19 research outputs found
Micropillar compression study of the influence of size and internal boundary on the strength of HT9 tempered martensitic steel
No full textHT9, a ferritic/martensitic steel, is a candidate structural material for next-generation advanced reactors. Its microstructure is a typical tempered martensite showing a hierarchical lath-block-and-packet structure. We investigate the specimen size effect and strengthening contribution of various microstructural boundaries manifested in the compression tests of micropillars with diameters ranging from 0.5 to 17 mu m. It is observed that micropillars with diameters larger than 3 mu m show uniform deformation and plastic flow curves comparable to the bulk flow curve. Localized deformation by a few pronounced slip bands occurs in micropillars with diameters smaller than 1 mu m, and the yield strength is reduced. Careful examination of the sizes of the microstructural features and cross-sections of the micropillars shows that the block boundaries are the most effective strengthening boundaries in tempered martensitic microstructure. The bulk mechanical properties of HT9 can be evaluated from a micropillar with diameter as low as 3 mu m. (c) 2018 Elsevier B.V. All rights reserved.
Anisotropic He-ion irradiation damages in nanocolumnarWthin films
No full textThe effects of He-ion irradiation on the microstructures and the mechanical, thermal properties of sputter-deposited nanocolumnar tungsten thin films have been studied. 200 keV He+ ion irradiation with a fluence of 2x1017 ions/cm2 was performed in the growth direction of the W thin films. Small scale mechanical testing methods, such as nanoindentation and square membrane deflection experiments, were carried out, and the thermal conductivity measurement was performed based on the electrical resistivity measurement and the Wiedemann–Franz law for the unirradiated and irradiated W thin films. It was revealed that the properties in the out-of-plane direction are not changed much, but a significant degradation occurs in the in-plane direction after the He-ion irradiation. The microstructure of the film and the distribution of He-ion induced damages are responsible for the anisotropic property changes by He-ion irradiation.
3D discrete dislocation dynamics applied to dislocation-precipitate interactions
No full textLa dynamique des dislocations discrètes (DDD) a été appliquée pour examiner les effets des précipités sur la plasticité des monocristaux de structure CFC.Les précipités sont modélisés par un assemblage de facettes franchissable pour une contrainte donnée. Afin de tenir compte des interactions élastiques entre les dislocations et les particules, un couplage avec la méthode des éléments finis (MEF) a été utilisé. Afin d'accélérer les temps de calculs, la 'méthode des boites' a été revisitée et une version parallèle du code a été développée en utilisant le standard du programmation 'Message Passing Interface (MPI)'.Dans un premier temps, les contraintes images créées par une particule 3D ont été calculées grâce un couplage entre la MEF et le code de DDD. Les résultats numériques ont été comparés aux solutions analytiques correspondantes. L'effet de la différence des modules d'Young sur la limite élastique et le comportement durcissant qui en découle ont ensuite été étudiés numériquement. Nous avons montré que les contraintes image ont un effet significatif sur le durcissement et les événements locaux tels que le glissement dévié et la montée. Finalement, la fatigue des matériaux durcis par des précipités cisaillables et non-cisaillables a été simulée avec le nouveau code parallèle de DDD. Les résultats obtenus grâce à nos simulations sont en accord avec nos observations experimentales et les données de la littérature. Un mécanisme de formation des bandes de glissement intense a été proposé à partir de l'observation des microstructures obtenues par simulation.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF
Modification of Electrical and Switching Characteristics of a Non Punch-Through Insulated Gate Bipolar Transistor by Gamma Irradiation
No full textFast neutron irradiation using nuclear reactors is an effective method to improve switching loss and short circuit durability of power semiconductor (insulated gate bipolar transistors (IGBT) and insulated gate transistors (IGT), etc.). However, not only fast neutrons but also thermal neutrons, epithermal neutrons and gamma exist in the nuclear reactor. And the electrical properties of the IGBT may be deteriorated by the irradiation of gamma. Gamma irradiation damages are known to be caused by Total Ionizing Dose (TID) effect and Single Event Effect (SEE), Displacement Damage. Especially, the TID effect deteriorated the electrical properties such as leakage current and threshold voltage of a power semiconductor. This work can confirm the effect of the gamma irradiation on the electrical properties of 600 V NPT-IGBT. Irradiation of gamma forms lattice defects in the gate oxide and Si-SiO2 interface of the IGBT. It was confirmed that this lattice defect acts on the center of the trap and affects the threshold voltage, thereby negatively shifted the threshold voltage according to TID. In addition to the change in the carrier mobility, the conductivity modulation decreases in the n-drift region, indicating a negative influence that the forward voltage drop decreases. The turn-off delay time of the device before irradiation was 212 ns. Those of 2.5, 10, 30, 70 and 100 kRad(Si) were 225, 258, 311, 328, and 350 ns, respectively. The gamma irradiation increased the turn-off delay time of the IGBT by approximately 65%, and the switching characteristics deteriorated
Effects of gamma irradiation on the electrical characteristics of trench-gate non-punch-through insulated gate bipolar transistor
No full textWe studied the effects of gamma-ray irradiation on the static and dynamic electrical characteristics of a trench-gate non-punch-through insulated gate bipolar transistor (IGBT) device. The threshold voltage and breakdown voltage decreased after irradiation, whereas the collector leakage current increased. Turn-on and turn-off switching times decreased and increased, respectively, with the irradiation dose. The irradiation-induced changes in the electrical characteristics can be attributed to the buildup of the positive oxide-trapped charges in the gate oxide by gamma irradiation. The evaluated characteristics data were compared with the data for gamma-irradiated planar-gate IGBTs reported in the literature. It was found that a significant degradation occurred in the shift of the threshold voltage and switching times for trench-gate IGBTs
Derivation of the Depth Dependent Mechanical Characteristics of Ion-Irradiated F82H Steels by Using a Finite Element Analysis
No full textMicrostructures and high-temperature tensile properties of mechanically alloyed and spark plasma-sintered 304SS-CNT composites
No full text We fabricated and investigated a 304 stainless steel and carbon nanotube (304SS-CNT) composite with an aim to study its microstructures and high-temperature tensile properties. 304SS powders were mixed with carbon nanotubes using ball milling and consolidated using the spark plasma sintering technique. Tensile specimens made from the consolidated samples of 304SS-CNT were tested in a temperature range from 299 K (26℃) to 773 K (500℃). An induction coil was used for high-temperature tensile tests. The yield strength and the work hardening of the 304SS-CNT sample were found to be higher than those of a sample fabricated from 304SS without carbon nanotubes for all tested temperatures. Microstructure analysis carried out using optical microscopy, scanning electron microscopy, and transmission electron microscopy showed that the 304SS-CNT sample has a microstructure significantly different from the 304SS sample, e.g. reduced grain size and many small cuboidal particles. Composition analysis using energy-dispersive spectroscopy revealed that the cuboidal particles are chromium carbides, and the chromium content is reduced in the 304SS-CNT matrix. Retained carbon nanotubes could not be observed; it is thought that the carbon nanotubes may decompose, induce the reduced grain size and chromium carbides. </jats:p
Effect of irradiation temperature on microstructural changes in self-ion irradiated austenitic stainless steel
No full textCopper Alloy Design for Preventing Sulfur-Induced Embrittlement in Copper
No full textThis study presents an experimental approach to address sulfur-induced embrittlement in copper alloys. Building on recent theoretical insights, we identified specific solute elements, such as silicon and silver, known for their strong binding affinity with vacancies. Through experimental validation, we demonstrated the effectiveness of Si and Ag in preventing sulfur-induced embrittlement in copper, even though they are not typical sulfide formers such as zirconium. Additionally, our findings highlight the advantages of these elements over traditional solutes, such as their high solubility and propensity to accumulate along grain boundaries. This approach may have the potential to be applied to other metals prone to sulfur-induced embrittlement, including nickel, iron, and cobalt, offering broader implications for materials engineering strategies and alloy development
Synaptic characteristics with strong analog potentiation, depression, and short-term to long-term memory transition in a Pt/CeO2/Pt crossbar array structure
No full textA crossbar array of Pt/CeO2/Pt memristors exhibited the synaptic characteristics such as analog, reversible, and strong resistance change with a ratio of similar to 10(3), corresponding to wide dynamic range of synaptic weight modulation as potentiation and depression with respect to the voltage polarity. In addition, it presented timing-dependent responses such as paired-pulse facilitation and the short-term to long-term memory transition by increasing amplitude, width, and repetition number of voltage pulse and reducing the interval time between pulses. The memory loss with a time was fitted with a stretched exponential relaxation model, revealing the relation of memory stability with the input stimuli strength. The resistance change was further enhanced but its stability got worse as increasing measurement temperature, indicating that the resistance was changed as a result of voltage-and temperature-dependent electrical charging and discharging to alter the energy barrier for charge transport. These detailed synaptic characteristics demonstrated the potential of crossbar array of Pt/CeO2/Pt memristors as artificial synapses in highly connected neuron-synapse network
