Ag-rich Precipitates Research Articles

Hydrothermal Coating of the Biodegradable Mg-2Ag Alloy

Developing antibacterial biodegradable Mg alloys is of paramount importance to prevent infection and inflammation during the healing process. In this regard, the Mg-2Ag alloy is proposed as a suitable candidate with appropriate biocompatibility as well as antibacterial activity. However, its rapid degradation rate limits its clinical application. To tackle this problem, the hydrothermal coating technique was employed to synthesize a barrier coating to enhance the degradability of the Mg-2Ag alloy using distilled water as the reagent. Field emission scanning electron microscopy (FESEM) micrographs and X-ray diffraction (XRD) patterns showed that a hydroxide coating was formed on the studied samples. Furthermore, it was observed that the substrate microstructure plays an essential role in the obtained coating quality and hence, the degradation behavior. The dendritic microstructure with the nonuniform distribution of Ag-rich precipitates of the as-cast Mg-2Ag alloy lead to undesirable cracks and holes in the coating owing to Mg deficiency to form Mg(OH)2, whereas the solution-treated alloy with a homogenized microstructure resulted in the formation of a more compact, thick, and integrated coating, which remarkably improved the corrosion resistance of the alloy.

Effect of silver additions on the microstructure, mechanical properties and corrosion behavior of biodegradable Fe-30Mn-6Si

FeMn-based alloys are promising materials for vascular implant applications, especially due to their superior mechanical properties and excellent processability. However, a further increase of the biodegradation rate of these metallic materials is desired. The addition of silver was reported to be a promising approach for accelerating the corrosion rate of those FeMn-based alloys by promoting local corrosion due to galvanic coupling, besides improving their antibacterial properties. On the other hand, the corrosion mechanisms occurring due to silver addition in various FeMn-based systems have not been understood completely. In this study, the effect of different silver contents (0.6 wt% and 1.2 wt%) on the microstructure, mechanical and corrosion properties of a cast biodegradable Fe-30Mn-6Si (wt%) is presented. By silver addition, finely distributed Ag-rich precipitates are formed in the matrix composed of austenite and ɛ-martensite, which could be detected by investigations with scanning electron and transmission electron microscopy as well as X-ray diffraction. Furthermore, an enhanced ɛ-martensite fraction was observed with rising Ag content. These changes in the microstructure significantly influence the corrosion properties. By means of potentiodynamic polarization measurements in a simulated body fluid (SBF) at 37 °C, it was revealed that the Ag additions reduce the corrosion current density, which indicates a decreased corrosion rate in comparison to Fe-30Mn-6Si. However, the alloy modifications still show higher corrosion current densities than a cast Fe-30Mn reference system. In addition, higher yield strengths for Ag-added alloys were detected by quasi-static tensile and compression tests. Data availabilityThe processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

A first-principles study of the temperature-dependent diffusion coefficients of silver in the thermoelectric compound PbTe

PbTe-based compounds establish an important class of thermoelectric (TE) materials due to their relatively high thermal-to-electrical energy conversion efficiency at the mid-temperature range (600 – 800 K). The Pb-Te-Ag system is of prime interest due to its potential of forming Ag-rich precipitates dispersed in the PbTe-matrix. Investigation of the microstructure evolution of this system enables us to predict the applicability regime of this system in service temperatures, and this involves both thermodynamic and kinetic aspects. Herein, we apply first-principles calculations to evaluate the diffusion coefficients of Ag in a PbTe matrix. Based on unique combination between calculations of point defect formation energies, electronic density of states (DOS), and electron localization function (ELF), we conclude that diffusion of Ag in PbTe in the interstitial mechanism is preferable compared to the vacancy or substitutional mechanism. We determine both the activation energy for diffusion and the pre-exponential diffusion coefficient for an interstitial mechanism, applying the transition state theory (TST), to be 1.08 × 10−5 cm2·s−1 and 52.9 kJ·mole−1, respectively. This study provides us with practical tools to evaluate the kinetics of phase transformations in these compounds by improving our ability to control nucleation and growth of Ag-rich precipitates in PbTe. This, eventually, provides us with predictive information on the thermal stability of PbTe-based compounds.

Tailoring Thermoelectric Transport Properties of Ag-Alloyed PbTe: Effects of Microstructure Evolution.

Capturing and converting waste heat into electrical power through thermoelectric generators based on the Seebeck effect is a promising alternative energy source. Among thermoelectric compounds, PbTe can be alloyed and form precipitates by aging at elevated temperatures, thus reducing thermal conductivity by phonon scattering. Here, PbTe is alloyed with Ag to form Ag-rich precipitates having a number density controlled by heat treatments. We employ complementary scanning transmission electron microscopy and atom probe tomography to analyze the precipitate number density and the PbTe-matrix composition. We measure the temperature-dependent transport coefficients and correlate them with the microstructure. The thermal and electrical conductivities, as well as the Seebeck coefficients, are found to be highly sensitive to the microstructure and its temporal evolution, e.g., the number density of Ag-rich precipitates increases by ca. 3 orders of magnitude and reaches (1.68 ± 0.92) × 1024 m-3 upon aging at 380 °C for 6 h, which is pronounced by reduction in thermal conductivity to a value as low as 0.85 W m-1 K-1 at 300 °C. Our findings will help to guide predictive tools for further design of materials for energy harvesting.

Effect of the Processing Route on the Thermoelectric Performance of Nanostructured CuPb18SbTe20.

The quaternary AgPb18SbTe20 compound (abbreviated as LAST) is a prominent thermoelectric material with good performance. Endotaxially embedded nanoscale Ag-rich precipitates contribute significantly to decreased lattice thermal conductivity (κlatt) in LAST alloys. In this work, Ag in LAST alloys was completely replaced by the more economically available Cu. Herein, we conscientiously investigated the different routes of synthesizing CuPb18SbTe20 after vacuum-sealed-tube melt processing, including (i) slow cooling of the melt, (ii) quenching and annealing, and consolidation by (iii) spark plasma sintering (SPS) and also (iv) by the state-of-the-art flash SPS. Irrespective of the method of synthesis, the electrical (σ) and thermal (κtot) conductivities of the CuPb18SbTe20 samples were akin to those of LAST alloys. Both the flash-SPSed and slow-cooled CuPb18SbTe20 samples with nanoscale dislocations and Cu-rich nanoprecipitates exhibited an ultralow κlatt ∼ 0.58 W/m·K at 723 K, comparable with that of its Ag counterpart, regardless of the differences in the size of the precipitates, type of precipitate-matrix interfaces, and other nanoscopic architectures. The sample processed by flash SPS manifested higher figure of merit ( zT ∼ 0.9 at 723 K) because of better optimization and a trade-off between the transport properties by decreasing the carrier concentration and κlatt without degrading the carrier mobility. In spite of their comparable σ and κtot, zT of the Cu samples is low compared to that of the Ag samples because of their contrasting thermopower values. First-principles calculations attribute this variation in the Seebeck coefficient to dwindling of the energy gap (from 0.1 to 0.02 eV) between the valence and conduction bands in MPb18SbTe20 (M = Cu or Ag) when Cu replaces Ag.

Enhanced thermoelectric performance in Cu2GeSe3 via (Ag,Ga)-co-doping on cation sites

An enhancement on thermoelectric performance of Cu2GeSe3via simultaneously Ag-alloying on Cu sites and Ga-doping on Ge sites is achieved. The relatively high solubility (∼10%) of Ag on Cu sites allows for the strong point defect scattering for phonons, which causes remarkable reduction in lattice thermal conductivity. Ag-rich precipitates emerge when the amount of Ag is higher than the solubility on Cu site, which however do not have significant effect on the lattice thermal conductivity since it is already very close to the lower limit of kinetic theory. Ga-doping, an effective way to tune the hole concentration, leads to optimization of power factor in the whole temperature range. The maximal zT obtained in Cu1.9Ag0.1Ge0.997Ga0.003Se3 is 1.03@786 K, about 58% higher than that in previous report. In addition, the average zT in the temperature range from 320 K to 786 K is 0.58, implying great potential for fabrication of thermoelectric devices.

Effects of nanocrystalline microstructure on the dry sliding wear behavior of a Cu-10 at% Ag-10 at% W ternary alloy against stainless steel

To explore the W nanoparticles effects on the subsurface microstructural self-organization of Cu-Ag two-phase alloys and the resulting wear performance, we have fabricated bulk nanostructured Cu-10 at% Ag-10 at% W ternary alloys by high energy ball milling and warm pressing. During ball milling, Ag is dissolved into the Cu matrix but W remains immiscible. Compaction of the powders into bulk at 300 °C leads to the Ag precipitation. The as-pressed Cu-10 at% Ag-10 at% W ternary alloy has Ag-rich precipitates size (dAg) of 22 nm and W particles size (dw) of 26 nm. Further annealing at 600 °C for 1 h only increases dAg and dw to 52 nm and 41 nm, respectively. The fabricated Cu-10 at% Ag-10 at% W ternary alloys thus demonstrate enhanced coarsening resistance. The two alloys were then subjected to dry sliding wear against stainless steel disks. It is found that the initial length scale of Ag-rich precipitates and W particles has a profound influence on the microstructure evolution during wear and accordingly on the wear performance. For the as-pressed sample with small dAg and dW, severe plastic deformation (SPD) by wear forced the formation of Cu-Ag homogeneous solid solution, leading to low wear resistance. In contrast, for the annealed one with relatively large dAg and dW, the Cu, Ag and W three phases co-existed but the Ag-rich precipitates were transformed into wavy nanolayers, providing enhanced wear resistance. Compared with Cu-Ag alloys subjected to sliding wear, the presence of W nanoparticles was found to hinder the formation of self-organized nanolayered structure and leads to a small deformation depth. The obtained results provide deep insights into the plastic deformation mechanisms in ternary alloys and the design of wear resistant engineering materials.

TEM observations of reactive bonded Ti6Al4V alloy

Ni(V)/Al multilayer covered with Ag-based braze alloy (NanoFoil®) was applied for joining parts made of Ti6Al4V alloy. The reaction of multilayers was started with electric current pulse (0.5A, 9V). The transmission electron microscopy (TEM) observations of joint area, backed by analysis of the electron diffractions, showed that the initial Ni(V)/Al multilayer transformed into fine grain (~150nm) of B2-NiAl intermetallic phase. The silver-based braze alloy layer from top and bottom of Ni(V)/Al multilayer, located itself mostly at the B2-NiAl/Ti6Al4V interface as Ag-rich precipitates, but small amounts out-diffused into B2-NiAl as intergranular veins. The Ti6Al4V close to the B2-NiAl is subject to strain high enough to fill it with platelets of α″-Ti martensite.

Influence of Silver nanoparticles addition on the phase transformation, mechanical properties and corrosion behaviour of Cu–Al–Ni shape memory alloys

Incorporation of silver nanoparticles into Cu-based shape memory alloys is recommended to enhance their phase transformation behaviour. However, this incorporation can affect their transformation temperatures, mechanical, microstructural and corrosion characteristics. Four different phase reactions β, α, NiAl and γ2 were detected on a derivative curve during the solidification by-computer-aided cooling curve thermal analysis. The highest fraction solid (82%) was calculated for the parent phase (β) based on the Newtonian baseline method. The microstructural changes and mechanical properties were investigated using field emission scanning electron microscopy, X-ray diffraction tensile test and shape memory effect test. It was found that the addition of Ag can control the phase morphology and orientations along with the formation of the Ag-rich precipitates, and thus the tensile strength, elongation, fracture stress–strain, yield strength and shape memory effect are improved. Remarkably, the shape recovery ratio reached approximately 80% of the original shape. The corrosion behaviour of the Cu–Al–Ni shape memory alloy were investigated using electrochemical tests in NaCl solution and their results showed that the corrosion potential (Ecorr) of Cu–Al–Ni SMA is shifted towards the nobler direction from −307.4 to −277.1m VSCE with the addition of 0.25wt.% Ag.

Microstructure and corrosion behaviour of Cu-Al-Ni shape memory alloys with Ag nanoparticles

In this study, the effect of silver nanoparticles on the transformation temperatures, microstructural and corrosion characteristics of the ternary Cu–Al–Ni shape memory alloys (SMA) was investigated. It was subsequently observed that the addition of Ag nanoparticles controlled the phase morphology and orientations of the parent phases, along with the formation of the Ag-rich precipitates. Furthermore, it was shown that the addition of Ag nanoparticles can affect the martensitic transformation temperature, due to the microstructure changes that are associated with the formation of intermetallic compounds and/or precipitates. The corrosion behaviour of the Cu–Al–Ni shape memory alloy with and without nano-Ag addition were investigated using electrochemical tests in a NaCl solution, and their results showed that the corrosion potential of Cu–Al–Ni–Ag (nanoparticles) SMA (−277.1 mVSCE) was nobler than the Cu–Al–Ni SMA. It was also shown that there is a decline in corrosion current density from 6.93 to 5.61 µA/cm2 after the addition of nano-Ag to Cu–Al–Ni SMA. The combination of SEM, together with EDX and XRD, also showed that the formation of the corrosion products silver chloride, cuprous oxide, aluminium oxide/hydroxide and silver oxide act as a protective layer and improve the corrosion resistance of Cu–Al–Ni–Ag SMA.

Ag-rich precipitates formation in the Cu–11%Al–10%Mn–3%Ag alloy

The formation of Ag-rich precipitates in the Cu–11%Al–10%Mn–3%Ag alloy initially quenched from 1123K was analyzed. The results showed that nanoprecipitates of a Cu-rich phase are produced at about 523K. In higher temperatures these nanoparticles grow and the relative fraction of Ag dissolved in it is increased, thus forming the Ag-rich phase.

Kinetics of Ag-rich precipitates formation in Cu–Al–Ag alloys

The kinetics of Ag-rich precipitates formation in the Cu–2 wt.% Al alloy with additions of 2, 4, 6, 8, 10 and 12 wt.% Ag was studied using microhardness changes with temperature and time, differential scanning calorimetry (DSC), differential thermal analysis (DTA), scanning electron microscopy (SEM), optical microscopy (OM), energy dispersive X-ray analysis (EDX) and X-ray diffractometry (XRD). The results indicated that an increase in the Ag content decreases the activation energy for Ag-rich precipitates formation, and that it is possible to estimate the values of the diffusion and nucleation activation energies for the Ag precipitates.

Transient oxidation of alumina forming Ti–Al–Ag-based alloys and coatings studied by SEM, AFM, XPS and LRS

γ-TiAl based intermetallics possess poor oxidation properties at temperatures above approximately 700°C. Previous studies showed that protective alumina scale formation on γ-TiAl can be obtained by small additions (around 2 at.%) of Ag. Recently, this type of materials has therefore been proposed as oxidation resistant coatings for high strength TiAl alloys. In the present study, a number of cast Ti–Al–Ag alloys and magnetron sputtered Ti–Al–Ag coatings were investigated in relation to transient oxide formation in air at 800°C. After various oxidation times the oxide composition, microstructure and morphology were studied by combining a number of analysis techniques, such as SEM, ESCA, AFM and LIOS-RS. The γ-TiAl–Ag alloys and coatings appear to form an α-Al2O3 oxide scale from the beginning of the oxidation process, in spite of the relatively low oxidation temperature of 800°C. The formation of metastable alumina oxides seems to be related to the presence of Ag-rich precipitates in the alloy matrix.

Transient oxidation of alumina forming Ti–Al–Ag-based alloys and coatings studied by SEM, AFM, XPS and LRS

γ-TiAl based intermetallics possess poor oxidation properties at temperatures above approximately 700°C. Previous studies showed that protective alumina scale formation on γ-TiAl can be obtained by small additions (around 2 at.%) of Ag. Recently, this type of materials has therefore been proposed as oxidation resistant coatings for high strength TiAl alloys. In the present study, a number of cast Ti–Al–Ag alloys and magnetron sputtered Ti–Al–Ag coatings were investigated in relation to transient oxide formation in air at 800°C. After various oxidation times the oxide composition, microstructure and morphology were studied by combining a number of analysis techniques, such as SEM, ESCA, AFM and LIOS-RS. The γ-TiAl–Ag alloys and coatings appear to form an α-Al2O3 oxide scale from the beginning of the oxidation process, in spite of the relatively low oxidation temperature of 800°C. The formation of metastable alumina oxides seems to be related to the presence of Ag-rich precipitates in the alloy matrix.

Quantitative characterisation of chemical inhomogeneities in [formula omitted]–Ag using high-resolution Z-contrast STEM

The incoherent imaging model is applied to interpret high-resolution Z-contrast micrographs. A simple method for a column-by-column resolved characterisation of Ag-rich precipitates in Al –Ag is developed. No information on the detailed imaging process is required. Evaluating the high-angle scattering intensities of Al and Ag by image analysis, the number of Ag atoms contained in individual atomic columns can be determined accurately and moreover, the thickness of the thin foil can be calculated. Multislice simulations confirm the broad validity of the incoherent imaging model for Z-contrast STEM and are used to check the method presented. Finally, the image analysis is applied to experimental Z-contrast images of Guinier–Preston zones in Al– 3 at% Ag. The Ag content of the individual atomic columns can be determined with an accuracy better than ±10%.

Nucleation of <I>Ω</I> Phase in an Al–Cu–Mg Alloy Containing Small Additions of Ag

The role of small additions of Ag in increasing the nucleation frequency of a plate shaped precipitate having {111} A1 habit plane, designated Ω, in an Al-Cu-Mg alloy has been investigated using transmission electron microscopy (TEM), electron diffraction and energy dispersive X-ray spectroscopy. The results provide for the first time direct evidence showing association of Ω plates with small Ag-rich precipitates of the constituent binary Al-Ag system during the early stages of artificial aging. Given the strong attraction between Ag and Mg owing to the large difference in electronegativity between them, Mg simultaneously combines with the Ag-rich precipitates formed on the {111} A1 matrix planes, and makes such sites suitable for the heterogeneous nucleation of Ω phase.

Analysis of the interfacial structure of a twinned variant of Ag precipitate in CuAg alloys

There have been numerous studies of orientation relationships (ORs) between precipitate and matrix phases in alloys. In contrast, there have been very few reports in the literature of the hetero-twin OR, where the lattice planes in the precipitate phase are in twin orientation with respect to the corresponding lattice planes in the parent phase. Examples of such an OR have been observed in metal silicide/Si systems. When a Co film is deposited on (111) Si, the CoSi[sub 2] reaction product displays two variants. One of these variants was found to have the hetero-twin orientation with respect to the underlying substrate. Similar ORs have also been observed in the NiSi[sub 2] system. In this study the authors report experimental results obtained on a binary Cu-Ag alloy consisting of Ag-rich precipitates in a Cu-rich matrix. The OR between some of the precipitates and the matrix is the hetero-twin orientation. The structural ledge (SL) theory is used to analyze the OR and to predict the habit plane.