Cylindrical nano-indentation on metal film/elastic substrate system with discrete dislocation plasticity analysis: A simple model for nano-indentation size effect

AbstractThe cylindrical nano-indentation on metal film/elastic substrate is computationally studied using two-dimensional discrete dislocation plasticity combined with the commercial software ANSYS®, with a focus on the storage volume for geometrically necessary dislocations (GNDs) inside the films and the nano-indentation size effect (NISE). Our calculations show that almost all GNDs are stored in a rectangular area determined by the film thickness and the actual contact width. The variations of indentation contact width with indentation depth for various film thicknesses and indenter radii are fitted by an exponential relation, and then the GND density underneath the indenter is estimated. Based on the Taylor dislocation model and Tabor formula, a simple model for the dependence of the nano-indentation hardness of the film/substrate system on the indentation depth, the indenter radius and the film thickness is established, showing a good agreement with the present numerical results

Strain rate dependency of dislocation plasticity

Dislocation slip is a general deformation mode and governs the strength of metals. Via discrete dislocation dynamics and molecular dynamics simulations, we investigate the strain rate and dislocation density dependence of the strength of bulk copper single crystals using 192 simulations spanning over 10 orders of magnitude in strain rate and 9 orders of magnitude in dislocation density. Based on these large set of simulations and theoretical analysis, a new analytical relationship between material strength, dislocation density, strain rate and dislocation mobility is proposed, which is in excellent agreement with the current simulations as well as with experimental data. The results show that the material strength is a non-monotonic function of dislocation density and displays two universal regimes (first decreasing, then increasing) as the dislocation density increases. The first regime is a result of strain rate hardening, while the second regime is dominated by the classical Taylor forest hardening. Accordingly, the strength displays universally, as a function of strain rate, a rate-independent regime at low strain rates (governed by forest hardening) followed by a rate hardening regime at high strain rates (governed by strain rate hardening). All the results can be captured by a single scaling function. Finally, the fluctuations of dislocation flow are analyzed in terms of the strain rate dependent distribution of dislocation segment velocities. It is found that the fluctuations are governed by another universal scaling function and diverge in the rate independent limit, indicating a critical behavior. The current analysis provides a comprehensive understanding on how collective dislocation motions are governed by the competition between the internal elastic interactions of dislocations, and the stress required to drive dislocation fluxes at a given externally imposed strain rate.Comment: 31 pages, 6 figures, 71 conference

The effect of size, orientation and alloying on the deformation of AZ31 nanopillars

We conducted uniaxial compression of single crystalline Mg alloy, AZ31 (Al 3% wt. and Zn 1% wt.) nanopillars with diameters between 300–5000 nm with two distinct crystallographic orientations: (1) along the [0001] c-axis and (2) at an acute angle away from the c-axis, nominally oriented for basal slip. We observe single slip deformation for sub-micron samples nominally oriented for basal slip with the deformation commencing via a single set of parallel shear offsets. Samples compressed along the c-axis display an increase in yield strength compared to basal samples as well as significant hardening with the deformation being mostly homogeneous. We find that the “smaller is stronger” size effect in single crystals dominates any improvement in strength that may have arisen from solid solution strengthening. We employ 3D-discrete dislocation dynamics (DDD) to simulate compression along the [0001] and [1122] directions to elucidate the mechanisms of slip and evolution of dislocation microstructure. These simulations show qualitatively similar stress strain signatures to the experimentally obtained stress-strain data. Simulations of compression parallel to the [1122] direction reveal the activation and motion of only -type dislocations and virtually no dislocation junction formation. Computations of compression along [0001] show the activation and motion of both and dislocations along with a significant increase in the formation of junctions corresponding to the interaction of intersecting pyramidal planes. Both experiments and simulation show a size effect, with a differing exponent for basal and pyramidal slip. We postulate that this anisotropy in size effect is a result of the underlying anisotropic material properties only. We discuss these findings in the context of the effective resolved shear stress relative to the unit Burgers vector for each type of slip, which reveal that the mechanism that governs size effect in this Mg-alloy is equivalent in both orientations

Distributed Equivalent Substitution Training for Large-Scale Recommender Systems

We present Distributed Equivalent Substitution (DES) training, a novel distributed training framework for large-scale recommender systems with dynamic sparse features. DES introduces fully synchronous training to large-scale recommendation system for the first time by reducing communication, thus making the training of commercial recommender systems converge faster and reach better CTR. DES requires much less communication by substituting the weights-rich operators with the computationally equivalent sub-operators and aggregating partial results instead of transmitting the huge sparse weights directly through the network. Due to the use of synchronous training on large-scale Deep Learning Recommendation Models (DLRMs), DES achieves higher AUC(Area Under ROC). We successfully apply DES training on multiple popular DLRMs of industrial scenarios. Experiments show that our implementation outperforms the state-of-the-art PS-based training framework, achieving up to 68.7% communication savings and higher throughput compared to other PS-based recommender systems.Comment: Accepted by SIGIR '2020. Proceedings of the 43rd International ACM SIGIR Conference on Research and Development in Information Retrieval. 202

Functional Role of Cyclin-Dependent Kinase 5 in the Regulation of Melanogenesis and Epidermal Structure

The mammalian integumentary system plays important roles in body homeostasis, and dysfunction of melanogenesis or epidermal development may lead to a variety of skin diseases, including melanoma. Skin pigmentation in humans and coat color in fleece-producing animals are regulated by many genes. Among them, microphthalmia-associated transcription factor (MITF) and paired-box 3 (PAX3) are at the top of the cascade and regulate activities of many important melanogenic enzymes. Here, we report for the first time that cyclin-dependent kinase 5 (Cdk5) is an essential regulator of MITF and PAX3. Cdk5 knockdown in mice causes a lightened coat color, a polarized distribution of melanin and hyperproliferation of basal keratinocytes. Reduced expression of Keratin 10 (K10) resulting from Cdk5knockdown may be responsible for an abnormal epidermal structure. In contrast, overexpression of Cdk5 in sheep (Ovis aries) only produces brown patches on a white background, with no other observable abnormalities. Collectively, our findings show that Cdk5 has an important functional role in the regulation of melanin production and transportation and in normal development of the integumentary system

Skin transcriptome profiles associated with coat color in sheep

Background Previous molecular genetic studies of physiology and pigmentation of sheep skin have focused primarily on a limited number of genes and proteins. To identify additional genes that may play important roles in coat color regulation, Illumina sequencing technology was used to catalog global gene expression profiles in skin of sheep with white versus black coat color. Results There were 90,006 and 74,533 unigenes assembled from the reads obtained from white and black sheep skin, respectively. Genes encoding for the ribosomal proteins and keratin associated proteins were most highly expressed. A total of 2,235 known genes were differentially expressed in black versus white sheep skin, with 479 genes up-regulated and 1,756 genes down-regulated. A total of 845 novel genes were differentially expressed in black versus white sheep skin, consisting of 107 genes which were up-regulated (including 2 highly expressed genes exclusively expressed in black sheep skin) and 738 genes that were down-regulated. There was also a total of 49 known coat color genes expressed in sheep skin, from which 13 genes showed higher expression in black sheep skin. Many of these up-regulated genes, such as DCT, MATP, TYR and TYRP1, are members of the components of melanosomes and their precursor ontology category. Conclusion The white and black sheep skin transcriptome profiles obtained provide a valuable resource for future research to understand the network of gene expression controlling skin physiology and melanogenesis in sheep

Association between Vitamin D supplementation and mortality in critically ill patients: A systematic review and meta-analysis of randomized clinical trials.

BACKGROUND: Observational studies suggest that low 25-hydroxyvitamin D status is common and has been associated with higher mortality in critically ill patients. This study aim to investigate whether vitamin D supplementation is associated with lower mortality in critically ill patients. METHOD: We searched Medline, Embase, and Cochrane databases from inception to January 12, 2020, without language restrictions, for randomized controlled trials comparing the effect of vitamin D supplementation with placebo in critically ill patients. Two authors independently performed data extraction and assessed study quality. The primary outcome was all-cause mortality at the longest follow-up. RESULT: We identified nine trials with a total of 2066 patients. Vitamin D supplementation was not associated with reduced all-cause mortality at the longest follow-up (RR 0.90, 95% CI 0.74 to 1.09, I2 = 20%), at 30 days (RR 0.81, 95% CI 0.56 to 1.15), at 90 days (RR 1.15, 95% CI 0.92 to 1.44), and at 180 days (RR 0.82, 95% CI 0.65 to 1.03). Results were similar in the sensitivity analysis. The sample size met the optimum size in trial sequential analysis. Similarly, supplemental vitamin D was not associated with length of ICU stay, hospital stay, or mechanical ventilation. CONCLUSION: Vitamin D supplement was not associated with reduced all-cause mortality in critically ill patients. SYSTEMATIC REVIEW REGISTRATION: Open Science Framework https://osf.io/bgsjq