As-received Foils Research Articles

Micro incremental forming of thin SS304 foils and its microstructural study

Micro-forming of thin foils is challenging due to the size effect, friction effect, and tool design. The conventional macro-forming techniques cannot be directly applied to the thin foils. The incremental forming process (ISF) can be used for thin foils since it possesses higher formability compared to the conventional process. Hence, µ-ISF (micro incremental forming process) experiments were conducted on thin foils of SS304 stainless steel having 50, 100, and 200 µm thickness. As-received foils were further annealed at 950°C with a holding time of 20 mins to achieve strain-free microstructure. The Taguchi L18 orthogonal was applied to optimize the initial processing parameters for, µ-ISF experiments. Using the initial optimized parameters µ-ISF experiments were performed for all the thickness sheets up to the maximum wall angle. It was observed that the higher tool diameter and the lower step depth give the minimum roughness value in the deformed sheet. The tearing mode of failure was observed due to the formation of strain-induced martensite in the deformed sheet. The microstructure and bulk texture analysis relate the orientation and twin fraction with the deformation behaviour. From the texture evolution, it was observed that the brass {110} <112>, Cube {100} <001>, and goss {110} <001> texture component has a greater influence on the formability of SS304 stainless steel thin foils.

Low temperature hydrogen plasma permeation in palladium and its alloys for fuel recycling in fusion systems

Superpermeation of hydrogen isotopes through metal foils is a critical component for efficient fuel recycling in fusion power systems. In that context hydrogen permeation through foils of palladium and its alloys with silver and copper was studied at low temperature (60 - 200°C) under plasma exposure. These alloys differ significantly in both bulk and surface properties, and comparisons can provide mechanistic insights. Permeation was observed only during plasma operation, confirming the negligible contribution of molecular hydrogen to the observed flux. As-received foils required surface treatment to achieve top performance. For Pd and Pd75Ag25 an oxidation treatment increased permeation an order of magnitude, but proved unstable as this desirable surface was reduced under hydrogen plasma exposure. In contrast, an Ar plasma cleaning step provided both high and stable flux. As-received FCC phase Pd60Cu40 foils required annealing to transform it into the high permeability BCC phase that delivered top performance. All foils displayed similar temperature dependence with flux declining with temperature, suggesting that the primary rate-limiting step is absorption of superthermal hydrogen. Among these foils the hydrogen flux through PdCu was 3 - 5X greater than that of Pd or PdAg, which were similar. The superiority of BCC PdCu is attributed to its superior hydrogen desorption kinetics. Using PdCu 100% permeation of supplied hydrogen was achieved, and the flux saturated with increasing plasma power at values >10−2 mol H·m−2·s−1, and under these conditions permeation rates are equivalent with or without the membrane present. The fluxes achieved are the highest reported to date at these conditions, and the results highlight the important roles of both surface and bulk properties.

A Study of Improving the Formability of the Commercial Pure Titanium Foils

Abstract In order to obtain the optimum forming process for commercial pure titanium grade 2 foils, a series of tensile tests and micro scale limited dome height (µ-LDH) tests at four temperatures, and meso scale limited dome height tests (meso-LDH) with three punch speeds were conducted on the as-received foils with a thickness of 75 µm. The effects of temperature, geometry, and high-velocity impact were investigated to understand their influences on the formability of the foils. It has been found in the tensile tests that the formability can be improved by elevating temperatures; this has been validated by the µ-LDH tests. Based on forming limit diagrams (FLDs) of the meso-LDH specimens, the high-velocity impact forming process results in not only much better formability but also more uniform thickness distributions than the quasi-static. By analyzing the fractographical scanning electron microscope (SEM) pictures of the meso-LDH specimens, it has been proven that the formability of the foils by using high-velocity impact process is superior to the conventional process. Furthermore, high-velocity impact causes forming limit curve (FLC) to shift in the upper right direction on the right-hand side of FLD. Therefore, it is suggested forming the foils by using high-velocity impact forming process at the elevated temperature for obtaining a better formability and more uniform thickness distribution. It is also recommended to make the radius of the LDH hemisphere punch close to the smallest feature of the designed products for obtaining more accurate FLCs.

Ultrahigh-strength AISI-316 austenitic stainless steel foils through concentrated interstitial carbon

This research explores intrinsic properties of the carbon-rich subsurface zone (“case”) that low-temperature carburization generates in an austenitic stainless steel (AISI-316). Foils of this steel were carburized to obtain concentrated interstitially dissolved carbon distributed uniformly throughout their thickness. Compared to the as-received foils, such “through” carburization increases the ultimate tensile strength to 3 times, the yield strength to 4 times, and Young's modulus to 1.5 times, respectively. On the other hand, the strain to failure decreases to (9 ± 1) × 10−3. For comparison, foils with larger thickness were carburized as well. Those foils retained a core of low carbon level. Decreasing the ratio of mean carbon depth to foil thickness was found to decrease the ultimate tensile strength, the yield strength, and Young's modulus, while increasing the strain to failure. Tensile testing partially carburized foils to failure confirmed ductility in the core, but revealed reduced ductility in the carbon-rich zone near the surface. On the surface, intergranular cracks were observed to propagate perpendicular to the tensile direction, indicating brittle fracture initiated by crack nucleation at grain boundaries. The isolated concentrated solid solution of interstitial carbon in austenite can be regarded as a new material with exceptional properties, particularly ultrahigh tensile strength, high yield strength, and high Young's modulus.

X-ray diffraction study on the effects of hydrogen on Pd60Cu40 wt% foil membranes

In-situ variable temperature X-ray diffraction analysis was performed on two as-received Pd60Cu40 wt% foil samples containing the disordered face centred cubic (FCC) phase between 30 and 700°C. One foil sample was exposed to 445kPa of flowing helium and the other foil sample was exposed to 445kPa of flowing hydrogen. Generally, it was found that 445kPa of flowing hydrogen had the effect of expanding the temperature range over which the body centred cubic (BCC) phase in the foil sample was stable when compared to testing under 445kPa of flowing helium. This is likely due to dissolved hydrogen shifting the BCC|FCC + BCC and FCC + BCC|FCC phase boundaries to relatively higher temperatures and Pd contents.An as-received Pd60Cu40 wt% foil membrane had been cycled between 50 and 450°C under a 445kPa feed pressure and 100kPa permeate pressure of hydrogen. In the third cycle, this membrane achieved a noticeably low hydrogen permeability of 5.59 × 10−9molm−1s−1Pa−0.5 at 450°C. Moreover, the partial pressure exponent was found to deviate significantly from Sieverts' law between 400 and 450°C. Subsequently, this membrane was cycled twice between 250 and 700°C achieving a much higher hydrogen permeability of 1.19 × 10−8molm−1s−1Pa−0.5 which was measured at 450°C. The activation energy for permeation reduced by more than 60% and the partial pressure exponent decreased to 0.52. The initial poor hydrogen permeability of the membrane was attributed to coring as evidenced by the presence of the disordered FCC phase in the as-received foil. The improvement in hydrogen permeability was linked to the homogenisation effect of hydrogen under the conditions used in this study.

Effect of surface roughness, chemical composition, and native oxide crystallinity on the orientation of self-assembled GaN nanowires on Ti foils

We report on plasma-assisted molecular beam epitaxial growth of almost randomly oriented, uniformly tilted, and vertically aligned self-assembled GaN nanowires (NWs), respectively, on different types of polycrystalline Ti foils. The NW orientation with respect to the substrate normal, which is affected by an in situ treatment of the foil surface before NW growth, depends on the crystallinity of the native oxide. Direct growth on the as-received foils results in the formation of ensembles of nearly randomly oriented NWs due to the strong roughening of the surface induced by chemical reactions between the impinging elements and Ti. Surface nitridation preceding the NW growth is found to reduce this roughening by transformation of the uppermost layers into TiN and TiOxNy species. These compounds are more stable against chemical reactions and facilitate the growth of uniformly oriented GaN NW ensembles on the surface of the individual grains of the polycrystalline Ti foils. If an amorphous oxide layer is present at the foil surface, vertically oriented NWs are obtained all across the substrate because this layer blocks the transfering of the epitaxial information from the underlying grains. The control of NW orientation and the understanding behind the achievement of vertically oriented NWs obtained in this study represent an important step towards the realization of GaN NW-based bendable devices on polycrystalline metal foils.

Hardness measurements of accumulative roll-bonded Mo foils

The hardness of accumulative roll bonding (ARB)-processed multilayers has been measured in the past with microindentation studies. In some cases the lateral indent size of microindenters, however, is too large. For example, if multilayers are ARB-processed that consist of arrays of different elemental foils the hardness ratio of the layers has been reported as a key factor for the development of diffuse necking. To predict diffuse necking during ARB processing, flow stress data are usually taken from tensile tests. Flow stresses can be estimated of individual layers based on hardness measurements. In this work results are compared between hardness measurements on ARB-processed Mo multilayers with micro- and nanoindentation. Two sets of Mo multilayers were examined, one with as-received Mo foils and a second set with annealed Mo foils as starting material. The microhardness measurements show an increase in hardness with the first two passes before the hardness reaches a near plateau for the annealed Mo foils. The as-received foils that appear to be cold-worked yield a constant hardness for the first five passes that approaches the hardness of the annealed samples after four passes. The nanoindentation measurements were conducted at the center of Mo grains. If the indentation size effect (ISE) is taken into account the nanoindentation-based hardness measurements of the annealed Mo foil multilayers agree with the microhardness measurements. For the multilayer comprised of as-received Mo foils, agreement is obtained with the microhardness measurements even without taking into account an ISE. These observations are discussed with respect to the changes in microstructural length scales during ARB processing and the indent sizes. A main conclusion is that nanoindentation measurements yield reliable hardness data if the ISE is considered.

Behaviour of oxygen atoms near the surface of nanostructured Nb2O5

Recombination of neutral oxygen atoms on oxidized niobium foil was studied. Three sets of samples have been prepared: a set of niobium foils with a film of polycrystalline niobium oxide with a thickness of 40 nm, another one with a film thickness of about 2 µm and a set of foils covered with dense bundles of single-crystal Nb2O3 nanowires. All the samples were prepared by oxidation of a pure niobium foil. The samples with a thin oxide film were prepared by exposure of as-received foils to a flux of O-atoms, the samples with a thick polycrystalline niobium oxide were prepared by baking the foils in air at a temperature of 800 °C, while the samples covered with nanowires were prepared by oxidation in a highly reactive oxygen plasma. The samples were exposed to neutral oxygen atoms from a remote oxygen plasma source. Depending on discharge parameters, the O-atom density in the postglow chamber, as measured with a catalytic probe, was between 5 × 1020 and 8 × 1021 m−3. The O-atom density in the chamber without the samples was found rather independent of the probe position. The presence of the samples caused a decrease in the O-atom density. Depending on the distance from the samples, the O-atom density was decreased up to 5 times. The O-atom density also depended on the surface morphology of the samples. The strongest decrease in the O-atom density was observed with the samples covered with dense bundles of nanowires. The results clearly showed that niobium oxide nanowires exhibit excellent catalytic behaviour for neutral radicals and can be used as catalysts of exhaust radicals found in many applications.

The effect of surface state on the kinetics of cerium-hydride formation

We studied the effects of surface oxidation on the kinetics of hydrogen (H) absorption into two commercial samples of Ce: bulk samples and thin rolled foils. A Sieverts’ apparatus was used to measure the H absorption kinetics and X-ray photoelectron spectroscopy (XPS) was used to determine the surface oxidation state and the presence of impurities, notably carbon (C). In the as-received condition, the bulk Ce specimen was covered by a Ce 2O 3 film (sesquioxide) whereas the as-received foil was covered with a CeO 2 film (dioxide). In this condition, the H absorption rate at room temperature was approximately an order of magnitude faster in the bulk sample than in the foil. Vacuum annealing at P ≤ 1 × 10 −4 Torr and T = 773 K significantly increased the rate of H uptake, making it similar in both samples. This annealing reduced the Ce 2O 3 in the bulk samples so that the samples became more metallic, but produced little change in the XPS signal from the CeO 2 film on the foil sample. The study suggests that the differences in H uptake kinetics are affected not only by the nature of the oxide (sesquioxide versus dioxide) but also by the integrity of the surface oxide film, which may be affected by the thermal treatment.

Size effects in tensile testing of thin cold rolled and annealed Cu foils

Thin copper foils of varying thickness were tested in tension at room temperature. The thickness of the samples was between 10 and 250 μm, length and width were scaled according to the thickness. Two types of samples were tested: as-received samples and annealed samples. The microstructure of the foils was determined in detail by means of several methods. If samples of the same processing condition were compared, there was a clear dependence of the mechanical behavior on the thickness of the foils in the tensile test. When the thickness was reduced from 250 to 10 μm, the fracture strain decreased for the as-received foils from approximately 20% to 0.2% and for the samples with heat treatment from 35% to 15%. It has to be stressed that this size effect occurred in the absence of considerable strain gradients and for samples with a comparable microstructure.

Positron Annihilation Spectroscopy Study of Interfacial Defects Formed by Dissolution of Aluminum in Aqueous Sodium Hydroxide

High-purity aluminum foils were examined using positron annihilation spectroscopy (PAS) after dissolution for various times in 1 M NaOH at room temperature. Measurements of the S and W shape parameters of the annihilation photopeak at 511 keV show the presence of voids of at least nanometer dimension located at the metal-oxide film interface. The large S parameter suggests that the metallic surface of the void is free of oxide. Voids are found in as-received foils and are also produced by dissolution in NaOH, evidently by a solid-state interfacial process. Atomic force microscopy (AFM) images of NaOH-dissolved foils, after stripping the surface oxide film in chromic-phosphoric acid bath, reveal cavities on the order of 100 nm size. The average cavity depth is in quantitative agreement with the PAS-derived thickness of the interfacial void-containing layer, and the dissolution time dependence of the defect layer S parameter closely parallels that of the fractional coverage of the foil surface by cavities; thus, the cavities are believed to be interfacial voids created along with those detected by PAS. The cavity distribution on the surface closely resembles that of corrosion pits formed by anodic etching in 1 M HCl, thereby suggesting that the interfacial voids revealed by AFM serve as sites for pit initiation. © 2001 The Electrochemical Society. All rights reserved.

The effect of lead impurity on the DC-etching behaviour of aluminum foil for electrolytic capacitor usage

The effects of lead impurity on the etched morphology of high purity aluminum foils for electrolytic capacitor applications were investigated in this work. The lead impurity was either present in as-received aluminum foils or deposited purposely on the foil surface through an immersion-reduction reaction. The amount and distribution of deposited lead varies with the lead content in as-received foil. The as-received foil with higher lead content gave a higher concentration and a more uniform distribution of deposited lead. Uniformly distributed vertical tunnel etchings were obtained when high lead foil, formed by immersion as-received high lead foil in Pb(NO3)2 solution, was subject to DC-etching. For the as-received lower lead foils, the deposited lead was concentrated in rolling lines which resulted in surface etching along rolling lines. Both the surfacial and cross-sectional etching morphologies are presented in this study, together with the etching mechanism discussed.