Chemical composition dependence of the strength and ductility enhancement by solute hydrogen in Fe–Cr–Ni-based austenitic alloys
Introduction
As a solution to recently escalating environmental problems, hydrogen (H) is attracting great attention for its utilization as an alternative energy carrier. However, a technical obstruction against the widespread commercialization of H-related systems is the degradation of mechanical performance in structural metals via hydrogen occlusion: hydrogen embrittlement (HE) [[1], [2], [3]]. The most straightforward effect of HE appears as the reduction in strength and ductility during conventional tensile tests [4,5]. In addition, hydrogen also triggers decreases in fracture toughness [6,7] and in material durability under cyclic fatigue loading [[8], [9], [10]]. These factors raise a long-standing industrial demand for the prevention methods for HE [3,11] and, in turn, the development of new materials possessing unprecedented hydrogen compatibility [12,13].
Among a variety of structural metals, austenitic alloys are now most frequently used for components subjected to direct gaseous hydrogen exposure. Empirically, the austenite phase, γ, with a face-centered cubic (FCC) crystal structure is known to be HE-insensitive owing to its low internal diffusivity of H atoms at ambient temperature (i.e., 10−15–10−14 m2/s) [[14], [15], [16]]. Nevertheless, the diversity of alloying compositions of austenitic alloys leads to a complex combination of various plastic deformation mechanisms [12,[17], [18], [19]], occasionally giving rise to serious unanticipated mechanical degradation via, for instance, the following microscopic rationales:
- (i)
Transformation to body-centered cubic (BCC), α′, martensite [[20], [21], [22], [23]].
- (ii)
Deformation twinning [[24], [25], [26]].
- (iii)
Transformation to hexagonal-close-packed (HCP), ε, martensite [[27], [28], [29]].
- (iv)
Planar dislocation glide and strain localization [[30], [31], [32], [33]].
This phenomenon (i) typically predominates the HE fracture of AISI Type 304 or 316 steels when they are deformed in their FCC-metastable temperature range [20]. The keys to understanding the degradation in such cases are the prompt escalation of hydrogen diffusivity and decrease of hydrogen solubility in response to γ-to-α′ phase transformation. Upon the transformation of H-rich γ into α′ martensite, oversaturated hydrogen in the product BCC phase tends to sink toward surrounding austenite, possibly resulting in a temporal hydrogen excess along the γ/α′ interface [22,23]. Furthermore, the BCC phase generated on free surfaces or crack tips acts as an occlusion pathway for H atoms if the material is loaded under an external H supply, such as a gaseous environment [20,34]. Meanwhile, the detrimental effects of (ii)∼(iv) emerge in Fe–Mn-based TWIP (twinning-induced plasticity) or TRIP (transformation-induced plasticity) steels [24,27], low stacking fault energy (SFE) alloys [5,30], high N-alloyed steel [31] and precipitation-hardened austenitic alloys [32,33,35]. Stress/strain concentrations at the growing end of twin/ε platelets, head of planar dislocation arrays and dislocation slip bands, which all stem from the inherent deformation characteristics of these materials, can exert some harmful impacts. Heretofore, hydrogen dissolved in metals is known to weaken the interatomic cohesive force [36,37], to encompass localization of plasticity by altering the gliding character of dislocations [[38], [39], [40]] and to change the phase stability of the austenite matrix [[41], [42], [43], [44]]. These unique functions of hydrogen itself, with the aid of distinct deformation modes of FCC austenite, render the nuclei of HE (i.e., internal voids or microcracks) initiated when the stress/strain concentration and hydrogen segregation at specific microstructural weakest links (e.g., grain or phase boundaries) simultaneously reach critical levels [22,24,38,45]. Stress-assisted diffusion [46], migration through pathways of the α' phase [34,47,48], statistical trapping [49] or transport by moving dislocations [45,50] may act here as the driving force for hydrogen accumulation. The enhancement of intergranular or interphase fractures by hydrogen is particularly remarkable in high-manganese steels with >10 mass % Mn or other alloy systems exhibiting γ-to-ε transformation [12,28,51,52]. This tendency is potentially ascribed to the reduction of intrinsic grain boundary cohesion via Mn alloying [53] and the difficulty of plastic relaxation at stress concentration sites due to deformation anisotropy of the HCP phase with an insufficient number of slip systems [12,28].
At the other extreme, however, a positive influence of solute hydrogen has notably been found to augment the material strength, i.e., solid-solution hardening, as with other conventional interstitial elements, including C and N [54]. Hydrogen greatly increases yield and flow stresses in pure Ni [55,56], Ni-Cu [57], Fe–Cr–Ni systems [58] and high-entropy alloys [52], although the coincidence of premature failure accompanying intergranular fracture is a critical drawback. Likewise, an improvement in ductility via hydrogen infiltration has also been reported [59,60]. In Fe–30Mn-based TWIP steel [59] and equimolar Fe–Mn–Ni–Cr–Co high-entropy alloy [60], uniform and fracture elongations were, on occasion, slightly increased. As opposed to the recognition of the negative impact of deformation twinning [[24], [25], [26]], such a ductility improvement was believed to be rooted in hydrogen accelerating the nucleation and growth of deformation twins [59,60]. Increasing the twin density promotes the work-hardening capacity; thus, the onset of plastic instability is accordingly retarded [19,61]. The criteria determining whether deformation twinning works positively or negatively in H-related fracture processes are still opaque. Nonetheless, exploring an optimized alloying composition that enables maximum synergy of the two beneficial aspects of hydrogen, with an effective mitigation of various embrittlement mechanisms, can be one of the strategies for the development of epoch-making hydrogen-compatible austenitic materials.
Recently, the authors of the present article incidentally found a commercially available austenitic steel in which cooperation of all those required conditions was successfully satisfied, i.e., Fe–24Cr–19Ni alloy (AISI Type 310S) charged with ∼130 mass ppm (∼7200 at ppm) hydrogen [62]. Both the tensile strength and elongation were significantly improved, rendering their product augmented up to ∼130% of the tensile strength and elongation in the nonhydrogenated situation. However, the identified steel contained large amounts of expensive elements, a challenge for its utilization in real industrial applications for hydrogen service. The next step toward a proactive alloy design desterilizing hydrogen as a useful element relies on deepening our understanding of the species and ranges of alloying compositions wherein the same H-effects manifest. That is, an additional consideration of economic efficiency aside from hydrogen compatibility is now desired for industrially acceptable alloy design.
In this study, as an initiative to tackle the aforementioned problem, we focus on the mechanical responses to hydrogen dissolution of Fe–Cr–Ni ternary (i.e., the most fundamental constituent for austenitic stainless steels) and Fe–Ni binary systems. In addition to the Fe–24Cr–19Ni alloy that we previously targeted, four other commercial austenitic alloys were newly prepared. These materials were uniformly hydrogenated under a pressurized gaseous environment at elevated temperature and subsequently subjected to tensile testing under ambient conditions. Alterations in yield/flow stresses as well as tensile elongation were systematically evaluated combined with analyses of work-hardening behavior and deformation microstructures. Based on these investigations, the rationales and general compositional requirements for the emergence of H-induced strengthening and ductilization were elucidated.
Section snippets
Materials and specimen
Four commercially available Fe-based austenitic alloys with different Cr and Ni content (mass %), 23Cr–13Ni (AISI 309S), 17Cr–12Ni (AISI 316L), 18Cr–35Ni (UNS N08330), and 0Cr–36Ni (UNS K93600), were selected in addition to 24Cr–19Ni (AISI 310S), which has already been investigated in our previous research [62]. These alloys were selected for significantly varying the fractions of Cr and Ni and for keeping the amounts of other minor alloying elements as low as possible. An attention was also
Nominal stress-strain responses
The nominal stress-nominal strain (σN-εN) curves of the noncharged specimens of all five alloys are presented in Fig. 3 (a), displaying remarkable differences in their mechanical properties under nonhydrogenated conditions. In 23Cr–13Ni and 18Cr–35Ni, somewhat high yield strengths (i.e., 0.2% proof stress, σ0.2) of ≈ 300 MPa were measured, whereas those in other alloys were approximately 230 MPa. The tensile strength, σB, was also classified into distinct levels: 600–650 MPa in 23Cr–13Ni and
Discussion
The most valuable finding in the present study was that strengthening and ductilization via hydrogen dissolution, which has already been identified in Fe–24Cr–19Ni [62], is neither an incidental nor a nonuniversal outcome. Rather, the emergence of the same effects was identified even in other Fe–Cr–Ni alloys with diverse chemical compositions. This striking fact instigates our aim for the development of new hydrogen-compatible materials in which hydrogen is utilized as a new beneficial alloying
Conclusions
Five Fe–Cr–Ni-based alloys were tensile-tested after uniform hydrogen precharging in a pressurized high-temperature gaseous environment. Hydrogen solubility and the impacts of dissolved hydrogen on yield/flow stresses, work-hardening and ductility were investigated. Combining macroscale testing with microstructural characterizations, the mechanisms and chemical composition dependence of the H-related changes in mechanical properties were discussed. Primary focus was placed on the requirements
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
CRediT authorship contribution statement
Haruki Nishida: Investigation, Writing – original draft. Yuhei Ogawa: Conceptualization, Writing – original draft, Supervision, Funding acquisition. Kaneaki Tsuzaki: Conceptualization, Writing – review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was partially supported by JSPS KAKENHI (Grant Numbers: 21K14045 and 21K04702). Y.O. would also like to acknowledge financial support from JFE 21st Century Foundation and The Iwatani Naoji Foundation. The authors are grateful to Prof. Osamu Takakuwa at Kyushu University for his helpful comments on the development of our discussion in Section 4.1.
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