The grain boundary diffusion process (GBDP) is now widely used to increase coercivity in Nd-Fe-B sintered magnets with a more efficient use of heavy rare earth elements (Dy, Tb). This process leads ...to a typical core-shell structure for the grains consisting of (Nd,Dy)2Fe14B shells at the outer grain regions and Nd2Fe14B cores. The thickness of the (Nd,Dy)2Fe14B shells decreases from the diffusion surface to the magnet core. This inhomogeneous distribution in Dy content gives rise to a coercivity gradient within the magnet and leads therefore to a reduced squareness of the demagnetization curve. The purpose of this work is to provide a quantitative understanding of the influence of composition profiles after GBDP on the shape of the demagnetization curve of Nd-Fe-B sintered magnets diffused with the Dy63Co37 (at. %) intermetallic compound. SEM/X-EDS analyses along the Fisher diffusion model allow the estimation of the Dy concentration in grains and at different depths. Then, after ascribing to the grains some critical values for the switching field that are related to the local Dy content, a macroscopic finite element model is implemented to provide a better understanding of the grain reversal sequence in the graded magnets tested in closed-circuit. Grain reversal patterns show that demagnetization starts from the less coercive grains in the magnet core, remains constricted in this zone thanks to a shielding effect from the external surface, and then propagates towards outer layers via magnetostatic interactions. When the coercivity gradient is large, the coercivity of the whole magnet measured in closed-circuit could be 100–200 kA/m lower than the value expected without considering magnetostatic interactions, suggesting that the shielding effect from the diffusion affected layers could be limited and counterbalanced by magnetostatic interactions.
Atomic scale observations of the oxide formed on stainless steels, under simulated nuclear reactor conditions, are performed to estimate the oxide layer contribution on stress corrosion cracking ...(SCC) mechanisms. A duplex oxide composed of a chromium enriched inner layer (Fe
1.5
Cr
1.5
O
4
) and an outer layer composed of magnetite crystallites (Fe
3
O
4
) is found. The oxide layer structure evolves from amorphous, for oxidation times of 1 min, to nano-crystalline at 2 min and mono-crystalline after 5 h. IFFT images, calculated from Cs-corrected HRTEM images recorded on grains oriented in the 〈111〉 direction, highlight a double network of dislocations with ½ 〈10-1〉 and ½ 〈−110〉 Burgers vectors. This network leads to the decrease in non-relaxed deformation and favors an epitaxial growth between steel and oxide. Both crystal structure transformations and epitaxial relations between metal and oxide have provided relevant information which contributed to progress on SCC modeling.
Oxide dispersion-strengthened ferritic stainless steels are foreseen as fuel cladding tube materials for the new generation of sodium fast nuclear reactors. Those materials, which exhibit remarkable ...creep properties at high temperature, are reinforced by a dense precipitation of nanometric oxides. This precipitation is obtained by mechanical alloying of a powder and subsequent consolidation. Before consolidation, to obtain a fully dense material, the powder is vacuumed to outgas trapped gases and species adsorbed at the surface of the powder particles. This operation is commonly done at moderate to high temperature to evacuate as much as possible volatile species. This paper focuses on the influence of outgassing conditions on some properties of the further consolidated materials. Chemical composition and microstructural characterization of different materials obtained from various outgassing cycles are compared. Finally, impact toughness of those materials is evaluated by using Charpy testing. This study shows a significant influence of the outgassing conditions on the mechanical properties of the consolidated material. However, microstructure and oxygen contents seem poorly impacted by the various outgassing conditions.
Laser powder bed fusion (L-PBF) of copper and copper alloys remain a challenge because of its low optical absorption and high thermal conductivity. L-PBF experiments were performed to print 2D ...(patches) and 3D (cubes) objects with pure copper under a low laser power (270 W max.). Patches reveal different surface morphologies allowing to select the best printing conditions for cubes. The highest reached cube relative density is 87.9%. To increase this value, a 1064 nm-absorptive physical vapor deposition (PVD) CrZr coating was performed on the copper powder to enhance its optical absorption. X-ray Photoelectron Spectrometry (XPS) and Auger measurements reveal the coating is oxidized, but increases the resulting CuCrZr powder optical absorption from 39% (value for pure Cu) to 81.8%. The CuCrZr powder was then used in the L-PBF process, and patches and cubes were successfully printed. The highest CuCrZr cube relative density reached 94.3%, highlighting the positive influence of a powder coating to create hard-to-melt metals or hard-to-atomized alloys under low laser power.
En este artículo se analizan las diversas etapas en la realización de tres grandes plataformas marinas. Después de un estudio de las características principales de la estructura, ensayos en modelo ...reducido y cálculo de todos los elementos, se procede a la construcción de las plataformas en tres fases: — Construcción en seco de la base del casco. — Colocación en el agua y acabado de la estructura. — Inmersión y colocación del puente. Por último, se realizan las operaciones necesarias para llevar la plataforma a su ubicación definitiva. Las estructuras están formadas por un casco paralelepipédico de hormigón armado, dividido en compartimientos mediante tabiques verticales ortogonales y sobre el que se han colocado 2 ó 4 pilas que soportan el puente.
Abstract Improved fundamental understanding of how cells interpret microenvironmental signals is integral to designing better biomaterial therapies. YAP/TAZ are key mediators of mechanosensitive ...signaling; however, it is not clear how they are regulated by the complex interplay of microenvironmental factors (e.g., stiffness and degradability) and culture dimensionality. Using covalently crosslinked norbornene-functionalized hyaluronic acid (HA) hydrogels with controlled stiffness (via crosslink density) and degradability (via susceptibility of crosslinks to proteolysis), we found that human mesenchymal stem cells (MSCs) displayed increased spreading and YAP/TAZ nuclear localization when cultured atop stiffer hydrogels; however, the opposite trend was observed when MSCs were encapsulated within degradable hydrogels. When stiffness-matched hydrogels of reduced degradability were used, YAP/TAZ nuclear translocation was greater in cells that were able to spread, which was confirmed through pharmacological inhibition of YAP/TAZ and actin polymerization. Together, these data illustrate that YAP/TAZ signaling is responsive to hydrogel stiffness and degradability, but the outcome is dependent on the dimensionality of cell-biomaterial interactions.
The extracellular matrix (ECM) presents an evolving set of mechanical cues to resident cells. We developed methacrylated hyaluronic acid (MeHA) hydrogels containing both stable and hydrolytically ...degradable crosslinks to provide cells with a gradually softening (but not fully degradable) milieu, mimicking physiological events such as fibrosis regression. To demonstrate the utility of this cell culture system, we studied the phenotype of rat hepatic stellate cells, the major liver precursors of fibrogenic myofibroblasts, within this softening environment. Stellate cells that were mechanically primed on tissue culture plastic attained a myofibroblast phenotype, which persisted when seeded onto stiff (∼20 kPa) hydrogels. However, mechanically primed stellate cells on stiff-to-soft (∼20 to ∼3 kPa) hydrogels showed reversion of the myofibroblast phenotype over 14 days, with reductions in cell area, expression of the myofibroblast marker alpha-smooth muscle actin (α-SMA), and Yes-associated protein/Transcriptional coactivator with PDZ-binding motif (YAP/TAZ) nuclear localization when compared to stellate cells on stiff hydrogels. Cells on stiff-to-soft hydrogels did not fully revert, however. They displayed reduced expression of glial fibrillary acidic protein (GFAP), and underwent abnormally rapid re-activation to myofibroblasts in response to re-stiffening of the hydrogels through introduction of additional crosslinks. These features are typical of stellate cells with an intermediate phenotype, reported to occur in vivo with fibrosis regression and re-injury. Together, these data suggest that mechanics play an important role in fibrosis regression and that integrating dynamic mechanical cues into model systems helps capture cell behaviors observed in vivo.
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