Hydrogen-Assisted Cracking Behavior of Ni Alloy 718: Microstructure, H Testing Protocol, and Fractography

Abstract

Hydrogen-assisted cracking (HAC) behavior of peak-aged, as-received material of aerospace-grade, and over-aged, based on the API 6ACRA-718 standard, microstructural variants of Ni alloy 718 was studied using slow strain rate test (SSRT) under in-situ hydrogen (H) environment. The effect of cooling rate post solution annealing (SA) treatment on HAC susceptibility was also examined. The influence of H testing protocols with and without H pre-charging on HAC susceptibility was evaluated. The link between microstructural features and HAC fracture features was established using advanced electron microscopy. The results suggest that the microstructural variant of alloy 718, with sufficient initial ductility and clean grain boundaries, may be suitable for service applications in H environments. The over-aged variant obtained by air cooling post SA and followed by aging at 774 °C for 6 hours exhibited better HAC resistance due to the presence of δ phase sporadically decorated along the GBs. The slower cooling rates post SA increased HAC susceptibility. The yield strength of the over-aged variants aged at 802 °C for 8 hours, regardless of SA cooling rates, was lower than the minimum requirement expected by the API standard, and the HAC susceptibility was severe exhibiting intergranular fracture due to continuous decoration of δ phase along the GBs. The peak-aged variant demonstrated severe HAC susceptibility showing transgranular fracture. HAC susceptibility of all variants was greater for SSRT of non-H pre-charged specimens under in-situ H environment at 20 °C than at 80 °C. H pre-charging enhanced the HAC susceptibility further when tested at 20 °C and the role of microstructure was largely suppressed. HAC susceptibility was manifested as both transgranular and intergranular fracture modes depending on the slip localization activity and H availability.