Au-Cd Alloys Research Articles

Hard-sphere model of the B2 → B19’ phase transformation, and its application to predict the B19’ structure in NiTi alloys and the B19 structures in other binary alloys

The pseudoelastic and pseudoplastic properties of NiTi alloys result from the closeness of the structures between the B2 cubic austenite and the B19’ monoclinic martensite, and from the facility to transform one into each other. The paths followed by the atoms during the B2 → B19’ transformation are usually imagined as combinations of simple shears and shuffles. Here, we propose a simplified hard-sphere atomistic model of phase transformation decomposed into three distinct types of rotational movements. The inputs are the Ti and Ni atomic diameters and the monoclinic angle β. The outputs are the lattice parameters and atomic positions of the B19’ phase. The results are remarkably close to those reported in the literature. The model permits a better understanding of the B19’ structure and the way it is inherited from its parent B2. The value of the monoclinic angle corresponds to the highest molar volume; which suggests that it could be a consequence of a maximization of the vibrational entropy. The hard-sphere model also successfully predicts the B19 structures reported in the binary alloys AuTi, PdTi, and AuCd. The agreement is less satisfying for the B19 structure observed in the ternary NiTiCu alloys. It is concluded that the B19’ phase in NiTi alloys, and the B19 phase in AuTi, PdTi, and AuCd alloys can be considered as “simple” hard-sphere structures.

Cd Atom Goes into the Interior of Cluster Induced by Directional Consecutive Assembly of Tetrahedral Units on an Icosahedron Kernel.

"Core sliding" in metal nanoclusters drives the reconstruction of external structural units and provides an ideal platform for mapping their precise transformation mechanism and evolution pathway. However, observing the movement behavior of metal atoms in experiments is still challenging because of the uncertain stability of intermediates. In this work, a series of Au-Cd alloy nanoclusters with continuously assembled kernels (one icosahedral building block assembled with 0 to 3 tetrahedral units) were constructed. As the assembly continued, it eventually led to the Cd atom doping into the inner positions of the clusters. Importantly, the Cd doped into the interior of the cluster exhibits a different behavior than the surface or external Cd atoms (dispersion doping vs localized occupy), which provides experimental evidence of the sliding behavior in the nanocluster kernel. Furthermore, density functional theory (DFT) calculations reveal that this sliding behavior in the inner sites of nanoclusters is an energetically favorable process. In addition, these Au-Cd nanoclusters exhibit tunable optical properties with different assembly patterns in their kernels.

Construction of a new Au27Cd1(SAdm)14(DPPF)Cl nanocluster by surface engineering and insight into its structure–property correlation

Surface engineering with a functional DPPF ligand and Cd atom is employed on a Au38 nanocluster to obtain a Au–Cd alloy nanocluster, that is, Au27Cd1. The difference in properties between Au38 and Au27Cd1 indicates the importance of the surface structure.

Surface engineering of linearly fused Au13 units using diphosphine and Cd doping.

Herein, surface engineering was delicately performed to assemble two new Au-Cd alloy nanoclusters, including [Cd2Au17(S-c-C6H11)12(DPPP)2](BPh4) and Cd2Au29(TBBT)17(DPPF)2. Both the Au13 (in Cd2Au17) and Au25 (in Cd2Au29) cores were covered by two identical Au2Cd(SR)6 motifs and two diphosphine ligands. In addition, their optical properties were explored to give clues on the kernel- and surface-dependent electronic structures.

A brief review of shape memory effects and fabrication processes of NiTi shape memory alloys

After the discovery of martensitic in steel, people started research on the martensitic transformation of different materials. In 1932 discovery of reversal of this transformation was discovered in AuCd alloys. Due to this phenomenon, materials were able to gain their original shape and size before mechanical deformations and the concept of shape memory effect (SME) and shape memory alloy (SMA) was developed. After that, so many SMAs are developed but the incident which discovered Nitinol in 1961 enhanced the applicability of SMAs in real-life applications. It happened because of its cost effectiveness and its SME for a suitable range of working temperature (Generally in between −50 °C and 110 °C). Earlier liquid metallurgy route was used to produce the products. But in a few decades, powder metallurgy and additive manufacturing processes are dominated. NiTi phase is the only responsible phase in the NiTi alloys, which is capable of conversion of martensite to austenite and vice versa. At higher service temperature thermal-induced SME is not possible because it operates only when martensite can change to austenite. Service temperatures above austenitic finish temperature can only exhibit stress-induced SME.

Property Modifications in Ni-Ti Shape Memory Alloys

In the paper are shown some properties modifications in Ni-Ti shape memory alloys with the shape memory effect transformation. Recovery of deformation and pseudo elasticity are of great importance for shape memory alloys. Mechanical and electrical properties versus temperature are given in comparison for Ni-Ti and Au-Cd alloys.

Opportunities and challenges in the electrocatalysis of CO2 and CO reduction using bifunctional surfaces: A theoretical and experimental study of Au–Cd alloys

Electrolysis could enable the large-scale conversion of CO2 to fuels and small molecules. This perspective discusses the state-of-the-art understanding of CO2 and CO reduction electrocatalysis and provides an overview of the most promising approaches undertaken thus far. We set to explore “bifunctional” catalysts using Au–Cd based alloys inspired by theoretical modelling. Density functional theory calculations suggest more favourable thermodynamics for CO2 reduction to CO and methanol on mixed Au–Cd sites on Au3Cd relative to similar values on Au. We use various tools to test the bulk and surface alloys experimentally. We find that Au3Cd in neutral media exhibits lower CO evolution activity than Au, and Au–Cd alloys also show negligible activity for CO reduction in alkaline media. This indicates that the mixed Cd–Au sites predicted to be catalytically active are not present in the sample. The catalytic performance is most consistent with Au-terminated step surface on a bulk alloy, possibly formed through adsorbate-induced restructuring. We highlight that future bimetallic catalysts must consider potential-dependent surface restructuring effects caused by reaction intermediates.

EC-STM study of the initial stages of the electrochemical Au(1 1 1)–Cd alloy formation

We have studied the formation of an Au(1 1 1)–Cd alloy in a H2SO4 electrolyte by means of electrochemical STM (EC-STM). To this end, we first characterized the underpotential deposited (upd) Cd overlayers on Au(1 1 1) electrodes. We confirmed the existence of two upd phases on the reconstructed Au(1 1 1) surface, of which the first can be described with a 4×3 unit cell and the second one with a 3×3 unit cell in coexistence with a 2×3 unit cell. At more negative potentials, an alloy with the Au(1 1 1) substrate is formed. In order to obtain a deeper insight into the alloying process, we had to avoid further Cd deposition at these potentials. This was achieved by exchanging the electrolyte after Cd deposition for a Cd-free solution under potential control. We found that the Au–Cd alloy exhibits an atomic structure with a close to square unit cell and locally interferes with the pattern of the Au(1 1 1) “herringbone” reconstruction. This Au–Cd alloy increases the overpotential for the hydrogen evolution reaction (HER) by about 130mV.

Superelastic organic crystals.

Superelastic materials (crystal-to-crystal transformation pseudo elasticity) that consist of organic components have not been observed since superelasticity was discovered in a Au-Cd alloy in 1932. Superelastic materials have been exclusively developed in metallic or inorganic covalent solids, as represented by Ti-Ni alloys. Organosuperelasticity is now revealed in a pure organic crystal of terephthalamide, which precisely produces a large motion with high repetition and high energy storage efficiency. This process is driven by a small shear stress owing to the low density of strain energy related to the low lattice energy.

Martensite aging effects on the dynamic properties of Au–Cd shape memory alloys: Characteristics and modeling

We revisit the martensite aging effect in Au–Cd alloys, by studying the evolution of dynamic properties (i.e. storage modulus and internal friction) with aging time. We found that the storage modulus shows a gradual increase while the internal friction shows a gradual decrease with increasing aging time. Moreover, the evolution of dynamic properties with aging time obeys a simple relaxation function with activation energy of 0.46 eV in AuCd 49.5 alloy. The dependence of such evolution on temperature, defect concentration, frequency and displacement amplitude was also well characterized. All the observed characteristics are understood by the gradual stabilization of martensite phase during aging, driven by a symmetry-conforming short-range ordering tendency of point defects. Furthermore, we proposed a homogenous Landau-type model to describe the response of storage modulus during aging by introducing a time-dependent variable coupled with the order parameter. Such systematic investigation on the dynamic properties during martensite aging is crucial for applications under mechanical vibrations.

Modeling of (121) Twin Boundaries in 2H Martensite

The structures of {121} twin boundary in orthorhombic 2H martensite are modeled using Finnis-Sinclair type many-body interatomic potentials. The boundary corresponds to type-I twinning in 2H martensite of Cu-Al-Ni, Cu-Zn-Al and Au-Cd alloys. Three possible configurations of the {121} boundary are found. The structure with the lowest energy possesses a non-corrugated central {121} plane.

Long-range nature of chemical interaction in solid-phase reactions: Formation of martensite phases of an Au-Cd Alloy in Cd/Fe/Au film systems

Long-range nature of chemical interaction in solid-phase reactions: Formation of martensite phases of an Au-Cd Alloy in Cd/Fe/Au film systems

Defects and Diffusion in a Prototype of Shape Memory Alloys: β<sub>2</sub>' Au-Cd Alloys

The Au-Cd alloys continue to be important vehicles of research for the shape memory effect involving martensitic transformations and related phenomena.They transform to a variety of martensitic structures depending on the alloy composition and thermal history. Additionally, the Au-rich alloys display rubber like behavior involving thermo-elastic memory. Defects and diffusion play important roles in determining these properties. Defects and diffusion mechanisms in the Au- 47.5 - 50.5 at.% Cd alloys are examined. Diffusion in the nanometer regime and the states of defects are found to be important contributing factors to determine the shape memory effect, the variable martensitic transformations and the rubber like behavior, which are discussed in details.

New insights on the Cd UPD on Au(111)

AbstractThe Cd underpotential deposition (UPD) process on Au(111) was analyzed by means of combined electrochemical measurements and in situ scanning tunneling microscopy (STM). In the underpotential range 300⩽ΔE (mV) ⩽400, 2D Cd islands are formed on the fcc regions of the Au(111)‐(√3 × 22) reconstructed surface without lifting the reconstruction. At lower underpotentials, the 2D Cd islands grow and, simultaneously, new 2D islands nucleate and coalesce with the previous ones forming a complete condensed Cd monolayer (ML). STM images and long time polarization experiments performed at ΔE = 70 mV demonstrate the formation of an AuCd surface alloy. At ΔE = 10 mV, the formation of the complete Cd ML is accompanied by a significant AuCd surface alloying and the kinetic results reveal two different solid‐state diffusion processes. The first one, with a diffusion coefficient D1 = 4 × 10−17 cm2 s−1, could be ascribed to the mutual diffusion of Au and Cd atoms through a highly distorted (vacancy‐rich) AuCd alloy layer. The second and faster diffusion process (D2 = 7 × 10−16 cm2 s−1) is associated with the appearance of an additional peak in the anodic stripping curves and could be attributed to the formation of another CdzAux alloy phase. Copyright © 2008 John Wiley & Sons, Ltd.

Stabilization of the gamma phase in Fe–Pt alloy nanoparticles with low Pt contents prepared by chemical synthesis method

Nanometer-sized particles (nanoparticles) of Fe–Pt alloy having less than 25 at.% Pt were prepared by a chemical synthesis method which Sun et al. recently reported. In this method, iron pentacarbonyl (Fe(CO) 5) and platinum acetylacetonate (Pt(acac) 2) are used as raw materials for the component metals. Crystal structure and phase transformation of the alloy nanoparticles were examined by transmission electron microscopy and electron diffraction, and chemical composition by energy-dispersive X-ray spectroscopy as well. As a result, it was found that even the γ phase nanoparticles with an average composition of Fe–5.4 at.% Pt did not show any martensitic transformation upon cooling from an austenitizing temperature to room temperature. This marked stabilization is discussed from a thermodynamic point of view, referring to previous work on Fe–Ni and Au–Cd alloy nanoparticles.

Alloy formation in the system Au(1 1 1)/Cd during the UPD process

The Au–Cd alloy formation in the course of Cd underpotential deposition (UPD) in the system Au(1 1 1)/Cd 2+, SO 4 2− has been studied by conventional electrochemical techniques and in situ STM. The long time polarization experiments and the current–potential desorption spectra have indicated that the alloy can be formed at relatively high underpotentials with the formation of 2D Cd islands as was observed by in situ STM images. These islands grow and coalesce at a constant potential with the subsequent formation of new 2D islands on top of the partially formed first Cd monolayer. Atomically resolved images of these islands have revealed an hexagonal atomic structure with an interatomic distance of 0.29 ± 0.01 nm. The dissolution of these features occurs together with the formation of many holes of monatomic height which induces a surface roughening. The alloy formation is explained considering a place exchange process between the Au atoms and the adsorbed Cd atoms followed by solid state diffusion of these atoms through the alloyed phase.

The aging effect in Au-Cd alloys: A Mössbauer spectroscopy study

The martensite aging effect, observed in a number of alloy systems and through which fundamental properties for particular materials are altered by aging in the martensite state, has attracted considerable interest in recent years. It is important to understand and control the effect, particularly in shape-memory alloys for which stability of properties is fundamental to applications. The most likely model, at least for the Au-Cd system in which it has been most critically tested, is the symmetry conforming short-range order (SC-SRO) model first proposed by Ren and Otsika. 197 Au Mossbauer spectroscopy offers the possibility to observe the nature of defects associated with the gold sites. The spectra for single crystal and polycrystalline Au-Cd in both the aged martensite and quenched austenite conditions are presented and discussed. While some uncertainty in interpretation of the spectra remains, particularly in relation to any quadrupole splitting to be expected from either Au atoms on Cd-nearest-neighbour sites or the martensite lattice distortion, the best single-line fits provide evidence for two defect types.

Electrodeposition of Au-Cd alloy nanostructures on Au(111).

This report concerns an in-situ scanning tunneling microscopy study of the initial stages in the formation of a Au-Cd alloy on the Au(111) herringbone reconstruction. Although Au-Cd nanoclusters of alloy have been observed in sulfate electrolyte by this group, alloy "nanowires" were observed to form preferentially in the hcp regions between the sets of "soliton" walls of the reconstruction only in the presence of chloride. The nanowires were formed at -0.55 V versus 3 M Ag/AgCl, corresponding to Cd underpotential deposition (upd). Upd is electrodeposition at a potential prior to that needed to deposit the bulk element.

Transmission electron microscopy and electron diffraction studies on martensitic transformations in nanometre-sized particles of Au-Cd alloys of near-equiatomic compositions

Abstract Martensitic transformations in nanometre-sized particles of Au-Cd alloys of near-equiatomic compositions were studied by means of transmission electron microscopy and electron diffraction. The alloy particles with an average size of several nanometres were prepared by high-vacuum electron-beam deposition. In the as-deposited state the particles were crystalline and possessed a B2-type structure at room temperature, similar to that of the β2 phase in the corresponding bulk. The β2−phase particles with an average composition of Au-44.5at.∼Cd were transformed at 76 K into an orthorhombic structure similar to that of the γ2 phase in the bulk, while those with an average composition of Au-50.2at.∼ Cd were transformed at 60K into a trigonal structure similar to that of the ζ2 phase in bulk. The temperature parameter in the Debye-Waller factor measured at room temperature for the β2−phase particles was roughly four times that for Au in the bulk. It was thus found that the M s temperatures for the β2 ζ γ‘2 and β2 → ζ2’ transformations in the nanometre-sized particles were lowered far below room temperature despite the increasing surface area and the enhanced lattice softening. Their reverse transformations, however, took place on heating to room temperature. It was thus seen that the lowering of the transformation temperatures was due to the decrease in the thermodynamic equilibrium temperatures T 0 between the β2 and γ2 and between the β, and ζ′2 phases. The decrease in T 0 indicated that the free energies of the β2, γ2 and ζ2 phases of the alloys in the small-particle state are definitely different from those in the bulk state.

Transmission electron microscopy and electron diffraction studies on martensitic transformations in nanometre-sized particles of Au-Cd alloys of near-equiatomic compositions

Transmission electron microscopy and electron diffraction studies on martensitic transformations in nanometre-sized particles of Au-Cd alloys of near-equiatomic compositions