Effect of grain size and precipitates on hydrogen embrittlement susceptibility of nickel alloy 718
Hamza Khalid, Vasanth Chakravarthy Shunmugasamy, Ryan DeMott, Khalid Hattar, Bilal Mansoor
Abstract
The present work reports on the effect of grain size and its combined effect with precipitation on hydrogen uptake and hydrogen embrittlement (HE) susceptibility of alloy 718. HE susceptibility of fine-grained microstructure obtained by friction-stir processing (FSP) is compared with standard heat-treated conditions. Melt extraction measurements on hydrogen pre-charged samples reveal that fine-grained FSP condition compared to solution-treated (ST) condition contains increased hydrogen content, indicating that higher vol% of grain boundaries and density of dislocations promote trapping. The presence of γ’ and γ” phases reduces hydrogen uptake in both fine-grained and aged (FSP-A) and peak-aged (PA) conditions, indicating a lack of affinity of these phases to hydrogen. Under tensile loading, for identical pre-charging conditions, the FSP (with fine grains) and ST (with coarse grains) conditions display similar susceptibility to HE. However, the FSP-A condition with refined grains and precipitates showed substantially reduced HE susceptibility than coarse grained PA condition, measured through % reduction in area. The finding that fine-grained microstructures absorb more hydrogen than larger grained microstructures yet have similar or reduced susceptibility to HE, demonstrates the inherent HE resistant character of fine-grained microstructures.