On the hydrogen embrittlement behavior of nickel-based alloys: Alloys 718 and 725

Nickel-based superalloys have attracted immense attention in the oil and gas industry due to their outstanding combination of mechanical properties and corrosion resistance. In corrosive service environment, hydrogen embrittlement is a severe issue. In the present work, the susceptibility of two precipitation-hardened nickel-based alloys, i.e., Alloy 718 and Alloy 725, to hydrogen embrittlement was studied using slow strain-rate tensile test and advanced characterization techniques. The mechanical properties and fracture behavior of these two alloys were compared in both hydrogen-free and hydrogen-charged conditions. In the presence of hydrogen, Alloy 718 failed prevalently through a combination of transgranular and intergranular cracking behavior, while Alloy 725 failed primarily through intergranular failure with a considerably lower resistance to hydrogen embrittlement. This distinction was attributed to their different microstructures and different types of precipitates along grain boundaries. Specifically, in Alloy 725, the decoration of (Cr, Mo)-rich precipitates at grain boundaries distort the local structures and cause such boundaries to be vulnerable to hydrogen attack, thus promoting intergranular cracking.