloy, consisting of a Ni-Nb solid solution gamma-matrix and a delta-Ni3Nb intermetallic phase.
The trapping states of H in the alloy were analyzed using Thermal Desorption Spectroscopy (TDS). Additionally, spatially resolved H-mapping was conducted using silver decoration, Scanning Kelvin Probe Force Microscopy (SKPFM) in conjunction with a Pd detection layer, and Secondary Ion Mass Spectrometry (SIMS): The Ag decoration method revealed rapid effusion of H at room temperature from the gamma-matrix. The corresponding kinetics was resolved both, spatially and temporally by the SKPFM measurements. At room temperature the H release from the gamma-matrix steadily decreased until about 100 h and then was taken over by the delta phase from which H was released much slower. For avoiding misinterpretation of H signals stemming from environmental effects we also charged specimens with deuterium. The deuterium distribution in the microstructure was studied by SIMS. Each individual technique used in this study has its advantageous characteristics as well as a specific range of spatial and /or temporal resolution. Combining these allows performing multi-scale mapping of H in the microstructure, ranging from the macroscale down to few-nanometers. The information obtained by clarifying the behavior of H desorption, binding energy between H and specific trapping sites, and the local H distribution, are used to discuss the H-assisted failure mechanism in this material. The combined results reveal that H dissolves more preferably inside the gamma-matrix and is diffusible at room temperature while the delta phase acts as a deeper trapping site for H. Correlating the obtained results with mechanical testing of the H-charged samples shows that hydrogen enhanced decohesion (HEDE) occurring at the delta/matrix interfaces promotes the embrittlement .
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