Hydrogen embrittlement effect observed by in-situ hydrogen plasma charging on a ferritic alloy

Abstract

Hydrogen embrittlement (HE) is a well-acknowledged phenomenon in metals. It draws much scientists'' and engineers'' attention because it can lead to catastrophic failure of an industrial structure.Researchers use different methods to charge a test piece with hydrogen (H) and study the behavior of the material during or after charging. A common method is to use H 2 gas as the environment, for example Vehoff [1,2] used gaseous H to study the H effect on the fracture behavior of ferritic steels. More examples can be found in Ref. [3][4][5][6][7][8][9][10][11][12]. Furthermore, electrochemical cathodic charging is also commonly used, for example Wang et al. [13] used cathodic charging with an in-situ tensile test to study the effect of H on mobile dislocations, and the H concentrations were also determined. Other examples can be found in Ref. [14][15][16][17][18][19][20][21][22] as well.There is also a smart method for special materials, for example, Deng et al. [23,24] and Rogne et al. [25] used water vapor to charge Fe-Al intermetallic alloys since the H 2 O molecule can react with Al, producing alumina and H to invade the matrix.Due to the high diffusivity of H in body-centered cubic (BCC) lattice [13], it is always critical to have H pre-charging on the test piece because H can diffuse out even during a short transferring procedure.Therefore, in-situ H-charging is favored for the HE study of BCC structures.Currently, in-situ charging of ferritic steel is either done in high pressure hydrogen gas or under electrochemical control by cathodic H-charging. This makes the high-resolution observation of the sample during deformation challenging.Environmental scanning electron microscopy (ESEM) combined with miniaturized tensile stages provides a unique possibility to test the effect of H on mechanical deformation, however the limited maximum pressure inside the chamber and restrictions on the possible gases at that pressure in the chamber from the manufacturer hinders the HE tests inside the ESEM. At low pressures, the fugacity of H is not enough to have adequate physisorption and dissociation of the H 2 molecules on the surface to observe HE within a reasonable time scale. Converting H 2 gas to H plasma before letting it enter the ESEM chamber will increase the H fugacity in the chamber and provide possibilities to observe the HE processes inside the ESEM.Narita [26] used H glow charging to study the embrittlement effect in a Fe-Si system with a tensile test on single crystals and concluded that this method is capable of charging the samples with enough H to reveal an embrittlement effect without severe surface damage. Kimura and Birnbaum [27] observed that that H-plasma charging can cause a softening effect in the flow stress of pure iron samples. A recent work from Malitckii et al. [28] used H-rich plasma at elevated temperatures to charge two kinds of steels and successfully revealed the HE effect on them.

Hydrogen-enhanced fatigue crack growth in a single-edge notched tensile specimen under in-situ hydrogen charging inside an environmental scanning electron microscope

“…8 was comparable with that under monotonic tensile loading in Ref. [31], with the only difference from fatigue striations. The characteristic "quasi-cleavage" facets were observed in the H-charged case in both works, c.f.…”

“…In a previous study [31], the HE effect on the same ferritic alloy was observed by Hplasma charging during slow strain rate tensile tests. With monotonic loading, the global elongation (final elongation value in percentage up to complete fracture) was reduced by about 5% in comparison with the reference test in Vac.…”

“…The H-charging method was the same as described in Ref. [31]. According to the report from XEI Scientific Inc. [32], the H radicals created by the plasma source contained both H atoms and excited hydrogen gas molecules.…”

“…The charging conditions were kept constant during the whole test, and the loading condition could be measured and controlled precisely by the miniature testing module, indicating a well-constrained setup for the H-uptake. According to a previous work from the authors' research group [31], the fugacity of the dissociated H atoms can reach a value larger than 80 Pa in the vacuum chamber, and can even reach several MPa locally [33]. However, due to limited thermodynamics data in the plasma phase and the missing physics in the H uptake mechanism from plasma phase, only indirect values of the H concentration could be assumed.…”

Understanding the Interaction between a Steel Microstructure and Hydrogen

“…These HSS show, however, a multiphase microstructure, complicating the interpretation of H related data. Therefore, the interaction between H and single-phase alloys is also of interest to get an improved understanding of the damaging mechanisms [ 34 , 35 , 36 , 37 ]. Additionally, the presence of carbides is generally considered to be beneficial to improve the HE resistance since they are assumed to trap H efficiently removing the detrimental mobile H from the microstructure [ 38 , 39 ].…”