The Hydrogen Trapping Potential of Cr-rich M23C6 and M7C3 Carbides in Hybrid Steel Alloys Investigated by Atom Probe Tomography

Hydrogen is a key element in the transition to a fossil-free society and the application of sustainable technologies. However, the presence of H presents challenges for a lot of materials. Hydrogen gas can dissociate on the material’s surface and be absorbed as atomic H and diffuse into the alloy. H is attracted to high stress areas and promotes crack formation and propagation in these regions. High-strength steels are especially prone to premature failure, which is called hydrogen embrittlement (HE) [1]. One way to prevent detrimental H diffusion is to introduce H traps [2]. Several microstructural features can act as traps and provide different ‘trapping strengths’, usually measured as de-trapping activation energy using thermal desorption mass spectrometry (TDMS). Dislocations and grain boundaries are weak traps and may release H back into the matrix during operation [3]. In contrast, carbides are known as strong H traps that can render H immobile, and thereby preventing its contribution to HE [4], [5], [6]. To gain spatial information of H trapping sites at nano-sized carbide precipitates, atom probe tomography (APT) is necessary. This method provides near atomic spatial resolution with high sensitivity of all elements including H [7], [8], [9]. H measurement is still challenging since H2 gas is the main residual gas in the ultra-high vacuum chamber and it is ever present in APT measurements [10]. To circumvent this, the heavier isotope deuterium (D) is used to get distinct information of filled H traps. A continuous cryogenic transfer between D charging and APT measurement was emphasized previously [11]. However, it was shown recently by the authors that strong enough traps can retain D even after a room temperature transfer [12]. Hybrid steel from Ovako AB contains two types of nanosized precipitates, intermetallic NiAl and Cr-rich carbides, in a martensitic matrix. Its alloy design allows for high mechanical strength with good fatigue and corrosion properties [13]. The carbides provide abundant trapping sites for H and should give the material good resilience against HE. APT specimens are made from Hybrid 60 and a Hybrid model alloy with no V and limited Mo as alloying elements. Heat treatments are performed to achieve similar sized precipitates in the two steels. The APT samples are electrolytically D charged in 0.1 M NaOH solution in heavy water at 2.2 V against a Pt counter electrode for 30 s. The samples are moved to the cryogenically cooled analysis stage of the APT instrument within 15 minutes after charging, thereby retaining D at the strong trapping sites. APT measurements allow the identification of prevalent carbides, namely Cr-rich M23C6, Cr-rich M7C3, and sporadic V-rich MC in the case of Hybrid 60. The carbides can be visualized by iso-concentration surfaces, and the concentrations within and near the precipitates are displayed by proximity histograms. In this way, the D concentration in connection to the carbides is illustrated and can be compared between Hybrid 60 and the model alloy. TDMS measurements of both materials allow for a comparison of the amount of trapped H as well as an estimation of de-trapping activation energies. In this way, the influence of V and Mo additions in Hybrid 60 and the mixed metal content in the carbides can be elucidated. A: APT reconstruction of a D charged Hybrid 60 sample; Intermetallic NiAl precipitates are depicted with green iso-surfaces and carbides with 4 at.% C iso-surfaces in brown. A carbide-decorated lath boundary is visible in the middle of the reconstruction. B: Selected range of the mass-spectrum showing H and D peaks as well as double-charged C. No peak at 2 Da is visible in uncharged measurements. C: Proximity histogram calculated from 4 at.% C iso-surfaces depicting the concentration of elements in connection with the carbides. The D concentration is elevated already near the interface and within the carbides.