The effect of the scarcely reported F phase on hydrogen-assisted cracking in nickel-based Alloy 725 was thoroughly studied by combining tensile tests, advanced characterization, and density functional theory (DFT) calculations. The results show grain boundary precipitate F phase promotes intergranular fracture in a hydrogen environment. DFT calculations further indicates hydrogen atoms lower the binding strength of the F phase and Ni matrix interfaces. More importantly, our study showed for the first time that the addition of approximately 0.01 wt.% boron can effectively suppress F phase precipitation, thereby elevating the hydrogen resistance of Alloy 725.

The effect of hydrogen on the surface morphology and nanomechanical properties of Ni-based Alloy 725 under solution-annealed (SA) and precipitation-hardened (API) conditions was thoroughly studied. The investigation involved in situ nanoindentation testing, microscopy characterization, statistical analysis, and numerical simulation approaches. The results showed the distinctive effects of hydrogen on the pop-in and hardness in the SA and API samples. For the SA sample, hydrogen mainly dissolved as solid solute in the matrix, causing enhanced lattice friction on the dislocation motion and increasing the internal stress via lattice expansion. Thus, an enhanced hardness, a reduced pop-in width/load ratio, and numerous surface steps were detected in the presence of hydrogen. For the API sample, the strengthening γ′′ phases were the stress concentrators, and the dislocations nucleated heterogeneously, demonstrating indistinctive pop-in phenomena. Furthermore, the precipitates in the API sample affected the trapping behavior of hydrogen, thereby resulting in the hardness change, which reflected the competition between solution hardening in the matrix and vacancy softening mechanism in precipitates.

Fatigue crack growth (FCG) tests were conducted on a medium-Mn steel annealed at two intercritical annealing temperatures, resulting in different austenite (γ) to ferrite (α) phase fractions and different γ (meta-)stabilities. Novel in-situ hydrogen plasma charging was combined with in-situ cyclic loading in an environmental scanning electron microscope (ESEM). The in-situ hydrogen plasma charging increased the fatigue crack growth rate (FCGR) by up to two times in comparison with the reference tests in vacuum. Fractographic investigations showed a brittle-like crack growth or boundary cracking manner in the hydrogen environment while a ductile transgranular manner in vacuum. For both materials, the plastic deformation zone showed a reduced size along the hydrogen-influenced fracture path in comparison with that in vacuum. The difference in the hydrogen-assisted FCG of the medium-Mn steel with different microstructures was explained in terms of phase fraction, phase stability, yielding strength and hydrogen distribution. This refined study can help to understand the FCG mechanism without or with hydrogen under in-situ hydrogen charging conditions and can provide some insights from the applications point of view.

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.