Function Of Alloy Composition Research Articles

Mechanical behavior and microstructural analysis of NiTi-40Au shape memory alloys exhibiting work output above 400 °C

Substituting Ni with Au in NiTi leads to dramatic increases in transformation temperatures, meeting one of the requirements for a viable high temperature actuator material. Consequently, four alloys containing between 49 and 51 at.% Ti, a fixed 40 at.% Au, and balance Ni, were prepared and investigated in detail using load-biased thermal cycling (LBTC), scanning electron microscopy (SEM), aberration corrected scanning transmission electron microscopy (STEM), and X-ray energy dispersive spectroscopy (XEDS). LBTC experiments demonstrated work output well above 400 °C, with full recovery up to 100 MPa. The alloys exhibit minimal variation in shape memory properties despite the relatively large composition range from Ti-lean to Ti-rich, in stark contrast to most other NiTi-based systems, which demonstrate extreme compositional sensitivity. Electron beam analysis revealed the presence of two types of secondary phases present in all compositions, which are subsequently characterized. Differences in secondary phase content as a function of alloy composition is shown to have a moderating effect on the transforming matrix composition - an important asset for this alloy system - potentially easing processing requirements and opening up shape memory alloys to new fabrication techniques. Unrecovered strain during cycling at higher loads is analyzed from a theoretical perspective to gain insight into the mechanisms of defect formation responsible for functional fatigue.

Bandgap tailoring in CdxZn1 − xS alloy films deposited by successive ionic layer adsorption and reaction

The paper reports tailoring of the optical bandgap in SILAR deposited CdxZn1-xS alloy films. Acetates and chlorides of zinc and cadmium were used to obtain the films. Two separate routes were employed to achieve desired alloy composition in the films. In the first route, the molar concentration of the precursors was varied. In the second route; various volumes of precursors (with fixed concentration) were used. The films were polycrystalline with predominant hexagonal phase. The bandgap of the films varied from 2.4eV to 3.5eV. This bandgap range is highly optimum for the window layer in solar cells and other optoelectronic applications. Variation in the bandgap was studied as a function of alloy composition. The bowing parameter of the bandgap variation was found to be different for two routes chosen to deposit the films.

Empirical expression for the composition and temperature dependence of the energy gap in InAlSb

An empirical expression for the energy bandgap as a function of alloy composition x and temperature for In1−xAlxSb was reported. The In1−xAlxSb epitaxial layers were grown by molecular beam epitaxy (MBE) on InSb(100) substrate, utilizing a p+–p+–n–n+ structure. High resolution X-ray diffraction was used to characterize the epitaxial layers. The Al composition of 2.8% was obtained by assuming the Bragg’s formula and Vegard’s law. Spectral response measurement of the diodes has been employed to investigate the temperature dependence of the band gap of In1−xAlxSb alloys in the range between 77K and 260K. The calculated results for energy gap of InAlSb were in good agreement with the available data and our experimental observation.

Phase transformation and grain growth behavior of a nanocrystalline 18/8 stainless steel

Fe-18Cr-8Ni and Fe-18Cr-8Ni-1Y (at%) stainless steel powders were nanostructured by mechanical alloying from elemental powders and subjected to 90min annealing treatments at various temperatures. The microstructural evolutions as a function of alloy compositions and temperatures were investigated by in-situ and ex-situ x-ray diffraction experiments, transmission electron microscopy and focused ion beam microscopy. The dependence of hardness on the microstructure was utilized to study the mechanical changes. It was found that the resulting microstructures by mechanical alloying were bcc solid solution, the so-called α’-martensite structure. The high temperature in-situ x-ray diffraction experiments showed that the martensite-to-austenite reverse phase transformation was completed above 800 and 900°C for Fe-18Cr-8Ni and Fe-18Cr-8Ni-1Y steels, respectively. A partial or complete retransformation to martensite was observed upon cooling to room temperature. Annealing of nanocrystalline Fe-18Cr-8Ni steel yielded grain growth reaching to micron sizes at 1100°C while addition of 1at% yttrium stabilized the microstructure around 160nm grain size and 6GPa hardness after 90min annealing at 1200°C.

Segregation of solute atoms to interphase boundaries in GdNi2

Lattice locations of 111In impurity probe atoms in intermetallic GdNi2 were studied as a function of alloy composition and temperature using perturbed angular correlation spectroscopy (PAC). Measurements were made on a pair of samples that were richer and poorer in Gd. Three nuclear quadrupole interaction signals were detected and their equilibrium site fractions were measured up to 700 ∘C. Two signals have well-defined electric field gradients (EFGs) and are attributed to In-probes on Gd- and Ni-sites in a well-ordered lattice.A third signal exhibiting strong inhomogeneous broadening was observed at low temperature in Gd-richer samples. This is attributed to segregation of the In-probes to phase boundaries (PB) of minor volume fractions of the neighboring GdNi phase. A measurement made on a stoichiometric GdNi sample exhibited the same inhomogeneously broadened signal, indicating that In-probes prefer to occupy PB and/or grain boundary sites in GdNi rather than the well-defined Gd- and Ni-sites. Changes in site fractions were reversible above 300 ∘C, indicating that probe atoms equilibrate among all lattice locations within a time period of one day. Thus, PB sites provide lower enthalpy environments for In-probes than either crystallographic site in GdNi2. Enthalpy differences between the levels were determined from measurements of temperature dependences of ratios of site fractions. The enthalpy of transfer of In-probes from the Gd- to Ni-sublattice, which is coupled to intrinsic disorder in the compound, was found to be much smaller in the Gd-richer sample than in the Gd-poorer sample. This can be explained by differing temperature dependences of intrinsic defect concentrations at the two compositions. Among those probes that remain within the GdNi2 phase, there is a temperature dependence of the ratio of site fractions of In-probes on Gd- and Ni-sites. Taking this into account, a macroscopic segregation enthalpy is derived from the measurements. Additionally, brief measurements on GdNi2 samples doped with 1 or 2 at.% Cu were carried out.

Dealloying at High Homologous Temperature: Morphology Diagrams

Dealloying under conditions of high homologous temperature, TH, (or high intrinsic diffusivity of the more electrochemically reactive component) is considerably more complicated than at low TH since solid-state mass transport is available to support this process. At low TH the only mechanism available for dealloying a solid is percolation dissolution, which results in a bicontinuous solid-void morphology for which nanoporous gold serves as the prototypical example. At high TH, there is a rich set of morphologies that can evolve depending on alloy composition and the imposed electrochemical conditions, including negative or void dendrites, Kirkendall voids and bi-continuous porous structures. We report on a study of morphology evolution upon delithiation of Li-Sn alloys, produced by the electrochemical lithiation of Sn sheets. Electrochemical titration and time of flight measurements were performed in order to determine the intrinsic diffusivity of Li, , as a function of alloy composition, which ranged from ∼5 × 10−8 – 4 × 10−12 cm2s−1. The activation energy for was measured in the temperature range 30–140°C and found to be 37.4, 37.9 and 22.5 kJ/mole, respectively for the phases Li2Sn5, LiSn and Li7Sn3. Morphology evolution was studied under conditions of fixed dealloying potential and fixed current density and our results are summarized by the introduction of dealloying morphology diagrams that reveal the electrochemical conditions for the evolution of the various morphologies.

Analysis of 2D Transport and Performance Characteristics for Lateral Power Devices Based on AlGaN Alloys

Predicted lateral power device performance as a function of alloy composition is characterized by a standard lateral device figure-of-merit (LFOM) that depends on mobility, critical electric field, and sheet carrier density. The paper presents calculations of AlGaN electron mobility in lateral devices such as HEMTs across the entire alloy composition range. Alloy scattering and optical polar phonon scattering are the dominant mechanisms limiting carrier mobility. Due to the significant degradation of mobility from alloy scattering, at room temperature Al fractions greater than about 85% are required for improved LFOM relative to GaN using a conservative sheet charge density of 1 × 1013 cm−2. However, at higher temperatures at which AlGaN power devices are anticipated to operate, this “breakeven” composition decreases to about 65% at 500 K, for example. For high-frequency applications, the Johnson figure-of-merit (JFOM) is the relevant metric to compare potential device performance across materials platforms. At room temperature, the JFOM for AlGaN alloys is predicted to surpass that of GaN for Al fractions greater than about 40%.

Rapid Solidification of Sn-Cu-Al Alloys for High-Reliability, Lead-Free Solder: Part I. Microstructural Characterization of Rapidly Solidified Solders

Particles of Cu x Al y in Sn-Cu-Al solders have previously been shown to nucleate the Cu6Sn5 phase during solidification. In this study, the number and size of Cu6Sn5 nucleation sites were controlled through the particle size refinement of Cu x Al y via rapid solidification processing and controlled cooling in a differential scanning calorimeter. Cooling rates spanning eight orders of magnitude were used to refine the average Cu x Al y and Cu6Sn5 particle sizes down to submicron ranges. The average particle sizes, particle size distributions, and morphologies in the microstructures were analyzed as a function of alloy composition and cooling rate. Deep etching of the samples revealed the three-dimensional microstructures and illuminated the epitaxial and morphological relationships between the Cu x Al y and Cu6Sn5 phases. Transitions in the Cu6Sn5 particle morphologies from faceted rods to nonfaceted, equiaxed particles were observed as a function of both cooling rate and composition. Initial solidification cooling rates within the range of 103 to 104 °C/s were found to be optimal for realizing particle size refinement and maintaining the Cu x Al y /Cu6Sn5 nucleant relationship. In addition, little evidence of the formation or decomposition of the ternary-β phase in the solidified alloys was noted. Solidification pathways omitting the formation of the ternary-β phase agreed well with observed room temperature microstructures.

Optimized Durability Prediction of Cast Iron Based on Local Microstructure

The microstructure of cast iron is decisively influenced by the composition, the applied metallurgy and the process conditions. Differences in microstructure mainly influence the local mechanical properties. In conventional durability predictions, local microstructure and local defects in the casting are not taken into account. The assumption of homogeneous material properties applying one single material dataset for the prediction of fatigue strength usually leads to a misinterpretation in the assessment of local risks. Furthermore, the material potential of the casting will not be used in full. Safety margins result in an over design and unnecessarily high weight of the component as well as increased casting part costs. The designer can only benefit from the potential of the material cast iron, if the real spectrum of local material properties is considered in durability calculations. Current casting process simulation tools allow the prediction of local microstructure and resulting static mechanical properties as a function of alloy composition, applied metallurgy and local solidification and cooling conditions. Within a German research project, the link between casting process simulation and cyclic material properties was established for different cast iron materials for the first time. S–N curves (Woehler curves), which depend on local microstructure close the gap between casting process simulation and lifetime analysis. This innovative approach leads to improvements in lifetime predictions. It was shown that the consideration of local fatigue strength based on this innovative approach leads as well to an increasing number of cycles to failure as to a correct prediction of the location of crack initiation. Compared to the conventional design of castings, a substantial qualitative and quantitative improvement in assessing the real performance of cast iron materials can be realized.

Development of a ReaxFF Reactive Force Field for the Pt-Ni Alloy Catalyst.

We developed the ReaxFF force field for Pt/Ni/C/H/O interactions, specifically targeted for heterogeneous catalysis application of the Pt-Ni alloy. The force field is trained using the DFT data for equations of state of Pt3Ni, PtNi3 and PtNi alloys, the surface energy of the PtxNi1-x(111) (x = 0.67-0.83), and binding energies of various atomic and molecular species (O, H, C, CH, CH2, CH3, CO, OH, and H2O) on these surfaces. The ReaxFF force field shows a Pt surface segregation at x ≥ 0.67 for the (111) surface and x ≥ 0.62 for the (100) surface in vacuum. In addition, from the investigation of the preferential alloy component of the adsorbates, it is expected that H and CH3 on the alloy surface to induce a segregation of Pt whereas the oxidation intermediates and products such as C, O, OH, H2O, CO, CH, and CH2 are found to induce Ni segregation. The relative order of binding strengths among adsorbates is a function of alloy composition and the force field is trained to describe the trend observed in DFT calculations, namely, H2 < H2O < CH3 ≈ O2 ≈ CO < OH < CH2 < C ≈ CH on Pt8Ni4, H2 < H2O < CO ≈ O2 < CH3 < OH < CH2 < CH < C on Pt9Ni3, and H2 < H2O < O2 < CO < CH3 < OH < CH2 < C ≈ CH on Pt10Ni2. Using this force field, we performed the grand-canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations for a Pt3Ni slab and a truncated cuboctahedral nanoparticle terminated by (111) and (100) faces, to examine the surface segregation trend under different gas environments. It is found that Pt segregates to the alloy surface when the surface is exposed to vacuum and/or H2 environment while Ni segregates under the O2 environment. These results suggest that the Pt/Ni alloy force field can be successfully used for the preparation of Pt-Ni nanobimetallic catalysts structure using GCMC and run MD simulations to investigate its role and the catalytic chemistry in catalytic oxidation, dehydrogenation and coupling reactions. The current Pt/Ni force field still is found to have difficulties in describing the observed segregation trend in Ni-rich alloy compositions (x < 0.6), suggesting the need for additional force field training and evaluation for its application to describe the characteristics and chemistry of Ni-rich alloys.

Dispersion of nonresonant third-order nonlinearities in GeSiSn ternary alloys.

Silicon (Si), tin (Sn), and germanium (Ge) alloys have attracted research attention as direct band gap semiconductors with applications in electronics and optoelectronics. In particular, GeSn field effect transistors can exhibit very high performance in terms of power reduction and operating speed because of the high electron drift mobility, while the SiGeSn system can be constructed using CMOS-compatible techniques to realize lasers, LED, and photodetectors. The wide Si, Ge and Sn transparencies allow the use of binary and ternary alloys extended to mid-IR wavelengths, where nonlinearities can also be employed. However, neither theoretical or experimental predictions of nonlinear features in SiGeSn alloys are reported in the literature. For the first time, a rigorous and detailed physical investigation is presented to estimate the two photon absorption (TPA) coefficient and the Kerr refractive index for the SiGeSn alloy up to 12 μm. The TPA spectrum, the effective TPA wavelength cut-off, and the Kerr nonlinear refractive index have been determined as a function of alloy compositions. The promising results achieved can pave the way to the demonstration of on-chip nonlinear-based applications, including mid-IR spectrometer-on-a-chip, all-optical wavelength down/up-conversion, frequency comb generation, quantum-correlated photon-pair source generation and supercontinuum source creation, as well as Raman lasing.

Knowledge-based artificial neural network model to predict the properties of alpha+ beta titanium alloys

In view of emerging applications of alpha+beta titanium alloys in aerospace and defense, we have aimed to develop a Back propagation neural network (BPNN) model capable of predicting the properties of these alloys as functions of alloy composition and/or thermomechanical processing parameters. The optimized BPNN model architecture was based on the sigmoid transfer function and has one hidden layer with ten nodes. The BPNN model showed excellent predictability of five properties: Tensile strength (r: 0.96), yield strength (r: 0.93), beta transus (r: 0.96), specific heat capacity (r: 1.00) and density (r: 0.99). The developed BPNN model was in agreement with the experimental data in demonstrating the individual effects of alloying elements in modulating the above properties. This model can serve as the platform for the design and development of new alpha+beta titanium alloys in order to attain desired strength, density and specific heat capacity.

Influence of Zircaloy cladding composition on hydride formation during aqueous hydrogen charging

Although hydrogen uptake in Zirconium (Zr) based claddings has been a topic of many studies, hydrogen uptake as a function of alloy composition has received little attention. In this work, commercial Zr-based cladding alloys (Zircaloy-2, Zircaloy-4 and ZIRLO™), differing in composition but with similar initial textures, grain sizes, and surface roughness, were aqueously charged with hydrogen for 100, 300, and 1000 s at nominally 90 °C to produce hydride layers of varying thicknesses. Transmission electron microscope characterization following aqueous charging showed hydride phase and orientation relationship were identical in all three alloys. However, elastic recoil detection measurements confirmed that surface hydride layers in Zircaloy-2 and Zircaloy-4 were an order of magnitude thicker relative to ZIRLO™.

Atom site preference and γ′/γ mismatch strain in Ni[sbnd]Al[sbnd]Co[sbnd]Ti superalloys

The structure and chemistry of NiCoAlTi quaternary superalloys are investigated at the microstructural and atomic scale. Atom probe tomograghy (APT) is used to quantify phase compositions and elemental partitioning behavior. Using aberration corrected electron microscopy, the site occupancy of the atoms within the A3 B structure (L12–type) γ′ phase is determined via atomic resolution energy dispersive X-ray spectroscopy (EDX). Ni is observed to preferentially occupy the A sub-lattice positions for all alloys, while Al and Ti occupy the B sub-lattice. In contrast, Co's site preference changes from a random distribution to the A sub-lattice as the alloy composition changes, in agreement with theoretical predictions from literature. Finally, the lattice strain across the γ′/γ interfaces is measured as a function of alloy composition.

How Intra-Particle Active Site Dispersion Impacts Electrocatalytic Activity

D-band position is often used to describe the reactivity of alloy catalysts. However, the spatial separation (dispersion) and geometric arrangement of catalytically active surface atoms can also control the system’s overall reactivity. My talk will discuss how the dispersion of catalytically active Rh atoms within 2-3 nm, ligand-protected Au/Rh alloy nanoparticles directly impacts electrocatalytic H2 oxidation reaction (HOR) activity. The HOR is an interesting model reaction for the Au/Rh system because Rh atoms catalyze H2 oxidation, whereas Au atoms do not. This provides the opportunity to study the composition-dependent evolution of HOR activity as a function of alloy composition (0-100% Rh). We find that HOR activity follows a volcano-type trend. Pure Au particles do not oxidize H2, and peak HOR activity was found for particles containing 21% and 34% Rh. Beyond this optimum Rh content the HOR activity decreased towards that of 100% Rh particles and a commercially available Rh/C catalyst. In fact, HOR activity decreased as a function of (%Rh)-1 beyond peak activity. We rationalize this trend through the dispersion of Rh-containing active sites on the surface of the Au/Rh alloy NPs. Analysis of literature results has identified similar inverse relationships between catalytic activity and composition. First-principles density functional theory (DFT) studies were also used to model H-binding at realistic Au/Rh alloy nanoparticles. H-binding energy is a common metric for predicting HOR activity. We found that Rh incorporation increased the d-band position of Rh containing particles compared with Au. However, did could not identify any relationship between H-biding and d-band energy. These results are in contrast to what would be expected from a purely electronic d-band model, and they provide strong evidence that intra-particle active site dispersion can tune the catalytic activity of alloy nanocatalysts. We hypothesize this phenomenon should also extend to other alloy systems, and active site dispersion provides another tool for controlling the nano-scale chemistry of electrocatalysts.

Modeling solute segregation during the solidification of γ-phase U-Mo alloys

Using first principles calculations, it is demonstrated that solute segregation during U-Mo solidification can be modeled using the classic Brody-Fleming limited diffusion framework. The necessary supporting equations specific to the U-Mo alloy, along with careful verification of the assumptions underpinning the Brody-Fleming model are developed, allowing for concentration profile predictions as a function of alloy composition and cooling rate. The resulting model is compared to experimental solute concentration profiles, showing excellent agreement. Combined with complementary modeling of dendritic feature sizes, the solute segregation model can be used to predict the complete microstructural state of individual U-Mo volume elements based upon cooling rates, informing ideal processing routes.

A density functional theory study of hydrogen occupation in VNiTi alloys used for dense metal membranes

Attempting to further the development of non-noble dense metal membranes for H2 separation we conduct a density functional theory study of hydrogen occupancy in V-based alloys with Ni and Ti substitutional solutes. Clusters consisting of 19 quasi-randomly coordinated metal atoms are built to model body-centred cubic VNi and VNiTi alloys with different stoichiometry. The total energy of the target systems is calculated using spatially localised functions. The disposition of a pair of hydrogen atoms within the metal lattice is explored and the binding energy in both tetrahedral and octahedral interstices is evaluated. Large spatial distance between absorbed H atoms is favoured for each of the interstitial sites, rejecting the idea of H clustering in the investigated solid solutions. Moreover, simultaneous occupation of both tetrahedral and octahedral interstices is found to be energetically feasible despite the common believe for solely tetrahedral occupancy in metals with body-centred cubic structure. Nonetheless, the most favourable absorption site depends on the solute concentration in the V-based alloys. Calculations of the binding energy using cluster models with different metal atomic ratio provide information on the hydrogen absorption affinity as a function of alloy composition. Enhancement of the absorption affinity with added Ti until certain limit is found, while Ni solutes influence this property in the opposite direction. The applied methodology can be used further in high-throughput calculations to screen various metal alloys for hydrogen separation membranes.

Exploring the electrochemistry of 6xxx series aluminium alloys as a function of Si to Mg ratio, Cu content, ageing conditions and microstructure

The electrochemical response of 6xxx series Al-alloys, which are based on the Al-Mg-Si-(Cu) system, was systematically studied. Nine custom Al-Mg-Si-(Cu) alloys were produced on the basis of varying Si to Mg ratio and Cu content. The alloys were further aged at four different ageing conditions, and analysed by potentiodynamic polarisation and careful quantification of metastable pitting events from potentiostatic transient testing. The relevant alloy microstructures were characterised using transmission electron microscopy, revealing differences in nanoscale precipitates that evolve as a function of alloy composition and ageing conditions. It was revealed that the pitting events depend primarily on the thickness of the precipitates, rather than their length or number density. In the case of Cu-containing 6xxx alloys, precipitates were found to be surrounded by an apparently single atomic layer of segregated Cu atoms, which appeared to inhibit the precipitate growth, thus inhibiting the number of pits occurring. This study provides significant insights into both the understanding of, and the control of, the genesis of pitting corrosion in 6xxx series Al alloys.

Effects of alloy composition on gain and bandwidth of Si/SiGe and Si/GeSn avalanche photodiodes

Abstract In this paper, effects of alloy composition (x or y) on gain and bandwidth of Si/Si1−xGex and Si/Ge1−ySny heterostructure avalanche photodiodes (APDs) have been investigated. Photocurrent calculation in APD is based on the consideration of dead-space effect and carrier confinement at heterointerface. Impact ionization parameters of Si1−xGex and Ge1−ySny are either calculated or interpolated from available data. The band-offsets also become a function of alloy composition and affect the carrier confinement at heterointerfaces. The photocurrent is computed using the composition dependent parameters and, hence, gain and bandwidth of the APDs are obtained. The results show that the gain in Si/Si1−xGex APD and Si/Ge1−ySny APD increases as the Ge-content and the Sn-content respectively increases. The transit-time limited bandwidth and RC-limited bandwidth of the APDs are shown to behave oppositely with change in alloy composition.

Growth of epitaxial Pt1-xPbx alloys by surface limited redox replacement and study of their adsorption properties.

The surface limited redox replacement (SLRR) method has been used to design two-dimensional Pt-Pb nanoalloys with controlled thickness, composition, and structure. The electrochemical behavior of these alloys has been systematically studied as a function of alloy composition. A single-cell, two-step SLRR protocol based on the galvanic replacement of underpotentially deposited monolayers of Pb with Pt was used to grow epitaxial Pt1-xPbx (x < 0.1) alloys of up to 10 ML thickness on Au substrates. It is shown that by varying the terminating potential of the galvanic replacement step, the Pb atomic content can be controlled in the films. Electrochemical analysis of the alloys showed that the adsorption of both H and CO exhibits similar, and systematic, decreases with small increases in the Pb content. These measurements, commonly used in electrocatalysis for the determination of active surface areas of Pt, suggested area values much lower than those expected based on the net Pt composition in the alloy as measured by XPS. These results show that Pb has a strong screening effect on the adsorption of both H and CO. Moreover, changes in alloy composition result in a negative shift in the potential of the peaks of CO oxidation that scales with the increase of Pb content. The results suggest electronic and bifunctional effects of incorporated Pb on the electrochemical behavior of Pt. The study illustrates the potential of the SLRR methodology, which could be employed in the design of 2-dimensional bimetallic Pt nanoalloys for fundamental studies of electrocatalytic behavior in fuel cell reactions dependent on the nature of alloying metal and its composition.