An investigation of micro-mechanisms in hydrogen induced cracking in nickel-based superalloy 718

Abstract

Superalloys are widely used in a variety of applications including aerospace and land based gas turbine engines and nuclear power plants. The aerospace industry has experienced hydrogen embrittlement problems for many years. Hydrogen embrittlement is a delayed failure mechanism related to loss of ductility due to the presence of hydrogen in the material. Hydrogen embrittlement of the nickel-iron based superalloy 718 has been investigated using slow strain rate tests for pre-charged material and also in-situ hydrogen charging during testing. Fractography analyses have been carried using scanning electron microscopy, electron back-scattering diffraction and orientation image microscopy concentrating on the influence of microstructural features and associated micro-mechanisms leading to hydrogen induced cracking and embrittlement. It was observed that hydrogen induced transgranular cracking initiates at micro-voids in the crystal lattice. Similar behaviour has been observed in multi-scale finite element chemo-mechanical numerical simulations. In contrast, hydrogen induced localized slip intergranular cracking was associated with the formation of micro-voids in intergranular regions. The effects of grain boundary and triple junction character on intergranular hydrogen embrittlement were also investigated. It was observed that low end high angle misorientations (LHAM), 15°<θ≤35°, and critical high angle misorientations (CHAM), 35° < θ ≤ 50°, are preferential sites for hydrogen induced cracking. In contrast, few or no hydrogen induced cracks were observed at low angle misorientations (LAM), 0°≤ θ≤15°, high end high angle misorientations (HHAM), 50°<θ≤55°, or special GB misorientations (SGB), θ>55°. Finally, the use of grain boundary engineering techniques to increase the resistance of super alloy 718 to hydrogen induced cracking and embrittlement is discussed.

Hydrogen enhanced cracking studies on Fe–3wt%Si single and bi-crystal microcantilevers

“…Among all the proposed HE mechanisms, it seems the H-enhanced decohesion (HEDE) mechanism to be the most relevant one when the GBs are involved. The intergranular failure by H is explained in terms of reduction of the cohesive strength of GB by first-principles studies [24][25][26][27] and is shown experimentally on different materials [23,[28][29][30][31][32]. However, the work done by McMahon [33] showed that H-induced intergranular brittleness is strongly depended on the amount of impurities segregated to the GB.…”

Localized microstructural characterization of a dissimilar metal electron beam weld joint from an aerospace component

“…Several EBSD runs were carried out at various locations within the joint, namely, along the dissimilar weld joint root, above the weld root, within the In718 HAZ and base metal and within the PP-Ni HAZ and base metal. A step size of 0.2 µm and 4 µm was used in the EBSD analyses and more detailed information about post processing of the EBSD results and applying these data in multiscale modelling can be found elsewhere [19,20].…”

Prediction of surface roughness and material removal rate in wire electrical discharge machining on aluminum based alloys/composites using Taguchi coupled Grey Relational Analysis and Artificial Neural Networks

“…Panda put forward a hybrid optimization technique namely Neuro-Grey Modeling which carries out the parameter design and optimization of multi-machining characteristics of WEDM process and concluded that the model was reliable and can be implemented fast when compared with genetic algorithm [32]. It has been reported that the localized microstructural parameters such as phase distribution, grain size, distribution & grain structure also plays an important role on the effect of process parameters [35][36][37][38][39][40][41][42]. It also has been reported that the machine learning based ANN model has been used to predict the mechanical properties of metals & alloys [33,34,43].…”