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Highlights • Amount of Mo 2 C precipitates is proportional to the amount of SiC in the initial powder mixture. • Mo 2 C precipitates readily dissolve into FLiNaK molten salt leaving behind empty voids. • The corrosion resistance of NiMo-SiC alloys is strongly dependent on the amount of SiC in the initial powder mixture. • The corrosion resistance of NiMo-SiC alloys with a low SiC content (<2 wt.%) is comparable to Hastelloy-N®. Abstract The corrosion performance of Ni-16%wt.Mo-X%wt.SiC (X = 0.5, 1.5, 2.0, 2.5 and 3.0) alloys prepared via mechanical alloying followed by consolidation using spark plasma sintering (SPS) from pure Ni, Mo and SiC powders is investigated. Corrosion testing at 650 °C/200 h in FLiNaK molten salt showed that increasing the volume fraction of SiC in the initial Ni-Mo-SiC powder mixture leads to formation of large amount of Mo 2 C precipitates, which readily dissolve into FLiNaK molten salt. Hence, only the corrosion resistance of NiMo-SiC alloys with a low SiC content (<2 wt.%) is comparable to that of Hastelloy-N® alloy. [ABSTRACT FROM AUTHOR]

The NeT TG4 benchmark specimen consists of a three-pass type 316L Tungsten Inert Gas slot weld in a AISI type 316L(N) plate. Phase one of the finite element simulation round robin performed on TG4 by the NeT network made the assumption that weld metal exhibits the same mechanical behaviour as parent material. A comprehensive series of material characterisation tests on weld metal was performed by NeT participants, and these allowed the derivation of several sets of mixed isotropic-kinematic material model parameters specifically for weld metal. The derived models have been used to improve the predicted stresses in the TG4 benchmark specimen. This paper first reviews the weld metal materials testing programme, and then discusses the optimum material hardening model fitting strategy to use for austenitic weld metal. The derived material models are tested by using them to predict residual stresses in the TG4 benchmark, and validating the predictions against the extensive database of measured residual stresses. [ABSTRACT FROM AUTHOR]

A new generation of alloys, which rely on a combination of various strengthening mechanisms, has been developed for application in molten salt nuclear reactors. In the current study, a battery of dispersion and precipitation-strengthened (DPS) NiMo-based alloys containing varying amounts of SiC (0.5-2.5 wt %) were prepared from Ni-Mo-SiC powder mixture via a mechanical alloying (MA) route followed by spark plasma sintering (SPS) and rapid cooling. Neutron Powder Diffraction (NPD), Electron Back Scattering Diffraction (EBSD), and Transmission Electron Microscopy (TEM) were employed in the characterization of the microstructural properties of these in-house prepared NiMo-SiC DPS alloys. The study showed that uniformly-dispersed SiC particles provide dispersion strengthening, the precipitation of nano-scale Ni3Si particles provides precipitation strengthening, and the solid-solution of Mo in the Ni matrix provides solid-solution strengthening. It was further shown that the milling time has significant effects on the microstructural characteristics of these alloys. Increasedmilling time seems to limit the grain growth of theNiMomatrix by producing well-dispersed Mo2C particles during sintering. The amount of grain boundaries greatly increases the Hall-Petch strengthening, resulting in significantly higher strength in the case of 48-h-milled NiMo-SiC DPS alloys compared with the 8-h-milled alloys. However, it was also shown that the total elongation is considerably reduced in the 48-h-milled NiMo-SiC DPS alloy due to high porosity. The porosity is a result of cold welding of the powder mixture during the extended milling process. [ABSTRACT FROM AUTHOR]

The effects of strain rate on the mechanical behaviour and microstructure evolution of extruded Mg AZ31 alloy are investigated here for strain rates ranging from 0.0001 s −1 to 4000 s −1 . High strain rate compression testing was performed using a split Hopkinson pressure bar apparatus. The microstructure and texture evolution were characterised by using electron backscatter diffraction (EBSD) and neutron diffraction. The results show that the yield strength increases by roughly 55% from the quasi-static to the high strain rate regime while the plateau stress increases by roughly 35%. After compression along the extrusion direction, the deformed textures of both quasi-static and high strain rate loading samples are similar, showing that a large portion of grains have been reoriented through the activation of tensile twinning. However, a detail EBSD analysis suggests that there is a higher fraction of secondary compressive twins in high strain rate deformed specimens at 16% strain. A visco-plastic self-consistent (VPSC) model, which incorporates the composite grain scheme to account for twinning, is employed to model the mechanical response and texture evolution, and to elucidate the collaborative nature of the slip-twinning deformation. Using only a single set of parameters, the VPSC model successfully reproduces the stress-strain curves at all tested strain rates, and more importantly, captures the texture and twin volume fraction evolutions. The simulation results suggest that under quasi-static and high strain rate loadings, the dominant slip systems are different in both untwinned grains and tensile twins. The results also suggest that twins are formed at a lower strain at high strain rates when compared to quasi-static loading. [ABSTRACT FROM AUTHOR]

A new class of materials, which rely on the dispersion strengthening of SiC particles in addition to precipitation strengthening by nano-precipitates is being developed for the application in molten salt nuclear reactors. A battery of dispersion and precipitation strengthened (DPS) NiMo-based alloys containing varying amount of SiC (0.5–2.5 wt.%) was prepared via a mechanical alloying (MA) route followed by spark plasma sintering (SPS), rapid cooling, high-temperature annealing and water quenching. Lab X-ray Diffraction (XRD), Electron Back Scattering Diffraction (EBSD), and Transmission Electron Microscopy (TEM) were employed in the microstructural characterization of this new type of alloys. It is shown that the NiMo matrix of these alloys is effectively reinforced by dispersion of SiC from the initial powder mixture and nano-Ni 3 Si precipitates, which precipitated during the sintering/annealing process. Furthermore, the matrix is strengthened by solid-solution of Mo in Ni. As a result, these newly developed NiMo alloys take advantage of dispersion, precipitation and solid solution strengthening, which leads to their superior mechanical properties. [ABSTRACT FROM AUTHOR]

Numerical finite element analyses were combined with experimental observation of a single-pass autogenous beam weld in SA508 Gr.3 Cl.1 ferritic steel. Two weldment sets were prepared using different weld heat inputs, resulting in different post-weld residual stress and ferritic phase distributions. Neutron diffraction was employed to measure the residual stress distribution while microhardness measurements were used to measure the post-weld phase distribution in each weldment. In both cases, the numerical model accurately predicts the ferritic phase distribution and residual stress field. Model predictions illustrate how the higher cooling rates associated with a faster torch speed result in an increased martensite volume fraction within the weldment. Consideration of both the transformation kinetics and transformation plasticity are proven to significantly improve model accuracy when comparing measured and predicted residual stress profiles. [ABSTRACT FROM AUTHOR]

• The state of the alloy's microstructure and its in-service evolution affects its corrosion behaviour. • High-temperature creep leads to the storage of dislocation and development of dislocation substructure. • Dislocation substructure promotes intragranular diffusion of alloying elements to the salt-alloy interface. • Accelerated diffusion of alloying elements leads to an increased corrosion attack of the alloy. Molten-salt-based energy-generation and energy-storage systems are expected to function under plasticity-imparting in-service operating conditions. It is thus of utmost importance to understand the effect of these operating conditions on the corrosion performance of molten-salt-facing structural alloys. Here, we investigate the impact of high-temperature creep on the molten salt corrosion of a NiMoCr alloy in FLiNaK molten salt. We show that high-temperature creep leads to an increased susceptibility of the alloy to molten salt corrosion via the development of a dislocation substructure, which promotes mass diffusion of alloying elements towards the salt–alloy interface (exposed surface). In addition, the corrosion-affected near-surface layer undergoes recrystallization leading to a significant increase in the amount of grain boundaries, which further promotes ongoing diffusion processes. This then causes increased molten salt corrosion attack resulting in an accelerated mass loss during the alloy's exposure to the molten salt. The present results thus unambiguously highlight the importance of the state of the microstructure as well as the evolution of the microstructure under in-service operating conditions on the alloy's molten salt corrosion performance. [ABSTRACT FROM AUTHOR]

This study aims to predict the creep response of Alloy 617 and assess the accuracy of three established creep damage models in predicting the creep time-to-failure ( t f ) and strain-to-failure ( ε f ) in the temperature range of 800–1000 °C. Idaho National Laboratory (INL) creep data were used to calibrate (1) Ductility Exhaustion (DE), (2) Stress-Modified Ductility Exhaustion (SMDE), and (3) Strain-Energy Density (SED) creep damage models using multiple linear regression (LR) and reverse damage (RD) approaches. In order to predict the creep response of Alloy 617 these creep damage models are then coupled with (secondary) creep strain model calibrated using Idaho National Laboratory (INL), Korean Atomic Energy Research Institute (KAERI), and Argonne National Laboratory (ANL) experimental data. Direct comparisons made between the investigated creep damage models revealed that the SED model with parameters calibrated through RD approach captures the creep response of Alloy 617 the most accurately and it thus produces the most accurate prediction of time-to-failure ( t f ) and strain-to-failure ( ε f ) across different temperature/stress (creep) conditions. However, it is also shown that none of the employed creep damage models are able to fully capture the material creep response at 1000 °C. This is attributed the strong oxidation of Alloy 617 at 1000 °C (tested in air) leading to the formation of a thick oxidation layer, which might affect the failure mechanism of the alloy at this temperature. • The most accurate prediction is achieved using Strain-Energy Density (SED) model. • Reverse damage approach yields more accurate results than linear regression approach. • No creep damage model was able to capture the creep behaviour at 1000 °C. [ABSTRACT FROM AUTHOR]

Interactions between stainless steel 316L and eutectic Li 2 CO 3 + K 2 CO 3 + Na 2 CO 3 at 450 °C were investigated for thermal energy storage. Scanning Electron Microscopy (SEM), Electron Back-Scatter Diffraction (EBSD), Scanning Kelvin Probe Force Microscopy (SKPFM), X-ray Photoelectron Spectroscopy (XPS), neutron diffraction pattern, material loss, micro-hardness, polarization and impedance measurements were used to compare the alloy's response in unrolled (0%) condition versus 20% and 30% cold-rolled conditions. Cold-rolling increased the number of grains, grain boundaries and density of dislocations. Initially, faster corrosion accompanied by more areas of localized attack was confirmed by Volta potential measurements. However, recovery and the formation of a surface film were found to decelerate corrosion for longer times. Cold-rolling-induced dislocations were found to facilitate carbon diffusion and subsequently carburize the material leading to increased corrosion resistance. Consequently, the overall long-time corrosion rate was not noticeably affected by cold-rolling. Image 1 • Interaction of cold-rolled SS316L with a molten carbonate salt was studied. • Cold-rolling increased grains, grain boundaries and density of dislocations. • Volta potential confirmed initial fast corrosion rate due to localized attack. • Recovery and formation of an oxide layer decelerated corrosion rate after 1000 h. • Dislocations facilitated carburization leading to increased corrosion resistance. [ABSTRACT FROM AUTHOR]

By time-of-flight (TOF) neutron diffraction experiments, the influence of segregation-induced microstructure bands of austenite (γ) and martensite (α′) phases on the partitioning of stress and strain between these phases was investigated. Initially, tensile specimens of a Co-added stainless steel were heat treated by quenching and partitioning (Q&P) processing. Tensile specimens were subsequently loaded at 350 °C parallel to the length of the bands within the apparent elastic limit of the phase mixture. Lattice parameters in both axial and transverse directions were simultaneously measured for both phases. The observation of a lattice expansion for the γ phase in the transverse direction indicated a constraint on the free transverse straining of γ arising from the banded microstructure. The lateral contraction of α′ imposed an interphase tensile microstress in the transverse direction of the γ phase. The multiaxial stress state developed in the γ phase resulted in a large deviation from the level of plastic strain expected for uniaxial loading of single phase γ. Since segregation-induced banded microstructures commonly occur in many engineering alloys, the analysis of stress and strain partitioning with the present Q&P steel can be used to interpret the observations made for further engineering alloys with two-phase microstructures. [ABSTRACT FROM AUTHOR]

Introducing coarse grains into nanocrystalline (NC) and ultrafine grained (UFG) metallic materials can lead to an excellent combination of strength and ductility. In this study, we produced Al 2 O 3 dispersion-strengthened (Al 2 O 3 -DS) Cu with heterogeneous microstructure by mechanical alloying (MA) of Cu-Al 2 O 3 powder mixture followed by spark plasma sintering (SPS) and hot extrusion (HE). For direct comparison, one sample was prepared by hot extrusion only without prior SPS of mechanically alloyed (MAed) powders. The microstructure of materials produced via a combination of SPS and HE possessed the heterogeneous microstructure consisting of Al 2 O 3 -free coarse grained (CG) Cu bands and Al 2 O 3 -rich UFG Cu regions. These different microstructural regions were found to play different roles on the tensile and electrical properties and their presence can be tuned to achieve desired mechanical and electrical properties. We also discuss the possible mechanisms responsible for the formation of the heterogeneous microstructure in the samples produced by a combined process of SPS and HE. [ABSTRACT FROM AUTHOR]

Introducing coarse grains into nanocrystalline (NC) and ultrafine grained (UFG) metallic materials can lead to an excellent combination of strength and ductility. In this study, we produced Al 2 O 3 dispersion-strengthened (Al 2 O 3 -DS) Cu with heterogeneous microstructure by mechanical alloying (MA) of Cu-Al 2 O 3 powder mixture followed by spark plasma sintering (SPS) and hot extrusion (HE). For direct comparison, one sample was prepared by hot extrusion only without prior SPS of mechanically alloyed (MAed) powders. The microstructure of materials produced via a combination of SPS and HE possessed the heterogeneous microstructure consisting of Al 2 O 3 -free coarse grained (CG) Cu bands and Al 2 O 3 -rich UFG Cu regions. These different microstructural regions were found to play different roles on the tensile and electrical properties and their presence can be tuned to achieve desired mechanical and electrical properties. We also discuss the possible mechanisms responsible for the formation of the heterogeneous microstructure in the samples produced by a combined process of SPS and HE. [ABSTRACT FROM AUTHOR]

A set of dispersion strengthened Ni-3 wt.% SiC samples have been prepared via a powder metallurgy route for an application in molten salt reactor (MSR) systems. A mixture of 97 wt.% Ni and 3 wt.% SiC powder was prepared by varying the ball-milling time (8, 24, 36 and 48 h). The ball-milling process was followed by powder consolidation using spark plasma sintering (SPS) and rapid cooling to room temperature. The samples were then annealed at different temperatures (300 °C, 700 °C and 850 °C). The fracture behaviour of the Ni-3 wt.% SiC samples was investigated using the Small Punch Test (SPT). A scanning electron microscope (SEM) with energy dispersive spectrometer (EDS) and electron back-scatter diffraction (EBSD), as well as transmission electron microscopy (TEM) were employed to examine the microstructure of the tested Ni-3 wt.% SiC samples. The obtained results show that the SPT fracture energy is strongly dependent on the grain size of Ni matrix, which is affected by ball-milling times and annealing temperature. It has been further found that the segregation of large SiC particles at grain boundaries promotes intergranular fracture and thus significantly reduces the fracture energy. The results indicate that the distribution and size of SiC particles at grain boundary is mainly affected by the ball-milling time. Hence, the ball-milling time governs the fracture toughness of Ni-3 wt.% SiC samples. [ABSTRACT FROM AUTHOR]

Interactions between neighboring grains influence the macroscale behavior of polycrystalline materials, particularly their deformation behavior, damage initiation and propagation mechanisms. However, mapping all of the critical material properties normally requires that several independent measurements are performed. Here we report the first grain mapping of a polycrystalline foil using a pixelated energy-dispersive X-ray area detector, simultaneously measuring X-ray fluorescence and diffraction with the Maia detector in order to determine grain orientation and estimate lattice strain. These results demonstrate the potential of the next generation of X-ray area detectors for materials characterization. By scanning the incident X-ray energy we investigate these detectors as a complete solution for simultaneously mapping the crystallographic and chemical properties of the sample. The extension of these techniques to broadband X-ray sources is also discussed. [ABSTRACT FROM AUTHOR]

Annealing usually softens Al–Mg based alloys due to grain coarsening. This work shows that annealing induces strengthening in bulk ultrafine-grained Al and Al-(2.5, 5 and 7.5) at.% Mg samples fabricated by mechanical alloying and rapid powder extrusion. Experimental investigation of the microstructure of the annealed samples reveals that the annealing promotes in-situ formation of nanoscale dispersoids which strongly suppresses grain growth and recrystallization. The in-situ formed nanodispersoids warrant high thermal stability of the ultrafine-grained matrix microstructure and improve the strength of the as-extruded samples while maintaining their good ductility. The present findings offer an exciting pathway in developing thermally stable ultrafine-grained Al–Mg based alloys with a notable combination of high strength and good ductility. [ABSTRACT FROM AUTHOR]

The GH3535 alloy samples were irradiated using 15-MeV Te4+ ions at 650 °C to a dose of 0.5, 3.0, 10, and 20 dpa, respectively. The Te atoms distribution and microstructure evolution were examined by electron probe microanalysis (EPMA) and transmission electron microscopy (TEM). The nano-indenter was then used to measure the nano-hardness changes of samples before and after irradiation. TEM results showed the formation of dislocation loops in the irradiated samples. Their mean diameters increase with the increase of irradiation dose and tends to be saturated when irradiation dose exceeds 10 dpa. The ratio of yield strength increments calculated by dispersed barrier hardening (DBH) model is basically consistent with that of nano-hardness increments measured by nano-indenter. In addition, the relationship between the nano-hardness increments and dpa for the GH3535 alloy irradiated by Te ions has been revealed in the study. [ABSTRACT FROM AUTHOR]

The irradiation damage behavior was studied in the nano-grained Ni–Mo–Cr alloy (nano-grained GH3535), which was irradiated by He ion to various dose. The evolution of defects and hardness changes are characterized by transmission electron microscopy and nanoindentation to explore the irradiation tolerance of the nano-grained GH3535 and the coarse-grained GH3535 (annealed GH3535), where the later was chosen as reference material to make comparison with nano-grained GH3535. The results show that though both the average size and number density of He bubbles increase with an increase in the irradiation dose, the smaller volume fraction is found in the nano-grained GH3535 compared with the coarse-grained GH3535 under the same irradiation condition. This indicates that the nano-grained GH3535 possess better irradiation swelling resistance than the coarse-grained GH3535. However, the increase in the hardness of the nano-grained GH3535 is more significant than in the coarse-grained GH3535 under the same irradiation dose. This suggests stronger irradiation-induced hardening of the nano-grained alloy comparing to coarse-grained alloy, due to the impeding effect caused by grain boundaries decorated with He bubbles. This study provides insight into the design of irradiation-tolerant nickel-based alloys for nuclear industry applications. [ABSTRACT FROM AUTHOR]

An amendment to this paper has been published and can be accessed via a link at the top of the paper. [ABSTRACT FROM AUTHOR]

An amendment to this paper has been published and can be accessed via a link at the top of the paper. [ABSTRACT FROM AUTHOR]

A nano-grained microstructure of an α-Zr alloy (Zircaloy-4) was produced by high-pressure torsion, which shows evidence of a metastable ω-Zr phase, rather than β-Zr, determined by combining synchrotron X-ray diffraction and detailed electron microscopy observations. The ω-Zr phase is retained at ambient conditions and shows a new orientation relationship of [ 1 ¯ 011 ] α // [ 1 ¯ 100 ] ω and (1 ¯ 01 1 ¯) α // (1 ¯ 1 ¯ 20) ω with the α-Zr matrix but is thermally unstable, fully reverting back to α-Zr phase upon heating above 350 °C. [Display omitted] [ABSTRACT FROM AUTHOR]

Post-processing is often inevitable for most additively manufactured components in order to improve material properties and product quality. In this study, the influence of post-heat treatments (PHTs) at 871 °C and 1177 °C on the microstructure and mechanical properties of a nickel-base Hastelloy C276 alloy prepared using wire arc additive manufacturing (WAAM) were investigated. The results showed that after a PHT at 871 °C, the as-built alloy was strengthened due to the formation of a large amount of Mo-rich nano-sized μ phase in the interdendritic areas. This was at the expense of a significant ductility loss. In contrast, no μ phase precipitates were observed after PHT at 1177 °C. Furthermore, the 1177 °C treatment led to the dissolution of the Mo-rich p phase which was present in the as-built sample, increased solid-solution strengthening, and improvements in both strength and ductility concurrently. This study enables an improved understanding of post-processing-microstructure-property inter-relationships for Hastelloy C276 alloy prepared by WAAM, providing guidelines for further microstructure optimization through PHT to improve the material's mechanical properties. • Hastelloy C276 using WAAM can be strengthened by post heat treatment (PHT). • PHT at 871 °C leads to the highest strength but lowest ductility due to intermetallics. • PHT at 1177 °C shows a combination of increased strength and lowered anisotropy. [ABSTRACT FROM AUTHOR]

A new Powder-bed Arc Additive Manufacturing (PAAM) processing which includes on-line remelting of deposited material has been developed for the manufacturing of high entropy alloys (HEAs) based on an existing AlCoCrFeNi 2.1 pseudo-eutectic system. The remelting process is typically applied in the arc melting process to improve the homogeneity of prepared material. We investigated the microstructure and mechanical properties of produced AlCoCrFeNi 2.1 HEA after applying a remelting process (1, 3, and 6 times) on each deposited layer. The results show the formation of the pseudo-eutectic microstructure, which consists of relatively large columnar grains of the dominant FCC phase (~90 wt%) and fine dendritic grains of the minor BCC phase (~10 wt%). The applied layer-remelting process shows negligible effects on the phase fractions and their compositions, however, it significantly degraded the tensile strength and ductility of prepared alloys. Particularly, the ductility of the alloy reduced dramatically from about 27% after one time layer-remelting to only about 3% after 3 times layer-remelting. This is rationalised by the significant localisation of thermally induced plasticity caused by repeated remelting of deposited material. We also show that this thermally induced plasticity leads to an increased amount of local misorientation in both constitute phases, which suggests an increased amount of stored dislocations in the microstructure. Despite the potentially strain hardening due to this accumulation of the thermally induced plasticity, the appreciable growth and constrained dendritic morphology of BCC grains that developed after remelting play a prevailing role on the materials strength, which limit the interfacial strengthening of the eutectic microstructure and consequently result in the loss of the tensile strength. The obtained results will assist in the further development and microstructure optimisation of novel HEAs using powder-based additive manufacturing processes. [ABSTRACT FROM AUTHOR]

High strength has always been pursued in binary Al–Mg and 5xxx series Al alloys. However, these alloys usually provide medium strength due to lack of multiple strengthening methods. Here, this study reports on multiple strengthening effects existing in a binary ultrafine-grained (UFG) Al-7.5at.%Mg alloy prepared by mechanical alloying and hot extrusion. Different strengthening mechanisms, in particular, dispersoid strengthening and stacking fault strengthening, are analyzed and evaluated in terms of their respective contribution to the 511 MPa yield strength of the alloy. The strengthening analysis proves that the concurrent generation of nanoscale dispersoids and stacking faults (SFs) can be used as a new microstructural design in developing novel Al–Mg-based alloys with higher strength and enhanced ductility. [ABSTRACT FROM AUTHOR]