Gas-atomized Al 6061 powder was studied in as-atomized, heat-treated, and homogenized conditions, and low-coverage cold spray deposits were produced from these powders to allow individual splats to ...be investigated. The microstructures were investigated by electron microscopy, and the mechanical behavior of the particle/splat interiors was evaluated by the compression of micropillars milled from the samples. The as-atomized and heat-treated powders exhibit cell-like solidification microstructures with Mg-Si and Fe-Al-Si secondary phases around the cell boundaries. These powders have flow stresses of ≈ 185 MPa with pronounced strain bursts due to rapid, stochastic dislocation motion confined within the cells. In the homogenized powders, the Mg-Si phase has dissolved, the Fe-Al-Si has coarsened, and the grain boundaries have unpinned. The stress–strain curves are smoother due to the dissolution of cell boundary obstacles, but the flow stress is about 30 MPa higher due to the increased solute content giving more sluggish dislocation motion. The microstructures of the splat interiors resemble those of the powders closely, but the flow stresses measured are all much higher (280-290 MPa) with little effect of powder microstructure or deposition pressure. This is consistent with the high dislocation density reaching a saturation value in all of the splats studied.
Micropillar compression experiments on 001-oriented CaFe2As2 single crystals have recently revealed the existence of superelasticity with a remarkably high elastic limit of over 10%. The collapsed ...tetragonal phase transition, which is a uni-axial contraction process in which As-As bonds are formed across an intervening Ca-plane, is the main mechanism of superelasticity. Usually, superelasticity and the related structural transitions are affected strongly by both the microstructure and the temperature. As such, in this study, we investigated how the microstructure and temperature affect the superelasticity of 001-oriented CaFe2As2 micropillars cut from solution-grown single crystals, by performing a combination of in-situ cryogenic micromechanical testing and transmission electron microscopy studies. Our results show that the microstructure of CaFe2As2 is influenced strongly by the crystal growth conditions and by subsequent heat treatment. The presence of Ca and As vacancies and FeAs nanoprecipitates affect the mechanical behavior significantly. In addition, the onset stress for the collapsed tetragonal transition decreases gradually as the temperature decreases. These experimental results are discussed primarily in terms of the formation of As-As bonds, which is the essential feature of this mechanism for superelasticity. Our research outcomes provide a more fundamental understanding of the superelasticity exhibited by CaFe2As2 under uni-axial compression.
The solidification microstructures in three alloys from the Ag.sub.3Sn-Cu.sub.3Sn pseudo-binary section in the Ag-Cu-Sn system have been studied using a combination of X-ray diffraction, electron ...microscopy, differential scanning calorimetry, and quenching experiments. The three alloys have Ag:Cu ratios of 50:50, 40:60, and 30:70. In each case, the as-cast structures exhibit the equilibrium phases θ-Ag.sub.3Sn and ζ.sub.1-Cu.sub.3Sn, with a little η-Cu.sub.6Sn.sub.5. There is no evidence of the metastable high-temperature phases that are so prevalent in as-cast structures of the corresponding binary alloys. The differential scanning calorimetry data obtained on heating the alloy samples are consistent with the transformations expected on the basis of the published ternary Ag-Cu-Sn diagrams. It is proposed that the solidification microstructures observed experimentally in such alloys must correspond to the nucleation of the high-temperature phases being kinetically limited upon cooling for these compositions. This leads to the direct formation of the equilibrium low-temperature phases by eutectic-type co-operative growth.
The solidification microstructures in three alloys from the Ag sub(3)Sn-Cu sub(3)Sn pseudo-binary section in the Ag-Cu-Sn system have been studied using a combination of X-ray diffraction, electron ...microscopy, differential scanning calorimetry, and quenching experiments. The three alloys have Ag:Cu ratios of 50:50, 40:60, and 30:70. In each case, the as-cast structures exhibit the equilibrium phases thetas-Ag sub(3)Sn and epsilon sub(1)-Cu sub(3)Sn, with a little eta -Cu sub(6)Sn sub(5). There is no evidence of the metastable high-temperature phases that are so prevalent in as-cast structures of the corresponding binary alloys. The differential scanning calorimetry data obtained on heating the alloy samples are consistent with the transformations expected on the basis of the published ternary Ag-Cu-Sn diagrams. It is proposed that the solidification microstructures observed experimentally in such alloys must correspond to the nucleation of the high-temperature phases being kinetically limited upon cooling for these compositions. This leads to the direct formation of the equilibrium low-temperature phases by eutectic-type co-operative growth.
Regenerative Electroless Etching of Silicon Kolasinski, Kurt W.; Gimbar, Nathan J.; Yu, Haibo ...
Angewandte Chemie (International ed.),
January 9, 2017, Letnik:
56, Številka:
2
Journal Article
Recenzirano
Regenerative electroless etching (ReEtching), described herein for the first time, is a method of producing nanostructured semiconductors in which an oxidant (Ox1) is used as a catalytic agent to ...facilitate the reaction between a semiconductor and a second oxidant (Ox2) that would be unreactive in the primary reaction. Ox2 is used to regenerate Ox1, which is capable of initiating etching by injecting holes into the semiconductor valence band. Therefore, the extent of reaction is controlled by the amount of Ox2 added, and the rate of reaction is controlled by the injection rate of Ox2. This general strategy is demonstrated specifically for the production of highly luminescent, nanocrystalline porous Si from the reaction of V2O5 in HF(aq) as Ox1 and H2O2(aq) as Ox2 with Si powder and wafers.
Nothing to k(V)etch about: H2O2 regenerates V in a 5+ oxidation state, which initiates etching to produce porous Si powders containing fully etched particles.
Nanocomposite powders with equal volume fractions of
Y
2
O
3
and
MgO
have been produced by the thermal decomposition of precursor mixtures of yttrium nitrate and magnesium nitrate. Solutions of the ...precursor salts were mixed with ammonium acetate fuel, dried to form a gel‐like substance, and then calcined to give nanocrystalline ceramic powders. The amount of ammonium acetate added to the metal nitrate precursors was varied systematically, and the morphology and distribution of the component phases in consolidated compacts of the resultant ceramic powders were examined by a combination of focused ion beam sectioning, scanning, and transmission electron microscopy. The dispersion of the
Y
2
O
3
and
MgO
phases within the synthesized powders improved, and the sizes of the phase domains reduced, with increasing ammonium acetate content up to the quantity required for a stoichiometric redox reaction with the metal nitrates. The addition of excess ammonium acetate gave no further improvement in phase domain dispersion or reduction in phase domain sizes. These phenomena are related to the thermal characteristics for the decomposition of the precursors and their effect on phase separation during oxide crystallization.
•Intermittent deposition allows thermal history engineering.•Tuning thermal history enables microstructure engineering in 3D-printed maraging steel.•A dual-phase structure of martensite and reverted ...austenite delivers enhanced strength and ductility.
Maraging steels are known for their exceptional strength but suffer from limited work hardening and ductility. Here, we report an intermittent printing strategy to tailor the microstructure and mechanical properties of maraging 250 steel via tuning the thermal history during wire-arc directed energy deposition. By introducing a dwell time between adjacent layers, the maraging 250 steel is cooled below the martensite start temperature, triggering thermally-driven martensitic transformation during the printing process. Thermal cycling during subsequent layer deposition results in the formation of reverted austenite which shows a refined microstructure and induces elemental segregation between martensite and reverted austenite. The Ni enrichment in the austenite promotes stabilization of the reverted austenite upon cooling to room temperature. The reverted austenite is metastable during deformation, leading to strain-induced martensitic transformation under loading. Specifically, a 3 min interlayer dwell time produces a maraging 250 steel with approximately 8% reverted austenite, resulting in improved work hardening via martensitic transformation induced plasticity during deformation. Meanwhile, the higher cooling rate and refined prior austenite grains lead to substantially refined martensitic grains (by approximately fivefold) together with an increased dislocation density. With 3 min interlayer dwell time, the yield strength of the printed maraging 250 steel increases from 836 MPa to 990 MPa, and the uniform elongation is doubled from 3.2% to 6.5%. This intermittent deposition strategy demonstrates the potential to tune the microstructure of maraging steels for achieving strength-ductility synergy by engineering the thermal history during additive manufacturing.
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Single-crystalline intermetallic compound CaFe2As2 are known to exhibit both superelasticity and micaceous plasticity, but these phenomena have not been studied in detail. Here we report a study of ...the elastic and plastic deformation behaviors for CaFe2As2 single crystals grown in Sn-flux solution. The anisotropic mechanical response has been studied using micropillar compression tests performed in-situ within a scanning electron microscope. These data are compared with density functional theory calculations to develop an understanding of micaceous plasticity in terms of strongly anisotropic generalized stacking fault energies. The results of this work will offer design guidelines to produce mechanically robust superelastic structures.
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Gas atomization is the most common approach used to produce powders of metallic alloys, and the high cooling rates involved frequently lead to the formation of non-equilibrium microstructures and ...phases. The transformations that occur in the powders upon heating are of great interest but are challenging to study experimentally. Here we use a novel focused ion beam-based specimen preparation protocol to obtain cross sections through individual gas-atomized powder particles of three different aluminum alloys: solid solution-strengthened Al5056, precipitation-hardenable Al6061, and an Al–Cr–Mn–Co–Zr alloy which contains icosahedral quasicrystal dispersoids. In situ scanning transmission electron microscopy heating experiments were performed on these cross-sectional specimens to investigate the changes that occur in the metastable phases and non-equilibrium microstructures upon heating. The experiments reveal the details of a wide variety of thermally activated processes occurring in the particles including: solute redistribution to eliminate micro-segregation; dissolution, coarsening, transformation and decomposition of secondary phases; and precipitation within the aluminum matrix.
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