The mechanical behavior and microstructural evolution of a single body-centered-cubic (BCC) phase NbTaTiV refractory high-entropy alloy (RHEA) were studied over a wide range of strain-rates (from 1 × 10-4 to 2 × 103 s-1) and temperatures [from room temperature (RT) to 850 °C]. The evolution of yield strength as a function of strain rate and temperature showed that the present RHEA had less strain-rate dependence and a strong resistance to softening at high temperatures. The formation of thin type-I twins was observed during high-strain rate deformation. The present work also showed that the thin twins did not significantly affect the work-hardening/softening rate during high strain-rate testing at all temperatures. Contrary to the high strain-rate behavior, substantial strain-hardening was observed at quasi-static rates, when the materials were tested at cryogenic temperatures. This difference in behavior was attributed to the formation of “thick” twins at cryogenic temperatures, which not only act as stronger barriers to dislocation motion but also interact with each other leading to an increase in the work hardening rate. The temperature increase during high strain-rate testing was estimated by theoretical calculations. The relatively low stacking fault energies with high twinning stress for NbTaTiV were predicted by density functional theory calculations. Moreover, a modified Zerilli-Armstrong constitutive model was also fit to represent the strength of this material at varying strain rates and temperatures.

Damage from low-temperature irradiation and the subsequent degradation of materials performance pose sig-nificant challenges for the storage of radioactive materials and for peripheral components in some nuclear reactor designs. Fully understanding the mechanical behavior of such materials requires test data for strain rates in both the quasi-static (< 10/s) and dynamic (>> 10/s) regimes. While dynamic testing has generally been avoided in the past for neutron irradiated (contamination concerns) and ion irradiated (insufficient volume) materials, surface-sensitive Richtmyer-Meshkov instability (RMI) tests were used in the present work to overcome these limitations. Here, nanopillar compression, nanoindentation, and RMI testing data from a helium implanted surface layer (similar to 10 mu m thick) were compiled to explore the effects of helium bubbles on the materials strength of high-purity copper at strain rates of 0.001/s - 10(8)/s. While nano-mechanical testing revealed increases in yield strength and hardness with increasing helium dose from 1000 to 4000 appm He, RMI indicated no significant changes in strength as compared to unimplanted copper. This discrepancy in behavior was rationalized through a combination of recent literature and follow-on molecular dynamics (MD) simulations, leading to the conclusion that the nanoscale helium bubbles acting as dispersed barriers to dislocation motion at quasi-static strain rates collapse under shock loading and cease to be effective barriers at high strain rates.

The grade 91 ferritic/martensitic steel is considered a promising structural or cladding material for various nuclear reactor applications. Here, grade 91 was fabricated via the Directed Energy Deposition Laser technique. This alternative manufacturing process potentially enables tailoring of the mechanical properties through increased control of the product's microstructure. Aimed at linking fabrication to performance via defining the process-structure-property relationships, the current research includes macro and up to nano-scale mechanical testing using microhardness, tensile, and in-situ nanoindentation hardness, coupled with electron diffraction-based microstructure characterization. Mapping of the product structure and properties was conducted by testing miniature-sized samples, parallel to the built direction ('Z' direction) and perpendicular ('X' direction) at constant distances. We found the majority of the microstructure consists of fine and coarsened-size lath-type martensite grains, with up to 15% d-phase, preferentially observed at the melt pool boundaries. Most intriguing was the gradual decrease found in observed metallurgical pores alongside softening at farthest distances from the cold build platform. Here, these changes were successfully explained in terms of phase composition, 'grain-like' size effects of the lath-type martensite , geometry necessary dislocation density. In final-izing this work, several competing strengthening mechanisms were addressed, , their activity was considered owing to fabrication-related mechanisms.& COPY; 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

A series of plate impact experiments have been performed on the three principal orientations of single crystal aluminium: [100], [110] and [111]. Experiments were designed to probe the Hugoniot Elastic Limit, spall strength and Equation of state as well as understand the microstructural response, via transmission electron microscopy, Transmission Kikuchi Diffraction and mechanical testing of shocked material. Results from this work, show that orientation affects the HEL and spall strength in a similar manner to the quasi-static uniaxial stress compressive response, with [100] and [110] being similar and [111] significantly stronger. This correlates with expectations from considerations using the Schmid analysis. However we have noted that similar studies in copper yield a different ordering of the HELs. We have reconciled those differences using an analysis based on second order elastic constants. However the shocked microstructures and post shock quasi-static mechanical response are strongly influenced by orientation. The [100] orientation shows a response typical of high stacking fault face centred cubic metals, consisting of equiaxed subgrains and a strong post shock hardening during quasi-static mechanical testing, similar to results in polycrystalline aluminium. In contrast, the [110] and [111] orientations have a microstructural response more similar to polycrystalline aluminium after repeated shock loading excursions, combined with much lower post shock hardening during mechanical testing. This suggests that these orientations require much greater dislocation activity to achieve the same plastic deformation as [100]. This would appear to agree with the much higher degree of work hardening observed in these orientations compared to [100].

Grade 91 steel forms martensite during additive manufacturing and the extent of tempering of martensite significantly affects the mechanical properties of parts. Currently, there is a lack of quantitative understanding of the tempering kinetics for Grade 91 steel, and as a result, the effects of repeated thermal cycles on properties for different processing conditions cannot be determined. Here we evaluate the tempering kinetics by determining the constant terms in the Johnson Mehl Avrami kinetic equation from the tempering data available in the literature and the thermal cycles computed using a rigorously-tested heat and fluid flow model of multi-layer additive manufacturing. The raw tempering data are cleaned using a neural network to enhance accuracy. The lower layers experience repeating cycles of heating and cooling when the upper layers are added. As a result, the hardness is reduced owing to the tempering of martensite. In contrast, martensite formed in the upper layers is not tempered to the same extent and the hardness remains high. Therefore, the hardness of the part increases with the distance from the substrate. Variations in the heat input at different laser powers and scanning speeds significantly affect the extent of tempering. Since the method used here can provide a quantitative understanding of the tempering of martensite and the spatial variation of hardness, it can be used to tailor the microstructure and hardness of heat treatable printed metallic parts.

Efforts to advance structural materials with improved properties and service life in support of next generation designs for nuclear reactor components have recently led to development of nano-ferritic alloys (NFAs) containing nano-oxides such as 14YWT. A key enabling technology to realizing the useful properties of NFAs during service involves preservation of the oxide dispersions during joining. Solid-state welding processes, such as projection-capacitor discharge resistance welding (P-CDRW) used here, are well suited for joining NFAs while retaining the oxides. Due to limitations in the supply of 14YWT NFA material, initial experiments were conducted using 430 stainless steel as an inexpensive surrogate material. The goal of the surrogate experiments was to scale suitable parameters from 430 welds to 14YWT using ratios of key properties for the two materials including flow stress at temperatures and strain rates relevant to hot working. Results indicated that weld displacement increased with increasing weld force and increasing weld energy for all other variables held constant. Weld energy appeared to have a larger effect on displacement than weld force for the sample geometry used here. Appropriate process parameters (no melting) were established for the two materials. The process window for the 430 material extended from 350 J to 600 J of energy for weld forces of 2.2 kN and 3.1 kN. Suitable parameters for 14YWT were similar in terms of energy but for force levels of 3.1 kN and 4.0 kN. Displacement for both materials ranged from 150 mu m to 300 mu m for welds that did not experience melting. Simple heat flow analysis confirmed that the extent of displacement was limited by the characteristic thermal distance determined from thermo-physical properties and the weld current rise time. The higher flow stresses of 14YWT relative to 430 were apparently offset by greater heating due to higher electrical resistivity near the projection tip and lesser heat conduction from the projection tip owing to lower thermal conductivity. Based on the results presented here and in our companion paper. The P-CDRW process appears capable of successfully joining the 14YWT NFA while retaining the microstructures and properties of the original material.

Joining nanostructured ferritic alloys (NFAs) has proved challenging, as the nano-oxides that provide superior strength, creep resistance, and radiation tolerance at high temperatures tend to agglomerate, redistribute, and coarsen during conventional fusion welding. In this study, capacitive discharge resistance welding (CDRW)—a solid-state variant of resistance welding—was used to join end caps and thin-walled cladding tubes of the NFA 14YWT. The resulting solid-state joints were found to be hermetically sealed and were characterized across the weld region using electron microscopy (macroscopic, microscopic, and nanometer scales) and nanoindentation. Microstructural evolution near the weld line was limited to narrow (~50–200 μm) thermo-mechanically affected zones (TMAZs) and to a reduction in pre-existing component textures. Dispersoid populations (i.e., nano-oxides and larger oxide particles) appeared unchanged by all but the highest energy and power CDRW condition, with this extreme producing only minor nano-oxide coarsening (~2 nm → ~5 nm Ø). Despite a minimal microstructural change, the TMAZs were found to be ~10% softer than the surrounding base material. These findings are considered in terms of past solid-state welding (SSW) efforts—cladding applications and NFA-like materials in particular—and in terms of strengthening mechanisms in NFAs and the potential impacts of localized temperature–strain conditions during SSW.

High-purity α-titanium was implanted with helium to observe cavity morphology – size, number density, and form (i.e., bubbles vs. voids) – effects on materials strength. Increasing implantation temperatures lead to an Arrhenius-type (i.e., exponential) increase in cavity size, a transition from spherical bubbles to faceted voids, and a marked increase in strength (∼20–50%). Implanted samples deformed uniformly in contrast to nano-compression of unimplanted or alloyed titanium, but pillars with the largest faceted voids developed a type of bulging localized deformation. Affected voids were repeatedly sheared, becoming rough and shrunken. Dislocation-cavity interactions and the role of helium content within cavities are considered.

Richtmyer-Meshkov instability (RMI) experiments offer a powerful, new tool for investigating materials strength at extreme strain rates, but obtaining peak velocities from the photon Doppler velocimetry (PDV) results of such tests is often subjective and time consuming. A new, semi-automated program, SAVER, was developed to address these concerns and was tested on data sets from previous RMI studies. Extreme surface velocities extracted using SAVER agreed well with human-determined values, and peak spike velocities – a key RMI metric – were reproduced to within ~ 8% in all cases. A re-analysis of materials strength based on these new SAVER values resulted in an estimated increase of ~ 20%, but no objective measure exists at this time to verify this result. While SAVER proved robust with regards to spectrogram quality, scatter in extracted velocities was seen to increase with decreasing signal quality – emphasizing the importance of PDV instrumentation and processing.

Additive manufacturing addresses all aspects of manufacturing. Aerospace, machining, mining, or even medical applications make use of metal based additive manufacturing. Naturally metal additive manufacturing is also used in nuclear applications. The efforts of deploying additive manufactured components in nuclear power applications have been increased and large testing campaigns of additive manufactured parts are exposed to neutron irradiation today through various DOE sponsored programs. properties of stainless to manufacture martensitic the microstructural characterization of powder bed laser fusion additively that the is largely ferritic and subsequent heat treatment is

The effect of helium (He) concentration on ejecta production in OFHC-Copper was investigated using Richtmyer–Meshkov Instability (RMI) experiments. The experiments involved complex samples with periodic surface perturbations machined onto the surface. Each of the four target was implanted with a unique helium concentration that varied from 0 to 4000 appm. The perturbation’s wavelengths were λ≈65μm, and their amplitudes h0 were varied to determine the wavenumber (2π/λ) amplitude product kh0 at which ejecta production beganfor Cu with and without He. The velocity and mass of the ejecta produced was quantified using Photon Doppler Velocimetry (PDV) and Lithium-Niobate (LN) pins, respectively. Our results show that there was an increase of 30% in the velocity at which the ejecta cloud was traveling in Copper with 4000 appm as compared to its unimplanted counterpart. Our work also shows that there was a finer cloud of ejecta particles that was not detected by the PDV probes but was detected by the early arrival of a “signal” at the LN pins. While the LN pins were not able to successfully quantify the mass produced due to it being in the solid state, they did provide information on timing. Our results show that ejecta was produced for a longer time in the 4000 appm copper.

Nanostructured ferritic alloys (NFA) with oxide dispersion strengthening are being developed for next generation reactor applications. Applications such as transmutation of long lived isotopes in used fuels require cladding materials that can withstand high irradiation doses, potentially in the hundreds of displacements per atom (dpa). Currently, however, there are no joining processes that retain the strength and fine distribution of oxide precipitates characteristic of ODS alloys. As part of a larger effort to adapt a solid state capacitive discharge welding technique for use with ODS cladding, presented will be characterization of the initial materials including 14YWT and MA956 ODS alloys.

Thirteen alloys including high- and low-strength nickel-base alloys, austenitic stainless steels, and ferritic alloys were irradiated using 2 MeV protons to a damage level of 2.5 dpa at 360 degrees C and assessed for their susceptibility to irradiation assisted stress corrosion cracking (IASCC) in both BWR normal water chemistry (NWC) and PWR primary water. Cracking susceptibility was highest for high strength nickel-base alloys, followed by the low strength nickel-base alloys and then the low strength iron-base alloys. Cracking in the nickel-based alloys was worst in normal water chemistry, which was reversed for the iron-based alloys. In general, cracking correlated with the degree of microstructure changes, though no single feature could be linked to cracking. IGSCC occurred in both the unirradiated and irradiated conditions in high strength nickel-base alloys with susceptibility being considerably higher following irradiation. In all cases, slip was planar, and the degree of slip localization correlated with the probability of IG crack initiation. Low strength nickel-base alloys showed the same dependence on environment as high strength alloys but were considerably less susceptible to IASCC initiation. Among the low strength iron-base alloys, alloy 800 was most susceptible to IASCC initiation in both BWR NWC and PWR primary water, which also correlated with grain boundary chromium depletion and silicon segregation. Across all alloys, cracking correlated with both the degree of localized deformation and the hardness in the irradiated condition. The agreement is expected as increased hardening also correlates with localized deformation, which is likely a necessary, though insufficient condition for cracking. (C) 2018 Elsevier B.V. All rights reserved.

Ten alloys including austenitic stainless steels 316L and 310; Ni-base alloys X750, 718, 725, 690, 625, and C22; and advanced ferritic alloys T92 (optimized) and 14YWT were irradiated in dual ion mode at 400 degrees C with similar to 5 MeV self-ions to a damage level of 150 dpa and with degraded 2 MeV helium to a concentration of similar to 13 appm He/dpa. Irradiation-induced dislocation loops and nanoscale cavities were observed across the alloys, but with only modest swelling. Pre-existing gamma' and gamma '' precipitates were dissolved or chemically disordered by irradiation, while irradiation-induced phases (e.g., Ni2Cr, G-phase) did not form. In terms of microstructural change, the ferritic alloys, as a class, showed the best radiation resistance while the austenitic stainless steels showed the worst. Radiation resistance among the Ni-base alloys varied significantly, with precipitation-hardened alloys performing worse and dislocation loop content increasing with iron content. These findings were in broad agreement, qualitatively and quantitatively, with past dual beam and in-reactor irradiations of structural materials, demonstrating the utility of dual ion irradiations to capture key evolution quickly and accurately. (C) 2019 Elsevier B.V. All rights reserved.

Thirteen austenitic stainless steels, nickel-base alloys, and ferritic alloys were irradiated using 2 MeV protons at 360 degrees C to a damage level of 2.5 displacements per atom (dpa). Comprehensive microstructural characterization was performed for irradiation-induced features, including dislocation loops, voids, precipitates, and radiation induced segregation (RIS). Dislocation loops formed in all alloys except 14YWT, while voids were observed in alloys 316 L, 310, C22, and 14YWT. Irradiation-induced formation of gamma' precipitates was observed in alloys 316 L, 310, 800, and 690; the irradiation-enhanced, long-range ordered Ni2Cr phase (Pt2Mo-type) was observed in alloys 690, C22, 625, 625Plus, 625DA, and 725; and G-phase was observed in alloy T92. No irradiation-induced precipitates were observed in alloys X750, 718 or 14YWT. Precipitation of the gamma' phase can be understood through segregation and clustering of Si, Al, and Ti. Overall, austenitic stainless steels are generally susceptible to irradiation damage in the form of loops, voids, precipitates, and RIS. Ni-base alloys have this same type of dislocation loops and RIS behaviors but are more resistant to void swelling. Ferritic alloys showed better resistance to loop formation, void swelling and irradiation-induced precipitation. From the degree of irradiation-induced microstructural change, alloy T92 was identified as the most radiation resistant among these alloys. (C) 2018 Elsevier B.V. All rights reserved.

Twelve commercial-grade austenitic alloys based on the Ni-Cr-Mo-Fe quaternary system were irradiated using 2 MeV protons at 360 degrees C to a damage level of 2.5 displacements per atom (dpa). Long-range ordered (Pt2Mo-type) precipitation under proton irradiation was observed, for the first time, in alloys C22, 625, 625P, 625D, 725, and 690. No relevant short-range ordering was observed. These irradiation enhanced long-range ordered precipitates are coherent with the matrix despite their irregular shape. Of the potential influences on long-range ordering (Ni:Cr, and Ni:(Cr + Mo) ratios, Mo, and iron concentration), Fe content was the strongest by far. The volume fraction of LRO decreases with increasing Fe content by virtue of its role as a stabilizer of the disordered FCC phase, thus reducing the energy savings from ordering. The observed effects of Fe on long-range ordering show qualitative agreement with predictions from thermodynamic modeling. Although solid state diffusion kinetics dominate long-range ordering under purely thermal conditions, ordering under irradiation here (similar to 2.5dpa) is controlled by the thermodynamic driving force. Proton irradiation thus offers a unique approach for studying the low temperature phase transformation in a thermodynamically favored, but kinetically constrained condition. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Atomic mixing by replacement collision sequences and other cascade effects is well known to create chemical disorder in irradiated alloys. Most studies of irradiation-induced disordering have focused on ex situ analysis of irradiated samples; however, fast in situ techniques are necessary to measure disordering at elevated temperatures without significant interference from concurrent re-ordering processes. In the present work, we use in situ electron diffraction with high speed data collection to measure the initial change in the long-range order parameter S with ion dose ϕ during 500 keV Ne+ irradiation of Cu3Au foils. The data reveal an unexpected and dramatic increase in the disordering rate as the critical order–disorder transition temperature TC is approached. Molecular dynamics simulations show that this increase is not due to temperature-dependent cascade mixing. We attribute the enhanced disordering, instead, to coupling between point defect fluxes and the chemical state of order.

KineticMonte Carlo simulations have been performed to investigate the evolution of ordered domains in model alloys under irradiation. The alloys investigated were equiatomic binary alloys on a simple square lattice with first and second nearest-neighbor interactions, chosen so that a 2 x 2 ordered structure is the equilibrium phase below a critical order-disorder transition temperature T-c. The ratio of second to first nearest-neighbor interactions R was varied from 0 to 0.45 to explore the effect of the thermodynamic frustrations induced by the proximity of the 2 x 1 phase boundary, which occurs at R = 0.5 for T = 0. The atomic mixing produced by nuclear collisions was modeled by forcing the ballistic exchange of pairs of atoms at a controlled rate Gamma b. This disordering process competed with thermodynamic reordering, resulting in nonequilibrium steady states. Two trivial steady states were found, a disordered state at high Gamma b and low T, and a long-range ordered state at low Gamma b and low T. In the R = 0.45 alloy, however, a third steady state was identified at intermediate Gamma b and T values, where multiple long-range ordered domains coexisted dynamically. It is shown that this state of patterning of order resulted from the coupling of the thermodynamic frustrations present in that alloy with the disorder introduced by irradiation. The practical relevance of this novel mechanism for patterning of order under irradiation is discussed in the context of recent observations of domain coexistence in irradiated Cu3Au.

This work reports on irradiation-induced creep (IIC) measured on nanolaminate (Cu-W and Ni-Ag) and nanocrystalline alloys (Cu-W) at room temperature using a combination of heavy ion irradiation and nanopillar compression performed concurrently in situ in a transmission electron microscope. Appreciable IIC is observed in multilayers with 50 nm layer thicknesses at high stress, the yield strength, but not in multilayers with only 5 nm layer thicknesses. (C) 2017 Elsevier B.V. All rights reserved.