Linear Composition Gradient Research Articles

Ballistic performance of functionally graded B4C/Al composites without abrupt interfaces: experiments and simulations

Functionally graded materials (FGMs) are promising protective structures for lightweight armor plates because of their features and designability. However, traditional discretely layered FGMs with macroscopic interfaces can generate unfavorable interlaminar stresses under ballistic attacks, eventually leading to delamination or interlayer tearing. Herein, we constructed new B4C/2024Al FGMs without sharp interfaces between layers. The FGM armor systems with varying through-the-thickness distributions were tested against 7.62 mm armor-piercing incendiary projectiles. The effects of compositional gradient exponent on the damage characteristics, stress distribution and energy evolution were numerically and experimentally investigated. The results indicated that the smooth transition within through-the-thickness microstructures effectively avoided the occurrence of interlayer delamination or tearing. The compositional gradient exponent played an essential role in the impact response of the FGM targets. The FGM having a linear composition gradient outperformed other nonlinear types with respect to energy absorption and ballistic properties due to the reduced tensile wave strength and prolonged projectile-target interaction time. Furthermore, the structural optimization problem with the objective of maximizing ballistic efficiency was solved using the conjugate gradient method, and the optimal design of the FGM targets was achieved. The current research provides a promising design guideline for developing and optimizing new advanced lightweight composite armor.

Computational design and optimization of multilayered and functionally graded corrosion coatings

This paper describes a computational approach to analysis and optimization of compositionally graded coatings for cathodic protection. Time-dependent galvanic corrosion is simulated by coupling a finite element electrochemical model with calculated rates of metal dissolution. A simulated annealing optimization algorithm is applied to the time-dependent corrosion model to determine coating structures that maximize desired protective qualities. This computational approach to coating design is applied to a hypothetical graded zinc-alloy coating with a circular defect on an iron substrate, in an aerated NaCl electrolyte. A linear compositional gradient increases the predicted duration of cathodic protection by 84% over an equivalent monolithic coating, while the optimized coating structure further improves protection time to a total increase of 112%. The optimized coating structure consists of a thin barrier layer adjacent to the substrate, with a thicker sacrificial layer on the exterior and a short region of graded composition in between. The overall approach to optimization of coating structure is shown to be robust, efficient, and produce non-obvious designs with significant improvement in coating performance, and thus has potential to be of significant utility in practical corrosion coating design.

Development and Validation of a ReaxFF Reactive Force Field for Fe/Al/Ni Alloys: Molecular Dynamics Study of Elastic Constants, Diffusion, and Segregation

We have developed a ReaxFF force field for Fe/Al/Ni binary alloys based on quantum mechanical (QM) calculations. In addition to the various bulk phases of the binary alloys, the (100), (110) and (111) surface energies and adatom binding energies were included in the training set for the force field parametrization of the Fe/Al/Ni binary alloys. To validate these optimized force fields, we studied (i) elastic constants of the binary alloys at finite temperatures, (ii) diffusivity of alloy components in Al/Ni alloy, and (iii) segregation on the binary alloy surfaces. First, we calculated linear elastic constants of FeAl, FeNi(3), and Ni(3)Al in the temperature range 300 to 1100 K. The temperature dependences of the elastic constants of these three alloys, showing a decrease in C(11), C(12), and C(44) as temperature increases, were in good agreement with the experimental results. We also performed ReaxFF molecular dynamics (MD) simulations for Al or Ni diffusion in the system modeled as Al/Ni mixed layers with the linear composition gradients. At 1000 K, Al diffusivity at the pure Al end was 2 orders of magnitude larger than that in the Al trace layers, probably explaining the nature of different diffusion behavior between molten metals and alloys. However, the diffusivity of Ni at the pure Ni end was only slightly larger than that in the Ni trace layers at the system temperature much lower than the melting temperature of Ni. Third, we investigated the surface segregation in L1(2)-Fe(3)Al, Fe(3)Ni, and Ni(3)Al clusters at high temperature (2500 K). From the analysis of composition distribution of the alloy components from the bulk to the surface layer, it was found that the degree of segregation depended on the chemical composition of the alloy. Al surface segregation occurred most strongly in Fe(3)Al, whereas it occurred most weakly in Ni(3)Al. These results may support the segregation mechanism that surface segregation results from the interplay between the energetic stability of the ordered bulk phase and the surface reconstruction. In addition, the surface segregation induced the depletion layers of segregating metal species (Al in Fe(3)Al and Ni(3)Al, and Ni in Fe(3)Ni) next to the segregation layers. These simulation results qualitatively agreed with early experimental observations of segregation in Fe/Al/Ni binary alloys.

Influence of various underlayers on [001] texture and magnetic properties in FePt and FexNi1 − xPt thin films

Perpendicular magnetic recording thin films require proper crystallographic growth to ensure that the magnetic easy-axis is aligned normal to the growth plane. The texture and in situ growth stresses of (001) FePt and FexNi(1−x)Pt thin films on various underlayer combinations are studied. These were correlated with the magnetic properties allowing self consistent direct comparisons of the various parameters to be made. It was found that the (001) texture, L10 chemical ordering and perpendicular magnetic alignment was improved with the smaller lattice misfit that accompanied a combination of Pt/Cr multilayered and compositionally graded underlayers on a (001) MgO substrate. A Cr underlayer allowed the Pt film to deposit in a relatively strain-free lattice condition from which the FePt based films could grow in a similar strain-free epitaxial condition. In situ stress measurements confirmed that these underlayers reduced the intrinsic compressive growth stresses to approximately one-fifth the FePt stress value for being directly deposited onto the MgO surface. The incorporation of Ni to the (001) growth of FePt facilitated in-plane and out-of-plane c-variant growth for the L10 structure. Finally, a linear compositional gradient FexNi0.48−xPt0.52 (0<x<0.48) thin film was deposited, which exhibited good [001] orientation and perpendicular magnetic alignment. This has been proposed as a magnetic switching architecture for high density, thermally stable but switchable magnetic media grains using current write head fields.

Composition Dependence of Piezoelectric Properties of Pb(Zr,Ti)O3 Films Prepared by Combinatorial Sputtering

We examined the composition and orientation dependences of the piezoelectric properties by combinatorial sputtering. PbTiO3 and PbZrO3 were simultaneously sputtered to prepare composition gradient Pb(Zr,Ti)O3 (PZT) films on Pt/Ti/Si substrates, and an almost linear composition gradient was observed in a range of Zr/(Zr+Ti) ratios from 0.33 to 0.76. Dielectric and piezoelectric properties attained maximum at the Zr/Ti composition close to the morphotropic phase boundary (MPB), which is consistent with that of PZT ceramics. 100-oriented PZT films showed a higher dielectric constant than the PZT films with the other orientations, while the PZT films with 111 + 100 mixed orientation showed larger values of e31f. These results suggest that the a-axis orientation is dominant in the 100-oriented PZT films. In this study, we demonstrated that the combinatorial sputtering method enables the precise evaluation of the composition dependence of piezoelectric and dielectric properties.

An in situ study of protein adsorption on combinatorial Cu–Al films using spectroscopic ellipsometry

An approach to quantifying adsorbed protein layers at the protein/metal interface through spectroscopic ellipsometry using an in situ technique is described. A combinatorial binary Cu 1− x Al x (0 ≤ x ≤ 1) linear composition gradient was prepared by magnetron sputter deposition. Studies on composition spread libraries of samples allow the screening of multiple compositions at once, resulting in less measurement time, less preparation time, better systematic experiments, increased reliability of data, and the opportunity for better understanding of the impact of composition on protein adsorption. Using a custom-designed tubular flow cell, the metallic thin films were exposed to protein solutions of fibrinogen and albumin at two concentrations. The thickness of adsorbed protein layers was monitored dynamically by spectroscopic ellipsometry. It was found that both fibrinogen and albumin preferentially adsorbed onto Cu-rich surfaces, and for an Al content above ∼21 at.%, a significantly lower protein thickness was observed for both fibrinogen and albumin. These amounts depended greatly on the Al 2O 3 content of the film surface, with a higher oxide content leading to a decreased protein thickness. Vastly different amounts of fibrinogen were found to adsorb on pure copper when measured using in situ or ex situ techniques due to a copper corrosion effect that occurs during ex situ measurements.

Mapping single-crystal dendritic microstructure and defects in nickel-base superalloys with synchrotron radiation

Solidification of single-crystal nickel-base superalloys introduces large-scale segregation of constituent elements and defects such as dislocations and mosaicity. By exploiting the energy tunability and interference capabilities of high-brilliance X-ray radiation, key structural features of the dendritic single crystals were mapped over large areas. Interference and diffraction of synchrotron X-rays revealed significant misorientations between individual dendrites in the as-solidified state. For the first time this mosaic structure was quantified for an array of dendrites and correlated with the density of “grown-in” dislocations whose density ranged from 10 7 to 10 8 cm −2. Absorption contrast permitted simultaneous mapping of the distribution of refractory metal additives (e.g. rhenium and tungsten), which segregated preferentially to the dendrite cores with a linear composition gradient toward the interdendritic regions. The results demonstrate that synchrotron X-ray imaging is promising for in situ studies of single-crystal structure and defects in nickel-base superalloys.

Combinatorial electrochemistry on Al–Fe alloys

Combinatorial material development was combined with high throughput microelectrochemistry to allow an efficient but comprehensive investigation of the interface chemistry of Al rich Al–Fe alloys as a function of their chemical composition. Composition spread thin films with a linear composition gradient were produced by thermal codeposition. A scanning droplet cell was used to determine the open circuit potential and to perform successive anodic oxide formation with intermittent impedance spectroscopy. The film formation factor, the relative permittivity of the oxides and the onset potential of oxide formation were determined quantitatively as function of the composition with a resolution of 0.5 at.%. An unexpected synergistic effect is found in a very narrow composition range between 9 and 12 at.% Fe. This effect, which shifts the onset potential by nearly 1 V, is discussed in terms of a local accumulation of Fe resulting in a redox stabilisation of space charge layer formation during high-field oxide growth. The results are supported by composition and depth dependent XPS measurements.

Phase formation and microstructure evolution in laser rapid forming of graded SS316L/Rene88DT alloy

The graded material of the SS316L stainless steel/Rene88DT superalloy was fabricated using laser rapid forming. A linear compositional gradient, from 100% SS316L stainless steel to 100% Rene88DT superalloy was achieved. The dendritic growth of the γ phase, the precipitation of the γ′ precipitate, as well as the formation of the η phase and the carbides along the compositional gradient zone were investigated. The growth morphology of primary γ-dendrites can be predicted by the microstructure selection models based on the maximum interface temperature criterion. The precipitation of the γ′ phase and η phase was to a large extent governed by the concentration of Ti and Al elements as well as by the Ti/Al ratio in the gradient zone. The size of the γ′ phase along the gradient zone can be explained by the LSW theory. In the deposit, apart from MC carbides, some M 6C carbides were obtained, and there was no evidence of M 23C 6 carbides. This differed from the predictions of the phase diagram. It was interesting to find that the MC carbides exhibited a cored structure. The formation of the cored structure as well as the M 6C carbides was discussed.

Laser rapid forming of SS316L/Rene88DT graded material

Abstract The stainless steel-SS316L/superalloy-Rene88DT graded material was successfully fabricated using laser rapid forming. A linear compositional gradient, from 100% SS316L stainless steel to 100% Rene88DT superalloy, was achieved within a thickness of 40 mm of laser multilayer deposition. The solidification behavior and the morphological evolution along the compositional gradient were investigated. It was found that, in the gradient zone, within the processing parameters of this study, there was continued epitaxial growth of the γ phase of the columnar dendrites, starting with 100% SS316L stainless steel to 100% Rene88DT superalloy, with the 〈1 0 0〉 crystallographic orientation parallel to the gradient direction. Clad layer bandings were found in the samples; however, the continuity of the growth of the columnar dendrites was not upset. The results are explained by the columnar to equiaxed transition theory and the criteria for planar interface instability and dendritic growth.

Sputter deposition of compositional gradient magnetostrictive FeCo based thin films

A thin film of Fe x Co (1− x) has been produced with a linear compositional gradient ranging from x=0.67 to x=0.40 via an RF sputtering technique. The film was deposited onto a glass slide from a target consisting of two semi-circles of pure Fe and Co. We show that such a compositional gradient combined with local magnetic probes allows easy and quick characterisation of the magnetic properties as a function of composition. The composition dependence of the coercivity, anisotropy and magnetostriction of Fe x Co (1− x) have been established using this method.

YBCO films with induced composition gradient: an accurate exploration of the film growth characteristics

The purpose of this study was to investigate the surface morphology and the structure of laser ablated YBa 2Cu 3O 7 (YBCO) films, as a function of cation composition in special samples presenting a wide range of compositions on the same substrate induced by off-axis geometry. This method ensures that the variations observed in the films are only influenced by the deviations from composition. The YBCO films studied had an almost linear composition gradient, which allowed the analysis in a continuous range of Cu/Ba compositions from 1.0 to 2.0 along a pseudo-binary line with Ba/Y∼1.4. The Y in excess induced the formation of Y 2O 3 inclusions in the film for all the Cu/Ba range. A dramatic change in the surface morphology, domain orientation, and c-axis parameter, explored through local X-ray diffraction measurements, was systematically observed when crossing the composition ratio Cu/Ba∼1.6, regardless of the oxygen pressure used during the deposition process. For Cu-rich compositions, Cu/Ba>1.6, the films show the presence of CuO precipitates onto a flat c-axis oriented YBCO matrix, whereas in the Cu-depleted region, Cu/Ba<1.6, a rough morphology starts to develop, associated to a progressive misorientation of the c-axis domains. The smoothest morphology, with total absence of outgrowth precipitates or deep holes, and rocking curve width below 0.1°, was obtained in a very narrow range of Cu/Ba composition ratio around 1.6. The abrupt transition from one regime to the other was related to a change in the growth mode, influenced by the balance between the formation of supplementary Cu–O planes and the presence of anti-phase boundaries. Some variations in the c-axis parameter of the films were also observed and were related to the oxygen diffusion mechanism during film preparation, and induced by the particular microstructure of the different regions.

Synthesis and characteristics of strontium–barium titanate graded thin films at low temperature using a sol–gel technique

Thin films (∼400 nm) of Ba x Sr 1− x TiO 3 (BST) with gradients in composition were formed on Si(100) and fused quartz substrates by a modified sol–gel technique. A special heating treatment procedure was employed in this study to form the uniform composition gradients at below 750°C. The results of Auger depth profiling indicate that the special process produces smooth, linear composition gradients. XRD diffraction clearly shows the formation of the perovskite structure. The (200) peaks of the BaTiO 3, SrTiO 3, and the graded BST films were analyzed; the peak from the graded film is broad and centered at 46.4°, while the intensity is slightly weaker than those of BaTiO 3 and SrTiO 3. The index of refraction and the band gap were determined at room temperature in the wavelength 200–900 nm from spectrophotometric measurements of the transmittance. The average value of the refractive index is found to be 2.45 for the graded BST films in the wavelength 400–800 nm; the higher values near ultraviolet are associated with the fundamental band gap absorption. The variation of the refractive index of the graded films is smaller than that of the other films. The direct band gap energy is found to be 3.73 eV.

Thermal residual stresses in plates and coatings composed of multi-layered and functionally graded materials

In this paper, thermal residual stresses in multi-layered and compositionally graded plates and coatings were analyzed. Systematic studies on effects of different material combinations, including the compositional gradient, the elastic modulus, the coefficient of thermal expansion, and the number of layers in the case of multi-layered materials on the magnitude and distribution of residual stresses were conducted. Geometry changes of plates and coatings due to these residual stresses were also investigated. It was found that the distribution and magnitude of thermal residual stresses within a plate can be adjusted by controlling the compositional gradient and selecting a proper combination of ceramic and metal constituents. When the compositional gradient of a plate is such that a rapid change in volume fraction and properties occurs near the ceramic face and a gradual change near the metal face, a residual compressive stress is produced on both faces of the plate. However, when the rapid change in volume fraction and properties occurs near the metal face and the gradual change near the ceramic face, a residual tensile stress is generated on both faces of the plate. Minimum residual stresses are obtained when the plate has a linear compositional gradient. As for compositionally graded coatings, the magnitude of the residual stresses on the surface of the coating cannot be decreased by introducing a compositional gradient. The gradient coating only alters the characteristics of the residual stress distribution.

Compositionally graded HgCdTe photodiodes: Prediction of spectral response from transmission spectrum and the impact of grading

We have studied the infrared transmission spectrum and the optical performance of HgCdTe photodiodes containing a linear composition gradient in the active layer. Our objectives were to enable the prediction of the optical performance of a photodiode at 77K, based on the easily and nondestructively measured transmission spectra, as well as to gain a better understanding of the effects of the grading on the optical performance. Consequently, we address three issues here. We first establish improved characterization techniques that can provide accurate values of the necessary material parameters such as gradient in composition. Second, we present a model that can predict the optical response of a diode, based on the material properties and the diode’s geometry. Third, we use the above-mentioned model for the theoretical calculations of the effects of the grading and the resulting built-in electric field on the diode’s optical response.

Demonstration of a linear composition gradient during water saturation of CO2 in supercritical fluid chromatography

Water was added to CO2 by saturation to increase the solvation power of the mobile phase in supercritical fluid chromatography. The saturation was performed at a temperature above the boiling point of water (100°C) to increase the amount of water which could be loaded homogeneously into the CO2 (2.5–3.0 mol% water as compared to about 0.25 mol% water at 25°C). A linear composition of water was produced by altering the density of the CO2 during saturation. Modifications to the injector and CO2 transfer lines prevented phase separation as a result of the instrumentation used in capillary supercritical fluid chromatography (SFC). After fitting vapor-liquid equilibria data to pressure, density, and temperature conditions, approximately 2.5–3.0 mol% of water was introduced in a linear gradient at 110°C. The effect of water on SFC performance was evaluated with standard steroid compounds. This paper provides further evidence for the need to examine vapor-liquid equilibria data prior to SFC.

Amorphization of Hf-Ni films by solid-state reaction

We report on a detailed study of a new process to produce amorphous binary alloys by solid-state reaction of the elemental constituents from an initially thin-layer configuration. The Ni-Hf system was selected on the basis of the criteria that a fast diffuser (Ni) and a large binary heat of mixing drives the solid-state reaction at temperatures so low (∼ 300 °C) that nucleation and growth of the crystalline phases are suppressed and the amorphous phase becomes the lowest accessible state of free energy. Backscattering spectrometry, x-ray diffraction, and transmission electron microscopy are used to monitor the atomic composition profile and the microstructure of the samples. The kinetics of the amorphous phase formation is consistent with the diffusion-limited growth of a laterally uniform amorphous layer. The substantial and approximately linear composition gradient of the amorphous layer reflects the low atomic mobility of the atoms in the amorphous phase and the broad existence range of the amorphous phase in the binary Ni-Hf system. The latter is consistent with predictions based on the calculated equilibrium free-energy diagram of the system.

Rhenium and rhenium-tungsten deposition by thermochemical reduction of the hexafluorides—A preliminary study

Near-optimum parameters were determined by the Bureau of Mines for vapor deposition of rhenium on heated copper substrates by hydrogen reduction of rhenium hexafluoride (ReF 6). High-purity deposits of nearly 90% theoretical density were obtained with the reaction chamber at 250 °C, with a H 2 to ReF 6 volume ratio of 25 to 1, and with a ReF 6 flow rate of 5 grams per hour. Some rhenium deposited as a loose, black powder. When the established near-optimum parameters were carefully followed, this could be minimized but never completely avoided. Varying the reaction chamber pressure, both above and below atmospheric, yielded excessive quantities of rhenium powder. Deposition efficiencies, based on only the coherent metal, ranged between 70 and 75%. Massive deposits contained only spectrographic traces of impurities. The average microhardness (Knoop, 100-gram load) of vapor-deposited rhenium was 907. Limited research on the deposition of rhenoium-tungsten alloys by reduction of the mixed hexafluorides produced deposits in which linear composition gradients were pronounced; this nonhomogeneity was attributed to the wide variation in optimum hydrogen-reduction temperatures of the two hexafluorides (250 °C for ReF 6 and 550 °C for WF 6). Using deposition temperatures between 250 ° and 550 °C did not eliminate the composition variations.