Search

Compositionally-graded austenitic 316L stainless steel (SS) samples with five different Hafnium (Hf) concentrations (up to 1 wt.%) were additively manufactured by directed energy deposition and then were irradiated using 5 MeV Fe2+ ions to 50 displacements per atom (dpa) at 500, 550, and 600 °C, respectively. A rastering beam was used to ensure homogenous irradiations for the large areas of ∼1.40 cm2. Composition-dependent void evolution was evaluated. Void size, void number density, and void swelling all tend to decrease with increasing Hf concentration at all three temperatures. When the nominal Hf concentration increases to 1 wt.% in the doped additively manufactured (AM) 316L SS, the void swelling is over an order of magnitude lower than that in pure AM 316L. Atom probe tomography results showed that about 0.14 wt.% Hf dissolved in the matrix at the 1 wt.%. Hf nominal concentration. The suppression of void swelling by Hf addition shows qualitative agreement with the numerical calculation based on the vacancy trapping mechanism, indicating that the oversized Hf reduces the steady state vacancy concentration, and hence the incubation period of swelling is extended, resulting in a dramatic difference in void swelling. Other factors including grain boundaries, dislocation and dislocation cells, Hf-rich particles, and delta ferrite are secondary to this mechanism. This work shows that additive manufacturing enabled microalloying is promising for developing void swelling resistant materials. [ABSTRACT FROM AUTHOR]

A uranium-10 wt% molybdenum (U10Mo) alloy is one of the primary candidates for metallic fuels that would use low-enriched uranium in place of highly enriched uranium, to support nuclear nonproliferation efforts. Optimal performance of a U-based metallic nuclear fuel can be achieved by retaining the high-temperature, body-centered cubic (bcc) allotrope (γ-U) at room temperature, which can be accomplished in the U10Mo alloy. However, presence of minor alloying elements can influence the final constitution of room-temperature phases in the U10Mo alloy, specifically, formation of α-U phase which results in anisotropic behavior of the fuel in reactor. Through a detailed transmission electron microscopy analysis, the present study reports the constituent phases that are present in a U10Mo alloy containing ~0.1 wt% Si after it is subjected to homogenization heat treatment and thermomechanical processing. For comparison, results from an undoped U10Mo alloy are also included. The experimental results reveal that γ-UMo solid solution is the major phase in a hot-rolled, Si-doped U10Mo alloy metallic fuel foil, along with U 2 MoSi 2 C, UC, and U 2 Mo, after isothermal annealing at 460 °C for 10 h. In contrast, after the same heat treatment, the undoped U10Mo alloy metallic fuel had formed a noticeable amount of α-U along prior γ-UMo grain boundaries through discontinuous precipitation (DP, area fraction: ~27.9 %) with characteristic lamellar morphology, together with γ-UMo, UC, and U 2 Mo. This result indicates that doping with Si could mitigate the DP reaction in U10Mo alloy and prevent formation of undesirable α-U. This work sheds light on optimizing Si-doping–dominated microstructure in U10Mo fuels and facilitates designing and tuning of microstructures of U10Mo alloys for tailoring the final designed performance of the fuel under irradiation. • Major phases in Si-doped homogenized U-10Mo sample include γ-UMo, U 2 Mo, UC, and U 2 MoSi 2 C. • After hot rolling, U 2 Mo, UC, and U 2 MoSi 2 C are there with a few α-U and UO 2 nanoparticles detected at GBs. • The area fraction of DP zones in undoped, hot-rolled U-10Mo is ~30%. Major phases after DP in hot-rolled undoped U-10Mo are γ-UMo, α-U, UC, and U 2 Mo. • Si preferentially segregates at the α-U lamellae/γ-UMo interfaces rather than at the γ΄-U 2 Mo/γ-UMo interface, and retards DP-based eutectoid transformation. [ABSTRACT FROM AUTHOR]

• The acceleration mechanism in oxidation kinetics ahead of crack tip was clarified. • Compact chromia forms along identical carbide/matrix interfaces at different rates. • Stress concentration near the crack tip only slightly increases the oxidation rate. • Deformation localization near the crack greatly promotes oxygen diffusivity. The acceleration in oxidation kinetics beyond a stress corrosion crack of alloy 690 in simulated PWR primary water was clarified by comparing the oxide depths at two identical carbide/matrix interfaces along a single grain boundary. Compact chromia layer formed along these two interfaces at vastly different rates. Stress concentration ahead of the crack tip only slightly increased the oxidation rate as the crack is still very shallow. Deformation localization near the crack plays a dominant role in accelerating oxidation kinetics as the associated high vacancy concentration can promote the diffusivity of oxygen by over an order of magnitude. [ABSTRACT FROM AUTHOR]

The creep behaviors of laser powder bed fusion (LPBF) additively manufactured (AM) 316L stainless steel (SS) and its recrystallized (Re) counterpart were investigated via uniaxial constant-load creep tests at 600 °C with nominal stress levels ranging from 235 to 360 MPa. Anisotropic creep behavior was observed in AM 316L SS, with superior creep resistance but inferior creep ductility in the horizontal sample (loaded perpendicular to the build direction (BD)) compared to the vertical sample (loaded parallel to the BD). This superior creep resistance was likely resulted from shorter dislocation slip distance and the inferior creep ductility was due to faster propagation of cracks along the columnar grain boundaries. Compared with both the Re counterpart and conventional 316L SS, AM 316L SS in this study exhibited an extraordinary creep resistance at various stress levels, with the minimum creep rate being two to three orders of magnitude lower and much longer creep life. This exceptional creep resistance of AM 316L SS was attributed to the presence of dislocation cells that impeded the deformation-induced dislocations. This led to a remarkably low rate of creep deformation and delayed the creep crack initiation, ultimately resulting in a long creep life. The gradual development of the precipitate films enriched with Mo, Si and Cr along high-angle grain boundaries, following prolonged exposure to high temperatures, was found to restrict the creep ductility in AM 316L under low stress conditions. Nevertheless, the study demonstrates that the stable dislocation cells are beneficial in enhancing the high-temperature creep resistance of AM 316L SS. • AM 316L SS shows better creep resistance than its recrystallized counterpart. • Printed microstructure induces anisotropic creep behavior in AM 316L SS. • Stable dislocation cells in AM 316L SS may reduce creep deformation rate. [ABSTRACT FROM AUTHOR]

Diverse polypods of silver phosphate form via mismatched but oriented interfaces, which are induced by formation of pyrophosphates and stabilized by rotation of phosphates and deformation of Ag-O bonds. [Display omitted] • Diverse Ag 3 PO 4 polypods form via mismatched but oriented interfaces. • Formation of pyrophosphates induces the formation of mismatched but oriented interfaces. • Rotation of phosphates across the interface compensates for the lattice mismatch at the interfaces. • Deformation of Ag-O bonds compensates for the lattice mismatch at the interface. Interfaces and their misfit defects determine the material properties of a wide range of applications, such as electronic devices, photocatalysts, and structural materials. However, current atomic-level understanding of interfacial structures is limited. Here we reveal that a special interfacial structure, a mismatched but oriented interface formed by two differently structured facets, gives rise to Ag 3 PO 4 polypods at room temperature in aqueous solution. Transmission and scanning electron microscopy results suggest that interfaces of {1 0 0} and {1 1 0}({1 0 0}/(1 1 0}), {1 0 0}/{1 1 1}, {1 1 0}/{1 1 1}, and {1 0 0}/{1 0 0} have certain orientation relationships, corresponding to the structural energy minima and coincident site lattices of interfacial atoms, as demonstrated by molecular dynamics simulations. Density functional theory calculations indicate that the formation of pyrophosphates and/or phosphates rotation across the interface, as well as deformation of Ag-O bonds, compensate for the lattice mismatch at the interfaces. Our work opens a new avenue for a much wider range of interfacial structures, allow for a higher diversity of structures, and shines light on tailoring crystal structures, morphologies, and their resulting properties. [ABSTRACT FROM AUTHOR]

The time dependence of proton irradiation effect on the intergranular oxidation of 316 L stainless steel in simulated PWR primary water was clarified for the first time by ruling out the interference from grain boundary structure dependence. Interestingly, proton irradiation has an acceleration effect on the intergranular oxidation of 316 L stainless steel after shorter-term oxidation (less than 500 h) but a mitigation effect at later stage (more than 500 h). It was found that Cr depletion at the pristine grain boundary due to radiation-induced segregation (RIS) plays a dominant role at the early stage of oxidation, resulting in a deeper intergranular oxidation than in the non-irradiated region. Nevertheless, the Cr content at the intergranular oxide tip in the irradiated region builds up faster than in the non-irradiated region with time due to the enhanced solute (especially Cr) diffusivity in the irradiated region. The positive effect of enhanced Cr diffusivity on resistance to intergranular oxidation gradually dominates after longer-term immersion and eventually leads to a shallower intergranular oxide penetration in the irradiated region. The faster intergranular oxidation in irradiated region due to RIS at the early stage partly explains the accelerated crack propagation of irradiated material as the oxidation condition at the crack tip is similar to that at the early stage. [Display omitted] [ABSTRACT FROM AUTHOR]

The irradiation accelerated corrosion within monolayer grains of a new AFA steel exposed to sCO 2 are studied. The presence of irradiation induced dislocation loops accelerate element diffusion and reduces corrosion resistance by ∼44%. Many oxide scale features are firstly found, including a two-phase inner layer consisting of Ni-rich austenite and Cr-rich spinel, and Al 2 O 3 /SiO 2 oxide precipitates. Irradiation leads to an additional Cr-rich oxide layer at the bottom of inner layer and a narrower Al 2 O 3 band in the internal oxidation layer. Irradiation also induces Cr depletion at grain boundaries, thereby impeding the formation of a protective Cr-rich oxide scale. • Coupling effects of irradiation and corrosion of a new AFA steel exposed to sCO 2 are firstly studied. • Irradiation induced dislocation loops accelerate the element diffusion, while it is not enough to form a protective Cr 2 O 3 /Al 2 O 3 in sCO 2. • A two-phase inner layer (Ni-rich austenite + Cr-rich spinel), and Al 2 O 3 /SiO 2 oxide precipitates are formed in sCO 2. • Irradiation results in an extra Cr-rich oxide layer and a narrower Al 2 O 3 band near the O/M interface. • Irradiation induces Cr depletion at grain boundaries, then the protective Cr-rich oxide scale cannot be easily formed here. [ABSTRACT FROM AUTHOR]

• As-built DED 316L SS exhibited intragranular IASCC. • Fine delta ferrite particles were found from as-built DED AM 316L SS. • Delta ferrite particles impeded dislocation gliding and prevented GB sliding. • Strain localization near GBs was confirmed by nanoindentation. In this study, irradiation-assisted stress corrosion cracking (IASCC) resistance of as-built AM 316L stainless steel (SS) made by direct energy deposition (DED) was evaluated in a simulated boiling water reactor (BWR) environment. Different from the intergranular cracking typically seen from 316L SS, unique intragranular cracking away from grain boundaries (GB) was observed. The presence of high-density micron-sized delta ferrite near the melt pool boundaries promoted the in-grain strain localization and crack nucleation. Delta ferrite particles can efficiently retard dislocations during deformation, altering the strain localization from the GB to its surrounding area and thus reducing the chance of GB cracking. The phenomenon was further confirmed by the higher nanoindentation hardness measured from the corresponding area, and by characterizing the evolution of microstructure, cracking morphology, and dislocation channels. [ABSTRACT FROM AUTHOR]

Irradiation-assisted stress corrosion cracks (IASCCs) from a proton-irradiated 316 L stainless steel (2.5 dpa) after constant load test in simulated PWR primary water were analyzed. The irradiation damages are consistent with those from neutron irradiations at similar dose. The IASCC cracks show microstructural and microchemical features similar to those reported from neutron-irradiated samples, confirming that proton is effective in emulating neutron for IASCC study. Si segregated at grain boundary gets oxidized preferentially beyond the crack tip and then dissolves into solution, resulting in enhanced porosity within the intergranular oxide. The Si effect is an important factor behind IASCC in reducing environment. • Proton irradiation induces similar IASCC features in SS as neutron irradiation. • Intergranular oxidation is a necessary precursor of IASCC of SS at low doses. • Si tends to get oxidized preferentially and dissolves during intergranular oxidation. • The Si effect is likely a key element of IASCC mechanism for stainless steel. [ABSTRACT FROM AUTHOR]