We present large-scale atomistic simulations that reveal triple junction (TJ) segregation in Pt–Au nanocrystalline alloys in agreement with experimental observations. While existing studies suggest ...grain boundary solute segregation as a route to thermally stabilize nanocrystalline materials with respect to grain coarsening, here we quantitatively show that it is specifically the segregation to TJs that dominates the observed stability of these alloys. Our results reveal that doping the TJs renders them immobile, thereby locking the grain boundary network and hindering its evolution. In dilute alloys, it is shown that grain boundary and TJ segregation are not as effective in mitigating grain coarsening, as the solute content is not sufficient to dope and pin all grain boundaries and TJs. Our work highlights the need to account for TJ segregation effects in order to understand and predict the evolution of nanocrystalline alloys under extreme environments.
The ability to change behavioural strategies in the face of a changing world has been linked to the integrity of medial prefrontal cortex (mPFC) function in several species. While recording studies ...have found that mPFC representations reflect the strategy being used, lesion studies suggest that mPFC is necessary for changing strategy. Here we examine the relationship between representational changes in mPFC and behavioural strategy changes in the rat. We found that on tasks with a forced change in reward criterion, strategy-related representational transitions in mPFC occurred after animals learned that the reward contingency had changed, but before their behaviour changed. On tasks in which animals made their own strategic decisions, representational transitions in mPFC preceded changes in behaviour. These results suggest that mPFC does not merely reflect the action-selection policy of the animal, but rather that mPFC processes information related to a need for a change in strategy.
Helium bubbles are known to form in nuclear reactor structural components when displacement damage occurs in conjunction with helium exposure and/or transmutation. If left unchecked, bubble ...production can cause swelling, blistering, and embrittlement, all of which substantially degrade materials and—moreover—diminish mechanical properties. On the mission to produce more robust materials, nanocrystalline (NC) metals show great potential and are postulated to exhibit superior radiation resistance due to their high defect and particle sink densities; however, much is still unknown about the mechanisms of defect evolution in these systems under extreme conditions. Here, the performances of NC nickel (Ni) and iron (Fe) are investigated under helium bombardment via transmission electron microscopy (TEM). Bubble density statistics are measured as a function of grain size in specimens implanted under similar conditions. While the overall trends revealed an increase in bubble density up to saturation in both samples, bubble density in Fe was over 300% greater than in Ni. To interrogate the kinetics of helium diffusion and trapping, a rate theory model is developed that substantiates that helium is more readily captured within grains in helium-vacancy complexes in NC Fe, whereas helium is more prone to traversing the grain matrices and migrating to GBs in NC Ni. Our results suggest that (1) grain boundaries can affect bubble swelling in grain matrices significantly and can have a dominant effect over crystal structure, and (2) an NC-Ni-based material can yield superior resistance to irradiation-induced bubble growth compared to an NC-Fe-based material and exhibits high potential for use in extreme environments where swelling due to He bubble formation is of significant concern.
•Defect morphology inspection is essential for radiation tolerance investigation.•Diffraction contrast imaging applied to nanocrystalline materials is inefficient.•Automated crystal orientation ...mapping can identify grains for quantitative analysis.•Defect areal density is a superior measurement than number density.
Transmission electron microscopy (TEM) is an established tool used for the investigation of defects in materials. Traditionally, diffraction contrast techniques—two-beam bright-field and weak-beam dark-field—have been used to image defects due to contrast sensitivity from weak lattice strains. Use of these methods entail an intricate tilt series of imaging using different diffracting vectors, g, to verify the g•b invisibility criterion relative to the different defect types and habit planes inherent to the material. Recently, the addition of down-zone imaging and STEM imaging has also proven to be effective imaging techniques for defect density analysis. Interest in nanocrystalline (NC) materials, spurred by their conjectured superior properties compared to their coarse-grain counterparts, has been thriving and the investigation of their defect morphologies is essential. Maneuvering within NC samples in the TEM adds another layer of difficulty making the aforementioned techniques not practical for application to specimens with complex microstructures. For this reason, we have devised a protocol for identifying NC grains optimally oriented for quantitative analysis using NanoMegas ASTAR automated crystal orientation mapping (ACOM) in the TEM. In this work, we conduct a series of experiments assessing the effectiveness of conventional two-beam bright-field, weak-beam dark-field, and down-zone STEM imaging. We also evaluate an ACOM-assisted multibeam imaging method and compare defect density results obtained using each technique in an irradiated nanocrystalline Au sample.
Abstract
Gold is a noble metal typically stable as a solid in a face-centered cubic (FCC) structure under ambient conditions; however, under particular circumstances aberrant allotropes have been ...synthesized. In this work, we document the phase transformation of 25 nm thick nanocrystalline (NC) free-standing gold thin-film via in situ ion irradiation studied using atomic-resolution transmission electron microscopy (TEM). Utilizing precession electron diffraction (PED) techniques, crystallographic orientation and the radiation-induced relative strains were measured and furthermore used to determine that a combination of surface and radiation-induced strains lead to an FCC to hexagonal close packed (HCP) crystallographic phase transformation upon a 10 dpa radiation dose of Au
4+
ions. Contrary to previous studies, HCP phase in nanostructures of gold was stabilized and did not transform back to FCC due to a combination of size effects and defects imparted by damage cascades.
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.
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•Irradiation induced defect densities did not demonstrate an immediate correlation to grain size.•Sample composition was found to affect defect evolution in nanocrystalline systems.•Average defect ...cluster size ubiquitously decreases as grain size decreases.•Solute addition inhibits defect kinetics and impedes defect agglomeration, i.e. smaller defect cluster size.
Grain boundaries (GBs) are considered sinks where mobile defects are attracted and annihilated thereby hampering irradiation damage accumulation. Nanocrystalline (NC) metals characteristically have greater densities of GBs relative to their coarse-grained counterparts hence they are postulated to provide enhanced resistance to irradiation damage.
The use of alloying as a means to impart synergistic properties such as corrosion resistance, increased toughness, or improved conductivity is well studied, yet the cooperative effects of solute addition and grain size in the nano-regime is not well understood. In this study, a combination of in situ ion irradiation, transmission electron microscopy (TEM), and automated crystal orientation mapping (ACOM) on model Ni, NiCr, Fe, and FeCr NC thin-films are used to provide experimental evidence that grain size and irradiation induced defect morphology (defect density and size) are not directly correlated due to defect agglomeration, annihilation at sinks, and saturation, while the addition of solute impedes defect mobility, altering the final damage state.
Minority women, particularly immigrants, have lower cancer screening rates than Caucasian women, but little else is known about cancer screening among immigrant women. Our objective was to assess ...breast, cervical, and colorectal cancer screening rates among immigrant women from Cambodia, Somalia, and Vietnam and explore screening barriers.
We measured screening rates by systematic chart review (N = 100) and qualitatively explored screening barriers via face-to-face questionnaire (N = 15) of women aged 50-75 from Cambodia, Somalia, and Vietnam attending a general medicine clinic (Portland, Maine, USA).
Chart Review - Somali women were at higher risk of being unscreened for breast, cervical, and colorectal cancer compared with Cambodian and Vietnamese women. A longer period of US residency was associated with being screened for colorectal cancer. We observed a 7% (OR 1.07, 95% CI 1.01-1.13, p = 0.01) increase in the odds that a woman would undergo a fecal occult blood test for each additional year in the US, and a 39% increase in the odds of a woman being screened by colonoscopy or flexible sigmoidoscopy for every five years of additional US residence (OR 1.39, 95% CI 1.21-1.61, p = 0.02). We did not observe statistically significant relationships between odds of being screened by mammography, clinical breast exam or papanicolaou test according to years in the US. Questionnaire - We identified several barriers to breast, cervical, and colorectal cancer screening, including discomfort with exams conducted by male physicians.
Somali women were less likely to be screened for breast, cervical, and colorectal cancer than Cambodian and Vietnamese women in this population, and uptake of colorectal cancer screening is associated with years of residency in this country. Future efforts to improve equity in cancer screening among immigrants may require both provider and community education.
Althoughthe prefrontal cortices, in particular the dorsolateral prefrontal cortex (DLPFC), have been examined in numerous imaging and neuropsychological studies, it has proved difficult to assign a ...specific function to this brain region. The aim of this study was therefore to delineate the function of the DLPFC first by using positron emission tomography with a word generation task and second by comparing these findings with a series of different circumstances in which the DLPFC is activated in association with response selection. Six healthy volunteers were presented with a sentence completion task based on the Hayling test. In each of two conditions, (A) response initiation and (B) response suppression, volunteers saw a sentence with the final word omitted. In condition A they had to provide a word that fitted at the end of the sentence and in condition B they had to provide a word that did not fit. The corpus of sentences used varied systematically in their level of contextual constraint, ranging from low to high. With all levels of constraint combined, significant greater activation was observed in the left DLPFC (BA46/9) under the suppression condition and in the medial orbitofrontal cortex (BA11) under the initiation condition. Under the high-constraint condition with both tasks combined, significant right middle temporal activity (BA21) was observed, whereas under low constraint, the left DLPFC was significantly activated. An interaction of task and constraint revealed that the left DLPFC was significantly more active in the suppression task at all levels of constraint, but only under the low-constraint conditions in the initiation task. The reaction time analyses mirrored the pattern of activity observed, with slower reaction times under conditions of suppression and under conditions of initiation with low constraint. By comparing these findings with other studies of response selection, we argue that the most likely single cognitive function of the DLPFC is to specify a set of responses suitable for a given task and to bias these for selection (sculpting the response space). This function resembles the biasing of competition between stimuli in the model of Desimone and Duncan and is analogous to the component of the Supervisory Attentional System that modulates the contention scheduling system.
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