PSI - Issue 2_A

G Sudhakar Rao et al. / Procedia Structural Integrity 2 (2016) 3399–3406 G.S. Rao et al./ Structural Integrity Procedia 00 (2016) 000–000

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active slip systems. Both, dislocation density and their mobility are increased in presence of hydrogen. Any variation in local hydrogen content will result in strain localization and vice versa. 3. Hydrogen-enhanced strain-induced vacancies (HESIV), Nagumo (2001): Hydrogen atoms reduce the formation energy of vacancies and facilitate their agglomeration/clustering and collapse to microvoids. Very high vacancy and hydrogen concentrations are observed in highly deformed, strain-hardened regions that affect the plastic deformation behavior (climbing of edge dislocations (softening), vacancy clusters and nanovoids as dislocations obstacles (hardening)) and enhance microvoid coalescence. Combinations and synergies of these interrelated mechanisms are possible in RPV steels under LWR conditions and their relative contributions depend on variables such as temperature, strain rate and microstructure, although most hydrogen effects are often believed to disappear above temperatures of 200 to 300 °C based on the typical hydrogen trapping/binding energies and very high mobility of hydrogen. HELP and HESIV are not only enhancing ductile failure by microvoid coalescence (MVC), but also facilitating brittle cleavage or quasi-cleavage cracking by various mechanism (such as due to dislocation pile-ups), at interfaces such as second phase particles or grain boundaries or along preferred highly active slip planes and their intersections. HEDE may explain the quasi-brittle features around oxide and MnS-inclusions or intergranular cracking. As soon as the local hydrogen concentration in the process zone reaches a critical value over a critical volume, ductile/shear or brittle crack initiation in the process zone at inclusions or strong trap centers may occur by various local hydrogen embrittlement mechanisms. The magnitude of embrittling effect is finally governed by the availability of hydrogen, its transport to susceptible locations and the nature and concentration of trap centers and defects in the susceptible location. The reduction in ultimate tensile strength (softening) by hydrogen may be rationalized by the shielding effect by hydrogen that is dependent on temperature, strain rate and strain, Birnbaum (1994), Robertson (2001), Reedhill (2004). The softening by shielding inherently results in strain localization and further hydrogen accumulation in these regions with positive feedback effects (HELP) that, depending on the extent of shear localization in the gauge section and the magnitude of softening effect of hydrogen on the dislocation mobility, can result in macroscopic hardening or softening. At the onset of yielding, shielding effects are absent due to the low dislocation density. With increasing plastic strain, dislocation density/distance increases/decreases and dislocations start to increasingly interact with each other and shielding and strain rate effects become more significant. This may finally result in early saturation of the capacity for uniform elongation (i.e. lower uniform strain). Subsequent formation of microvoids results in higher interaction of hydrogen and enhances localized plasticity in the intermediate regions of the voids. The stress concentration and the triaxial stress state in this region further amplify the hydrogen accumulation, which may increase the shear stress component through localization causing shear dominating failure of the material. This thus explains the more pronounced effect of hydrogen on the reduction of area than on yield stress/strength or uniform elongation strain. Additionally, the hydrogen enhanced formation of strain-induced vacancies, their agglomeration/clustering and collapse to microvoids (HESIV) may be a further contributing factor towards reducing ductility. The interaction of deformation bands with prior austenite grain boundaries may result in intergranular cracking, in particular, when the strength of the grain boundary is reduced due to hydrogen (HEDE) and/or metalloid segregation and/or if slip transfer across the boundary is impeded (e.g., by a large misorientation). Synergy with DSA probably arises through the formation of localized deformation bands by DSA, whereas the moderate increase of strength is believed to be a second order effect. Shielding effects by hydrogen might particularly occur in highly deformed shear bands with planar slip, high dislocations densities, highly mobile dislocations and very high local hydrogen concentrations and in dislocation pile-ups at obstacles in such bands. At 288 °C, hydrogen embrittlement due to softening in the RPV steel with low DSA susceptibility is only observed in a narrow strain rate range with a maximum at 10 -2 s -1 . The maximum in embrittlement might be related to shielding effects and the matching of hydrogen atom and dislocation mobility and the coincidence with the maximum in strain localization by DSA at this temperature strain rate combination. Furthermore, at the lower strain rates (  ~10 -4 s -1 ), hydrogen effusion might be too strong to keep a sufficiently high residual hydrogen content in the steel, although hydrogen could easily follow the dislocations. In the DSA range, materials with high susceptibility to DSA exhibit inhomogeneous plastic deformation with plastic strain localization (with macroscopic hardening) due to the negative strain rate sensitivity, an increase of planar deformation and dislocation density and in deformation bands that can act as regions for hydrogen accumulation, which may further amplify the localization of plastic deformation by hydrogen and hydrogen accumulation in these regions. In RPV steels with high DSA susceptibility, very high local