Hardening mechanism of commercially pure Mg processed by high pressure torsion at room temperature

Coarse-grained Mg in the as-cast condition and fine-grained Mg in the extruded condition were processed by high pressure torsion (HPT) at room temperature for up to 16 turns. Microstructure observation and texture analysis indicate that to fulfil the Von Mises criterion, the non-basal slip is activated in the as-cast Mg and tension twinning is activated in the as-extruded Mg. Although the deformation mechanism is different in the as-cast Mg and the as-extruded Mg during HPT, their hardening evolutions are similar, i.e. after 1/8 turn of HPT, microhardness of the as-cast Mg and the extruded Mg both show a significant increase and further HPT processing does not significantly further increase the microhardness. Texture strengthening can explain the rapid hardening. Hardness anisotropy and texture data results suggest that texture strengthening plays an important role for both types of samples. Texture strengthening weakens with decreasing grain size

Predicting grain refinement by cold severe plastic deformation in alloys using volume average dislocation generation

The grain refinement during severe plastic deformation (SPD) is predicted using volume averaged amount of dislocations generated. The model incorporates a new expansion of a model for hardening in the parabolic hardening regime, in which the work hardening depends on the effective dislocation free path related to the presence of non shearable particles and solute-solute nearest neighbour interactions. These two mechanisms give rise to dislocation multiplication in the form of generation of geometrically necessary dislocations and dislocations induced by local bond energies. The model predicts the volume averaged amount of dislocations generated and considers that they distribute to create cell walls and move to existing cell walls/grain boundaries where they increase in the grain boundary misorientation. The model predicts grain sizes of Al alloys subjected to SPD over 2 orders of magnitude. The model correctly predicts the considerable influence of Mg content and content of non-shearable particles on the grain refinement during SPD

Prediction of hardness of Al alloys processed by accumulative roll bonding

A recently developed grain refinement and strengthening model for oxide-free Al alloys processed by Severe Plastic Deformation (SPD) is tested and subsequently further developed to address Accumulative Roll Bonding (ARB) processed Al alloys. The model accurately predicts the grain refinement and the hardness of an ARB processed alloy. Compared to other SPD processes, oxide inclusions at the bonded surfaces cause additional grain refinement and hardening. The model also reveals that the hardness measured on the large surface of ARB processed Al alloys overestimates the volume averaged hardness of the material due to artifacts caused by the sample preparation and the indentation depth

Nitroxide-mediated polymerization of methacrylates in the presence of 4-vinyl pyridine as controlling comonomer

International audience• 4VP is an efficient controlling comonomer for NMP of MMA and PEOMA. • 4VP is unable to control the NMP of methacrylic acid. • Successful formation of 4VP-based block copolymers by chain extension. • Chain extension in the presence of silica particles yields raspberry-like composite colloids. A B S T R A C T The controlled polymerization of methacrylic monomers by nitroxide-mediated polymerization (NMP) still represents a challenge in polymer science. This issue can be circumvented by the addition of a small amount of a comonomer known to exhibit a controlled character in NMP, the most representative example being styrene. The purpose of this work is to explore the use of 4-vinyl pyridine (4VP) as controlling comonomer, while conferring at the same time some functionality to the polymer chains. The NMP of methyl methacrylate (MMA), poly (ethylene glycol) methyl ether methacrylate (PEOMA 950 , M n = 950 g mol −1) and methacrylic acid (MAA) using a small amount of 4VP under mild experimental conditions (< 90 °C, atmospheric pressure), was investigated in this work. The copolymerization of MMA or PEOMA 950 with 10 mol% 4VP, mediated by the use of the BlocBuilder ® alkoxyamine and a small amount of free nitroxide SG1, exhibited all the features of a controlled system in agreement with the favored incorporation of 4VP at the chain ends, leading to an efficient deactivation of the propagating radicals by the nitroxide SG1. In contrast, the polymerization of MAA was uncontrolled in the same conditions likely due to acid/base interaction between MAA and 4VP, affecting the reactivity of the functional comonomer. Consistently, the copolymerization of MAA with MMA also led to a progressive loss of control as the MAA content in the feed was increased. Among all the polymers synthesized, the P(MMA 72-co-4VP 10)-SG1 macroalkoyamine was then successfully used to reinitiate the copolymerization of n-butyl metha-crylate (BMA) and 4VP both in solution and via dispersion polymerization in a mixture of ethanol and water. Finally, raspberry-like polymer/silica composite particles were prepared through nitroxide-mediated dispersion polymerization performed in the presence of silica nanoparticles, taking benefit of the strong acid-base interaction between 4VP and silica

Microstructure and mechanical properties of a nanostructured Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr supersaturated solid solution prepared by high pressure torsion

In this study, a solution-treated Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr (wt.%) alloy was processed by high pressure torsion (HPT) at room temperature, with the aim of producing a material with exceptional mechanical properties. Transmission electron microscopy and X-ray diffraction line broadening analysis reveal that a nanostructured supersaturated solid solution with a mean grain size of ~48 nm and high dislocation density of ~4.7×1014 m-2 is achieved by HPT deformation. After HPT processing, the present solution-treated samples show finer grains and higher dislocation densities as compared to as-cast samples of this alloy, other solution-treated Mg-RE alloys and conventional Mg alloys deformed using similar HPT processing conditions. This could be attributed to retardation of the dislocation annihilation through enhanced solute-dislocation and / or dislocation-dislocation interactions. The higher dislocation density and nanosized grains within supersaturated solid solution cause the microhardness to increase to ~126 HV, which is higher than that of the as-cast Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr (wt.%) alloy processed by HPT, and is also higher than that of any conventional Mg-rich alloy deformed using HPT

Nonlinear properties of relativistically intense laser in plasmas

10.1063/1.2803769Physics of Plasmas1411-PHPA

Spectral analysis of bent fiber Bragg gratings: theory and experiment

This Letter presents a simple mathematical model developed from coupled-mode theory to describe the relationship between Bragg transmission loss (BTL), grating length, coupling coefficients, and bending loss in a bent fiber Bragg grating. In our investigation, the finding indicates that the decrement of BTL can be attributed to the increasing bending loss and degradation of both dc and ac coupling coefficients as the bending radius decreases. Besides, the center wavelength shifts as a result of coupling coefficients degradation. The validity of the proposed model is supported by experimental result

Achieving ultra-high hardness of nanostructured Mg-8.2Gd-3.2Y-1.0Zn-0.4Zr alloy produced by a combination of high pressure torsion and ageing treatment

Abstract A Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr (wt%) alloy is subjected to a series of thermal and mechanical treatments involving solution treatment, artificial ageing to peak hardness, high pressure torsion (HPT) and a second artificial ageing. During HPT precipitates dissolve and during the final post-HPT ageing, the supersaturated solid solution decomposes and solutes segregate at grain boundaries. By employing this T6 + HPT + T5 treatment, the hardness increases to 156 ± 1 HV, which is much higher than that achieved by any other reported combination of thermal and thermo-mechanical processing of Mg alloys. The ultra-high hardness is due to the combined effects of solute segregation, grain refinement and high dislocation density

Nonlinear output response of acousto-optic modulated distributed bragg reflector (DBR) fiber laser

This article presents a study on the output response of a distributed Bragg reflector (DBR) to acousto-optic wave. In the fabrication, the DBR is constructed with a 12 cm long EDF and two FBGs that share the same center wavelength. The behavior of the DBR under the influence of acousto-optic wave can be well explained based on the model of a single sinusoidal-modulated FBG. A nonlinear response has been observed for DBR output amplitude which is similar to the reflectivity variation at the center wavelength of an FBG impinged by acousto-optic wave. The potential for using DBR as a vibration sensor has been investigated in the frequency range of 120 kHz-190 kHz