The character, morphology and distribution of inclusions has been studied in additively manufactured 17-4PH stainless steel parts produced from gas-atomized powder by selective laser melting. A ...combination of advanced electron microscopy techniques has been used to show that such parts contain oxide inclusions ranging from a few nm to tens of μm across. The coarser inclusions have morphologies that mimic the oxides which accumulate between melt tracks at the build surface, suggesting that these inclusions are incorporated into the build during deposition of subsequent layers. Such features could have deleterious anisotropic effects upon the fatigue and fracture resistance of the built parts. The finer inclusions are equi-axed, reside preferentially at grain boundaries, and could inhibit grain growth during processing via Zener pinning effects. The chemistries of the inclusions include elements such as Al that are not part of the alloy specification. This indicates that the inclusions are exogenous defects that are entrained in the melt during gas atomization due to interactions with the crucible or nozzle materials. Examples of encapsulated oxide material in the powder feedstock support this explanation. These observations highlight the need for careful control of powder pedigree when using additive manufacturing for critical structural components.
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•Inclusions consisting of amorphous oxides of silicon and aluminum are observed in additively manufactured 17-4 PH samples.•The largest inclusions are many microns across with characteristic shapes, and contain crystalline alloy nanoparticles.•Oxide phases with similar morphologies are found between the tracks at the build surface of the samples.•The origin of the oxides is shown to be entrained refractory material within the powder feedstock.•These observations highlight the importance of powder pedigree in metal additive manufacturing.
Carbon-aerogel-supported ruthenium nanoparticles were synthesized by impregnating carbon aerogels with Ru(acac)3 or Ru(cod)(tmhd)2 from supercritical carbon dioxide (scCO2) solutions, followed by ...thermal reduction of these precursors. Two different carbon aerogels with pore diameters of 4 and 21 nm were synthesized. The kinetics and the thermodynamics of impregnation of carbon aerogels with the ruthenium coordination complexes were studied. The approach-to-equilibrium data indicated very fast adsorption, and the adsorption isotherms were found to follow the Langmuir model. The impregnated carbon aerogel complexes were reduced thermally at different temperatures between 300 and 1000 degrees C in the presence of nitrogen. The resulting nanocomposites were characterized using transmission electron microscopy (TEM) and hydrogen chemisorption. TEM micrographs showed that the ruthenium nanoparticles were dispersed homogeneously throughout the porous carbon aerogel matrix, and the average sizes obtained under different conditions ranged from 1.7 to 3.8 nm. Once complete decomposition of the precursor had been achieved, the mean size of the ruthenium particles increased with increasing reduction temperature.
Aerobic self‐esterification of primary alcohols catalyzed by mesoporous metal oxides (manganese and cobalt oxides) is reported under base and solvent free conditions. For a range of aliphatic ...alcohols, up to 90 % conversions to esters was achieved. The catalytic reaction is likewise applicable to neat aldehydes as substrates with yields of up to 86 %. High pressure batch reaction for ethanol to ethyl acetate led to 22 % yield. Isotope labeling studies indicated decarboxylation on the catalyst surface. Mechanistic and kinetic experiments implicate oxygen rebound and α‐carbon removal as intermediate steps. Mesoporous cobalt oxide showed about 20 % higher catalytic activity compared to mesoporous manganese oxide.
Mesoporous materials: Heterogeneous catalytic esterification in solvent free, base free, aerobic conditions is of interest from green chemistry standpoint. University of Connecticut synthesis (UCT) as a class of mesoporous materials has been shown to have broad applicability for catalysis. Here, we report the self‐esterification reaction for alcohols and aldehydes for a broad range of substrates by mesoporous manganese oxides (meso‐MnOx) and mesoporous cobalt oxides (meso‐CoOx).
Metal-assisted catalytic etching (MACE) using Ag nanoparticles as catalysts and H
O
as oxidant has been performed on single-crystal Si wafers, single-crystal electronics grade Si powders, and ...polycrystalline metallurgical grade Si powders. The temperature dependence of the etch kinetics has been measured over the range 5-37°C. Etching is found to proceed preferentially in a 〈001〉 direction with an activation energy of ~0.4 eV on substrates with (001), (110), and (111) orientations. A quantitative model to explain the preference for etching in the 〈001〉 direction is developed and found to be consistent with the measured activation energies. Etching of metallurgical grade powders produces particles, the surfaces of which are covered primarily with porous silicon (por-Si) in the form of interconnected ridges. Silicon nanowires (SiNW) and bundles of SiNW can be harvested from these porous particles by ultrasonic agitation. Analysis of the forces acting between the metal nanoparticle catalyst and the Si particle demonstrates that strongly attractive electrostatic and van der Waals interactions ensure that the metal nanoparticles remain in intimate contact with the Si particles throughout the etch process. These attractive forces draw the catalyst toward the interior of the particle and explain why the powder particles are etched equivalently on all the exposed faces.
Pt-based electrocatalysts were prepared on different carbon supports which are multiwall carbon nanotubes (MWCNTs), Vulcan XC 72R (VXR) and black pearl 2000 (BP2000) using a supercritical carbon ...dioxide (scCO sub(2) deposition technique. These catalysts were characterized by using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and cyclic voltammetry (CV). XRD and HRTEM results demonstrated that the scCO) sub(2) deposition technique enables a high surface area metal phase to be deposited, with the size of the Pt particles ranging from 1 to 2 nm. The electrochemical surface areas (ESAs) of the prepared electrocatalysts were compared to the surface areas of commercial ETEK Pt/C (10 wt% Pt) and Tanaka Pt/C (46.5 wt% Pt) catalysts. The CV data indicate that the ESAs of the prepared Pt/VXR and Pt/MWCNT catalysts are about three times larger than that of the commercial ETEK catalyst for similar (10 wt% Pt) loadings. Oxygen reduction activity was investigated by hydrodynamic voltammetry. From the slope of Koutecky-Levich plots, the average number of electrons transferred in the oxygen reduction reaction (ORR) was 3.5, 3.6 and 3.7 for Pt/BP2000, Pt/VXR and Pt/MWCNT, correspondingly, which indicated almost complete reduction of oxygen to water.
Ti-6Al-4V powder has been recycled 30 times in an electron beam melting system. A combination of electron microscopy techniques has been used to show that the recycled powder has a 35% higher oxygen ...content, and that the particles have a more irregular morphology, a narrower particle size distribution, and a much more variable microstructure than the virgin powder. The microstructures in the recycled powder particles vary from a martensitic α′ structure, which is identical to that in the virgin powder, to a two-phase α + β structure. This variability is related to the complex thermal history of the unmelted metal powder in the system. Despite these differences, all of the particles exhibit essentially the same surface oxide thickness; the excess oxygen in the recycled powders is instead located in the β phase. The possible consequences for the structure and properties of the resultant additively manufactured parts are discussed.
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•17-4PH stainless steel thin-walled samples were additively manufactured by SLM.•Samples parallel and at 45˚ to the scan axes gave beam path lengths of 19−0.8 mm.•Longer beam paths ...gave microstructures comprising mostly coarse-grained ferrite.•The shortest beam paths gave structures with mostly austenite and some martensite.•Re-heating of ferrite leads to austenite, with martensite forming during cooling.
Components with varying dimensions are found in numerous applications. The current work examines how microstructures and phases change for additively manufactured 17-4PH thin walls as a function of laser path length, path direction, and wall thickness. Two sample sets were designed, each consisting of four walls with thicknesses of 6.4 mm to 0.8 mm. In the first set, the wall axes were parallel to the scan axes, such that the laser path length varied from layer to layer with the laser path either being parallel or perpendicular to the wall. In the second set, the walls lay at 45° to the scan axes, such that the laser path had the same length in all layers and gradually decreased with wall thickness. Substantial changes in phase stability and microstructure are observed as the wall thickness decreases, with ferritic phases and coarse grains changing to fine grains and an increasing volume fraction of austenite. These changes are attributed to changes in the local temperature-time profile as the length of the laser paths change from 19 mm to 0.8 mm. These observations demonstrate the range of microstructure and phase control options available in additive manufacturing with judicious selections of part layouts on build plates and of laser beam directions.
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