In this article, the Kocks-Mecking formalism is employed to analyze the grain size and thickness effects which modify the mechanical behavior of nickel polycrystals. To this aim, a two-internal ...variable Kocks-Mecking model, based on the evolution of forest and mobile dislocation densities, is numerically optimized using a wide experimental database of tensile curves of nickel polycrystals already published. The original model has been modified to take into account the grain size contribution to strain hardening in addition to the conventional dislocation production and annihilation terms. Using this improved model, the tensile curves of samples with different grain sizes, thicknesses and number of grains across the thickness are well reproduced. By means of an analysis of the model parameters, i.e. dislocation mean free path, cross-slip rate and dislocation densities, the strain mechanisms of nickel polycrystals are investigated as a function of their microstructural characteristics. For specimens with few grains across the thickness, the rate of dislocation annihilation is increased which, in turn, decreases the forest dislocation density and stress level. These results show, first, that the well-known Kocks-Mecking model is able to reproduce size effect and, second, confirm previous assumptions about the mechanical behavior of miniaturized samples. Eventually, the modeling of the mechanical behavior for samples concerned by miniaturization taking into account the surface effect contribution is discussed.
•The Kocks-Mecking-Estrin model is modified to take into account the size effects.•Grain size and thickness effects are well predicted by the model for Ni polycrystals.•Dislocation density predictions are coherent with crystalline plasticity model ones.•Dislocation mean free path prediction is compared to dislocation cell measurements.
During Eugene Garfield’s (EG’s) lengthy career as information scientist, he published about 1500 papers. In this study, we use the impressive oeuvre of EG to introduce a new type of bibliometric ...networks: keyword co-occurrences networks based on the context of citations, which are referenced in a certain paper set (here: the papers published by EG). The citation context is defined by the words which are located around a specific citation. We retrieved the citation context from Microsoft Academic. To interpret and compare the results of the new network type, we generated two further networks: co-occurrence networks which are based on title and abstract keywords from (1) EG’s papers and (2) the papers citing EG’s publications. The comparison of the three networks suggests that papers of EG and citation contexts of papers citing EG are semantically more closely related to each other than to titles and abstracts of papers citing EG. This result accords with the use of citations in research evaluation that is based on the premise that citations reflect the cognitive influence of the cited on the citing publication.
Heat treatments performed on copper-clad aluminium thin wires increase the ductility of copper and aluminium but activate diffusion between both layers, creating brittle intermetallic compounds. ...Ductility reaches a maximum for an optimal intermetallic thickness related to appropriate annealing conditions in order to avoid Kirkendall void formation assisted by stress gradients.
The aim of this article is to provide experimental results in order to understand the microstructural size effects which occur with a decrease in the thickness of polycrystalline nickel samples from ...3.2
mm to 12.5
μm. The influence of the thickness, grain size and ratio thickness to grain size on the mechanical properties and strain hardening were investigated by mechanical tests and TEM observations. The results show the presence of three different domains of mechanical behaviour: polycrystalline, multicrystalline and quasi-single crystalline depending on the thickness and on the number of grains across the thickness. The transition between the three domains is due to the occurrence of surface effects involving a decrease in the long-range internal backstress revealed by the TEM observations.
In radiation therapy, a plan is robust if the calculated and the delivered dose are in agreement, even in the case of different uncertainties. The current practice is to use safety margins, expanding ...the clinical target volume sufficiently enough to account for treatment uncertainties. This, however, might not be ideal for proton therapy and in particular when using intensity modulated proton therapy (IMPT) plans as degradation in the dose conformity could also be found in the middle of the target resulting from misalignments of highly in-field dose gradients. Single field uniform dose (SFUD) and IMPT plans have been calculated for different anatomical sites and the need for margins has been assessed by analyzing plan robustness to set-up and range uncertainties. We found that the use of safety margins is a good way to improve plan robustness for SFUD and IMPT plans with low in-field dose gradients but not necessarily for highly modulated IMPT plans for which only a marginal improvement in plan robustness could be detected through the definition of a planning target volume.
In this work, 316L samples with submicrometric grain size were sintered by spark plasma sintering. To this aim, 316L powder was first ball-milled with different conditions to obtain nanostructured ...powder. The process control agent quantity and milling time were varied to check their influence on the crystallite size of milled powder. Samples were then sintered by spark plasma sintering using different sets of sintering parameters (temperature, dwell time and pressure). For each sample, grain size and density were systematically measured in order to investigate the influence of the sintering process on these two key microstructure parameters. Results show that suitable ball-milling and subsequent sintering can be employed to obtain austenitic stainless steel samples with grain sizes in the nanometer range with porosity lower than 3%. However, ball-milling and subsequent sintering enhance chromium carbides formation at the sample surface in addition to intragranular and intergranular oxides in the sample as revealed by X-ray diffraction and transmission electron microscopy. It has been shown that using Boron nitride together with graphite foils to protect the mold from powder welding prevent such carbide formation. For mechanical properties, results show that the grain size refinement strongly increases the hardness of the samples without deviation from Hall-Petch relationship despite the oxides formation. For corrosion resistance, grain sizes lower than a few micrometers involve a strong decrease in the pitting potential and a strong increase in passivation current. As a consequence, spark plasma sintering can be considered as a promising tool for ultra-fine grained austenitic stainless steel.
Powder metallurgy associated to spark plasma sintering was used to elaborate near full-dense samples of 316L austenitic stainless steel with unimodal or bimodal grain size distributions. To this aim, ...two different precursor powders were employed: a ball-milled one giving rise to ultrafine grains and a coarse one, as-received, for grains with conventional size. Sintered specimens were characterized in mechanical tension and their microstructure was revealed using transmission and scanning electron microscopy. Unimodal ultrafine grained samples show a large yield stress and a low ductility with a breakdown in the Hall-Petch relationship. For bimodal samples, a compromise between yield stress and ductility can be found. These features are then discussed in terms of strain mechanisms, grain size distribution and backstress. It is shown in particular that coarse grains contribute to enhance the ductility of the ultrafine grains matrix by modifying both the strain hardening mechanisms and the stress concentration areas.
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•Impacts of nanostructuration and irradiation on the properties of 316 stainless steels are reported.•Irradiation of nanostructured samples implies chromium depletion as than depicted ...in coarse grain specimens.•Hardness of nanocrystalline steels is only weakly affected by irradiation.•Corrosion resistance of the nanostructured and irradiated samples is less affected by the chromium depletion.
The influence of grain size and irradiation defects on the mechanical behavior and the corrosion resistance of a 316 stainless steel have been investigated. Nanostructured samples were obtained by severe plastic deformation using high pressure torsion. Both coarse grain and nanostructured samples were irradiated with 10MeV 56Fe5+ ions. Microstructures were characterized using transmission electron microscopy and atom probe tomography. Surface mechanical properties were evaluated thanks to hardness measurements and the corrosion resistance was studied in chloride environment. Nanostructuration by high pressure torsion followed by annealing leads to enrichment in chromium at grain boundaries. However, irradiation of nanostructured samples implies a chromium depletion of the same order than depicted in coarse grain specimens but without metallurgical damage like segregated dislocation loops or clusters. Potentiodynamic polarization tests highlight a definitive deterioration of the corrosion resistance of coarse grain steel with irradiation. Downsizing the grain to a few hundred of nanometers enhances the corrosion resistance of irradiated samples, despite the fact that the hardness of nanocrystalline austenitic steel is only weakly affected by irradiation. These new experimental results are discussed in the basis of couplings between mechanical and electrical properties of the passivated layer thanks to impedance spectroscopy measurements, hardness properties of the surfaces and local microstructure evolutions.
Little is known whether peer reviewers use the same evaluation criteria and how they integrate the criteria into their overall judgment. This study therefore proposed two assessment styles based on ...theoretical perspectives and normative positions. According to the case-by-case style, referees use many and different criteria, weight criteria on a case-by-case basis, and integrate criteria in a complex, non-mechanical way into their overall judgment. According to the uniform style, referees use a small fraction of the available criteria, apply the same criteria, weight the criteria in the same way, and integrate the criteria based on simple rules (i.e., fast-and-frugal heuristics). These two styles were examined using a unique dataset from a career funding scheme that contained a comparatively large number of evaluation criteria. A heuristic (fast-and-frugal trees) and a complex procedure (logistic regression) were employed to describe how referees integrate the criteria into their overall judgment. The logistic regression predicted the referees’ overall assessment with high accuracy and slightly more accurately than the fast-and-frugal trees. Overall, the results of this study support the uniform style but also indicate that the uniform style needs to be revised as follows: referees use many criteria and integrate the criteria using complex rules. However, and most importantly, the revised style could describe most—but not all—of the referees’ judgments. Future studies should therefore examine how referees’ judgments can be characterized in those cases where the uniform style failed. Moreover, the evaluation process of referees should be studied in more empirical and theoretical detail.
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