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Irradiation induced creep (IIC) compliance in NiCoFeCrMn high entropy alloys is measured as a function of grain size (30 < x < 80 nm) and temperature (23–500 °C). For 2.6 MeV Ag3+ irradiation at a dose rate of 1.5×10–3 dpa−1s−1 the transition from the recombination to sink limited regimes occurs at ∼ 100 °C. In the sink-limited regime, the IIC compliance scales inversely with grain size, consistent with a recently proposed model for grain boundary IIC. The thermal creep rate is also measured; it does not become comparable to the IIC rate, however, until ∼ 650 °C. The results are discussed in context of defect kinetics in irradiated HEA systems. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The evolution of radiation-induced defects in Ni and the single-phase concentrated solid solution alloy, NiCoCr, were investigated during in situ 2.8 MeV Au ion irradiation and post-irradiation analysis using transmission electron microscopy. Compared to Ni, both the size and area density of defect clusters decreased in NiCoCr under the same irradiation conditions, suggesting that the chemical complexity, i.e., randomness of lattice site occupations, of NiCoCr suppressed radiation-induced damage. One-dimensional glide of defect clusters was observed in Ni but not in the NiCoCr alloy. The structural nature of small defect clusters in NiCoCr were further investigated using high-angle annular dark field scanning transmission electron microscopy. • Smaller and fewer defects were generated in more chemically-complicated alloys. • One-dimensional gild of defect clusters was observed in Ni but not in NiCoCr. • Both vacancy-type and interstitial-type defect clusters were identified in NiCoCr. [ABSTRACT FROM AUTHOR]

High entropy alloys have gained significant interest due to several unique properties including enhanced radiation resistance. In this work, radiation induced segregation, a key phenomenon observed in alloys under irradiation, is examined for the first time at high angle grain boundaries under Ni heavy ion irradiation in the CoCrFeNiMn alloy. Our experimental study indicates significant Mn depletion and Co and Ni enrichment at grain boundaries. The segregation is discussed in the context of a proposed vacancy dominated radiation induced segregation mechanism and compared to existing models in conventional single core component alloys including stainless steels. [ABSTRACT FROM AUTHOR]

Radiation induced segregation in austenitic Fe-Ni-Cr stainless steels is a key detrimental microstructural modification experienced in the current generation of light water reactors. In particular, Cr depletion at grain boundaries can be a significant factor in irradiation-assisted stress corrosion cracking. Therefore, having a complete knowledge and mechanistic understanding of radiation induced segregation at high dose and after a long thermal history is desired for continued sustainability of existing reactors. Here, we examine a 12% cold worked AISI 316 stainless steel hexagonal duct exposed in the lower dose, outer blanket region of the EBR-II reactor, by using advanced characterization and analysis techniques including atom probe tomography and analytical scanning transmission electron microscopy. Contrary to existing literature, we observe an oscillatory w-shape Cr and M-shape Ni concentration profile at 31 dpa. The presence and characterization through advanced atom probe tomography analysis of the w-shape Cr RIS profile is discussed in the context of the localized GB plane interfacial excess of the other major and minor alloying elements. The key finding of a co-segregation phenomena coupling Cr, Mo, and C is discussed in the context of the existing solute segregation literature under irradiation with emphasis on improved spatial and chemical resolution of atom probe tomography. [ABSTRACT FROM AUTHOR]

Grain boundary engineering of Fe-based austenitic stainless steels and other materials has been successful in producing a large increase in twin and twin related grain boundaries from a wide range of thermomechanical treatments. However, the exact mechanisms and effective grain boundary network descriptors to create the heavily twinned microstructures are yet to be fully understood. In this study, we provide insight into the grain boundary engineering process by examining sequential progression of the same spatial location of a twin related microstructure through thermomechanical processing. The results show that clusters of twin related grain boundaries called twin related domains form during primary recrystallization. The size of the twin related domains increases as the level of strain falls toward the critical strain for recrystallization. Growth of twin related domains during recrystallization results in the formation of twin boundaries behind the migrating grain boundary front. Formation of higher order twin boundaries occurs when two separate grain boundary fronts of the same twin related domain impinge upon each other. We also present relevant microstructural descriptors with emphasis on twin related domain statistics to recrystallization phenomena in grain boundary engineering materials. [ABSTRACT FROM AUTHOR]

Abstract: Radiation-induced segregation (RIS) and subsequent depletion of chromium along grain boundaries has been shown to be an important factor in irradiation-assisted stress corrosion cracking in austenitic face-centered cubic (fcc)-based alloys used for nuclear energy systems. A full understanding of RIS requires examination of the effect of the grain boundary character on the segregation process. Understanding how specific grain boundary structures respond under irradiation would assist in developing or designing alloys that are more efficient at removing point defects, or reducing the overall rate of deleterious Cr segregation. This study shows that solute segregation is dependent not only on grain boundary misorientation, but also on the grain boundary plane, as highlighted by markedly different segregation behavior for the Σ3 incoherent and coherent grain boundaries. The link between RIS and atomistic modeling is also explored through molecular dynamic simulations of the interaction of vacancies at different grain boundary structures through defect energetics in a simple model system. A key insight from the coupled experimental RIS measurements and corresponding defect–grain boundary modeling is that grain boundary–vacancy formation energy may have a critical threshold value related to the major alloying elements’ solute segregation. [Copyright &y& Elsevier]

Abstract: Under REDD+, the enhancement of carbon stocks through reforestation and restoration of degraded forest landscapes offers considerable potential benefits. In the Asia–Pacific region, however, many previous reforestation and forest restoration initiatives have exacerbated existing inequities by concentrating resources among powerful political and economic actors, often to the detriment of forest-dependent communities. Both in design and implementation, tree-planting programs have been guided by forest rent distribution practices of state forestry bureaucracies and by corporate accumulation strategies linked to increasingly globalized commodity chains. This article examines the political economy of reforestation and forest restoration programs in Asia–Pacific and highlights governance challenges these pose. In various contexts, they have: [•] consolidated the control of state agencies and corporate actors over ‘degraded’ forest landscapes, often resulting in the displacement of rural communities; [•] exacerbated economic disparities by channeling large capital subsidies and resource rents to companies with close ties to state elites; [•] facilitated corruption and financial fraud, in some cases on a grand scale; [•] accelerated biodiversity loss by creating perverse incentives for the conversion of ‘degraded’ secondary forests; and [•] generated mixed results for rural small-holders, at times locking them in inequitable agreements with plantation companies, and in other cases, offering enhanced tenure security. To avoid the negative results of earlier reforestation schemes, REDD+ must incorporate: (1) rights-based spatial planning; (2) equitable and accountable distribution of financial incentives; (3) improved financial governance to prevent corruption and fraud; (4) policy reform to remove perverse incentives for forest conversion; (5) strengthening of economic benefits and safeguards for small-holders. [Copyright &y& Elsevier]

Small-scale in situ transmission electron microscopy-based Coble creep experiments were performed on Al 2 O 3 -GdAlO 3 composites using localized laser heating. A primary goal of the work was to isolate the Al 2 O 3 -GdAlO 3 interphase boundary diffusivity in order to understand how it contributes to the average properties of the composite. The diffusivities of the grain boundaries (GB) of GdAlO 3 (GAP) and Al 2 O 3 were measured to be D Gb , GAP = 15 ± 14 exp − 580,000 ± 62 , 000 J mol − 1 RT m 2 s − 1 , and D GB , Al 2 O 3 = 25 ± 21 exp − 542,000 ± 30 , 000 J mol − 1 RT m 2 s − 1 , respectively. The average interphase boundary (IPB) diffusivity exceeds that of the grain boundaries and was measured to be D IPB = 15 ± 14 exp − 559,000 ± 117 , 000 J mol − 1 RT m 2 s − 1 . Capillary smoothing experiments after creep were used to determine surface (S) diffusivities, D S , Al 2 O 3 = 3.2 x 10 3 ± 2.7 x 10 3 exp − 539,000 ± 26 , 000 J mol − 1 RT m 2 s − 1 and D S , GAP = 4.6 x 10 4 ± 7.7 x 10 − 6 exp − 625,000 ± 82 , 000 J mol − 1 RT m 2 s − 1 . This works demonstrates the feasibility of small-scale Coble creep experiments to directly measure individual components of grain boundary and IPB diffusivities in composites. [ABSTRACT FROM AUTHOR]

This work explores the development of a heterogeneous nanostructured material through leveraging abnormal recrystallization, which is a prominent phenomenon in coarse-grained Ni-based superalloys. Through synthesis of a sputtered Inconel 725 film with a heterogeneous distribution of stored energy and subsequent aging treatments at 730°C, a unique combination of grain sizes and morphologies was observed throughout the thickness of the material. Three distinct domains are formed in the aged microstructure, where abnormally large grains are observed in-between a nanocrystalline and a nanotwinned region. In order to investigate the transitions towards a heterogeneous structure, crystallographic orientation and elemental mapping at interval aging times up to 8 h revealed the microstructural evolution and precipitation behavior. From the experimental observations and the detailed analysis of this study, the current methodology can be utilized to further expand the design space of current heterogeneous nanostructured materials. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Nanocrystalline metals are promising radiation tolerant materials due to their large interfacial volume fraction, but irradiation-induced grain growth can eventually degrade any improvement in radiation tolerance. Therefore, methods to limit grain growth and simultaneously improve the radiation tolerance of nanocrystalline metals are needed. Amorphous intergranular films are unique grain boundary structures that are predicted to have improved sink efficiencies due to their increased thickness and amorphous structure, while also improving grain size stability. In this study, ball milled nanocrystalline Cu-Zr alloys are heat treated to either have only ordered grain boundaries or to contain amorphous intergranular films distributed within the grain boundary network, and are then subjected to in situ transmission electron microscopy irradiation and ex situ irradiation. Differences in defect density and grain growth due to grain boundary complexion type are then investigated. When amorphous intergranular films are incorporated within the material, fewer and smaller defect clusters are observed while grain growth is also limited, leading to nanocrystalline alloys with improved radiation tolerance. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Abstract Irradiation induced creep (IIC) rates are measured in compression on Ag nanopillar (square) beams in the sink-limited regime. The IIC rate increases linearly with stress at lower stresses, i.e. below ≈2/3 the high temperature yield stress and parabolically with pillar width, L, for L less than ≈300 nm. The data are obtained by combining in situ transmission electron imaging with simultaneous ion irradiation, laser heating, and nanopillar compression. Results in the larger width regime are consistent with prior literature. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

Recrystallization and subsequent twin formation in > 99.99% pure copper is investigated by in situ annealing in a scanning electron microscope. Distinct microstructural evolution mechanisms are observed for specimens deformed using 5% and 25% cold-rolled reductions. Upon annealing the former, growth and twinning of existing grains via strain-induced boundary migration is observed while the latter exhibits nucleation and growth. Serial sectioning indicates that recrystallization in the low strain condition is surface dominant while nucleation occurs through thickness in the high strain condition. Geometrically necessary dislocation densities are calculated and mapped to identify the localized driving pressures for each recrystallization mechanism. [ABSTRACT FROM AUTHOR]

Fatigue-induced grain growth has been shown to drive high-cycle fatigue crack initiation in nanocrystalline metals. As a potential strategy to suppress such grain growth, particular solute additions are known to stabilize grain boundaries through both thermodynamic and kinetic mechanisms. In previous work, Au additions in nanocrystalline Pt were confirmed to stabilize grain boundaries against thermally-driven grain boundary migration, however, the corresponding effect on fatigue resistance was unknown. The present work compared the high-cycle fatigue performance of stabilized Pt-10at%Au to reference pure Pt with a similar initial grain size, using a combination of ex-situ and in-situ scanning electron microscope fatigue tests. Ex-situ tests revealed that the Pt-10Au exhibits substantial improvement in overall fatigue life over Pt, including a 75% elevation of the fatigue endurance limit. The improvement was attributed to enhanced crack initiation resistance, since the Pt-10Au exhibited diminished resistance to crack propagation compared to Pt. Electron backscatter diffraction, transmission kikuchi diffraction, and transmission electron microscopy showed that the pure Pt exhibited extensive fatigue-induced abnormal grain growth, with the largest grains growing to 10 times the size of the initial grain size, whereas the Pt-10Au exhibited inhibited grain growth, with the largest grains growing by approximately a factor of two. This study provides clear evidence that thermodynamic strategies used to impart thermal stability can also contribute to improved high-cycle fatigue resistance via suppression of grain-growth induced crack initiation. [Display omitted] [ABSTRACT FROM AUTHOR]

This work uses a combination of stress dependent single grain boundary Coble creep and zero-creep experiments to measure interfacial energies, along with grain boundary point defect formation and migration volumes in cubic ZrO 2. These data, along with interfacial diffusivities measured in a companion paper are then applied to analyzing two-particle sintering. The analysis presented indicates that the large activation volume, v * = v f + v m primarily derives from a large migration volume and suggests that the grain boundary rate limiting defects are delocalized, possibly due to electrostatic interactions between charge compensating defects. The discrete nature of the sintering and creep process observed in the small-scale experiments supports the hypothesis that grain boundary dislocations serve as sources and sinks for grain boundary point defects and facilitate strain during sintering and Coble creep. Model two-particle sintering experiments demonstrate that initial-stage densification follows interface reaction rate-limited kinetics. Image, graphical abstract [ABSTRACT FROM AUTHOR]

This work demonstrates novel in situ transmission electron microscopy-based microscale single grain boundary Coble creep experiments used to grow nanowires through a solid-state process in cubic ZrO 2 between ≈ 1200 °C and ≈ 2100 °C. Experiments indicate Coble creep drives the formation of nanowires from asperity contacts during tensile displacement, which is confirmed by phase field simulations. The experiments also facilitate efficient measurement of grain boundary diffusivity and surface diffusivity. 10 mol% Sc 2 O 3 doped ZrO 2 is found to have a cation grain boundary diffusivity of D g b = (0.056 ± 0.05) exp (− 380 , 000 ± 41 , 000 R T) m 2 s − 1 , and surface diffusivity of D s = (0.10 ± 0.27) exp (− 380 , 000 ± 28 , 000 R T) m 2 s − 1 . Image, graphical abstract [ABSTRACT FROM AUTHOR]

Grain boundary (GB) properties and associated anisotropies due to the boundary's geometric degrees of freedom (DOF) greatly influence many of the salient features of polycrystalline aggregates, and as a consequence the observable properties of the material. Through theoretical analysis and atomistic simulations, we show that when considering the GB plane normal DOF, the GB interface stiffness tensor plays a paramount role in a wide range of GB dynamical processes. As a demonstration, we examine the interface stiffness tensor of Σ3 GBs in nickel and show that the stiffness can be much larger in magnitude and more anisotropic than the GB energy itself. Moreover, it is found that a wide range of inclinations exhibit negative stiffness values, a signature of a structural instability. This is the first study to consider the complete spatial description of the GB stiffness tensor, where both angular variations describing the interface plane normal are explored. In broad terms, our results highlight the integral role that the stiffness tensor plays in GB interfacial phenomena, such as curvature-driven flow and faceting instabilities. [ABSTRACT FROM AUTHOR]

Grain boundaries have complex structural features that influence strength, ductility and fracture in metals and alloys. Grain boundary misorientation angle has been identified as a key parameter that controls their mechanical behavior, but the effect of misorientation angle has been challenging to isolate in polycrystalline materials. Here, we describe the use of bicrystal Au thin films made using a rapid melt growth process to study deformation at a single grain boundary. Tensile testing is performed on bicrystals with different misorientation angles using in situ TEM, as well as on a single crystalline sample. Plastic deformation is initiated through dislocation nucleation from free surfaces. Grain boundary sliding is not observed, and failure occurs away from the grain boundary through plastic collapse in all cases. The failure behavior in these nanoscale bicrystals does not appear to depend on the misorientation angle or grain boundary energy but instead has a more complex dependence on sample surface structure and dislocation activity. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] Grain boundaries (GBs) play a critical role in the formation of microstructure during materials processing and its subsequent evolution under service conditions. While GB misorientation is commonly used to describe the boundary's properties, a more complete description should also account for the GB plane normal, in which the GB stiffness is the relevant property controlling many GB dynamical processes. Herein, we leverage published atomistic simulation data to construct the full GB energy–plane normal diagrams for Σ 5, 7, 9, and 11 GBs in Ni. The functional fits are used to obtain a complete mapping of the GB stiffness as a function of the plane normal, construct the equilibrium shapes, and determine the driving force for GB migration. It is shown that the GB stiffness can be larger in magnitude and more anisotropic than the energy itself. Further, many boundary inclinations are found to exhibit negative stiffness, indicating propensity for faceting. Results from our GB stiffness analysis are shown to be in qualitative agreement with experimental GB plane normal distributions in polycrystalline Ni. In broad terms, our results provide future avenues to account for the plane normal dependency of GB properties in mesoscale treatments of GB migration and microstructural evolution. [ABSTRACT FROM AUTHOR]

• Grain size reduction improves fatigue endurance limit, but not in the nanocrystalline regime. • Ultimate tensile strength does not provide a useful metric for scaling fatigue response. • Instead, work hardening behavior is shown to effectively scale with fatigue resistance. The high-cycle fatigue life of nanocrystalline and ultrafine-grained Ni-Fe was examined for five distinct grain sizes ranging from approximately 50–600 nm. The fatigue properties were strongly dependent on grain size, with the endurance limit changing by a factor of 4 over this narrow range of grain size. The dataset suggests a breakdown in fatigue improvement for the smallest grain sizes <100 nm, likely associated with a transition to grain coarsening as a dominant rate-limiting mechanism. The dataset also is used to explore fatigue prediction from monotonic tensile properties, suggesting that a characteristic flow strength is more meaningful than the widely-utilized ultimate tensile strength. [ABSTRACT FROM AUTHOR]