Phase composition and structure of aluminum Al–Cu–Si–Sn–Pb alloys

The structure and phase composition of cast and heat treated Al–Cu–Si–Sn–Pb alloys containing 6 wt % Sn, 2 wt % Pb, 0–4 wt % Cu, 0–10 wt % Si have been studied using calculations and experimental methods. Polythermal and isothermal sections are reported, which indicate the existence of two liquid phases. It was found that the low-melting phase is inhomogeneous and consists of individual leadand tin-based particles.     

Antifriction coatings of Pb–Sn–Cu alloy electro-deposited from methanesulphonate bath

Abstract

Electrodeposition of antifriction Pb–Sn–Cu coatings from a bath based on methanesulphonic acid (MSA) has been investigated. The bath contains two specially selected organic additives which modify both the structure and the appearance of deposits, prevent the oxidisation of Sn(II) ions and allow the increase of tin content in deposits. The antifriction lead (～90%)–tin (～8%)–copper (～2%) films are deposited from the MSA-electrolyte at a current density of 4 A dm^?2 and a bath temperature of about 20–25°C. Inhibition of the Sn²⁺ ions discharge reaction is observed when the ternary alloy is deposited on the cathode. The Cu²⁺ ions discharge proceeds at a diffusion limiting current. Simultaneous co-deposition of copper has no effect on the kinetics of both lead and tin electrodeposition. Wear characteristics of the deposits from the methanesulphonate bath do not differ from those obtained from the usual fluoroborate bath.     

Electrochemical Properties and Microstructure of Al/Pb–Ag and Al/Pb–Ag–Co Anodes for Zinc Electrowinning

The Al/Pb–0.8%Ag and Al/Pb–0.75%Ag–0.03%Co(in mass fraction) anodes used in zinc electrowinning are prepared through the electrodeposition of lead methanesulfonate electrolyte onto an aluminum matrix.The results of anode polarization curves,Tafel curves,and EIS characterizations indicated that the Al/Pb–0.75%Ag–0.03%Co anode has higher electrocatalytic activity and corrosion resistance than the Al/Pb–0.8%Ag anode.SEM observations on the fruit surfaces demonstrated the crystals on the Al/Pb–0.8%Ag anode are larger than on the Al/Pb–0.75%Ag–0.03%Co anode.After 24 h of anodic polarization,SEM observations and XRD analysis showed that the MnO2–PbO2layer on the Al/Pb–0.75%Ag–0.03%Co anode surface is characterized by dendritic crystals,and the PbSO4–PbO2layer under the MnO2–PbO2layer is characterized by uniform and chaotic orientation tetragonal symmetry crystallites of PbSO4.However,the MnO2–PbO2layer on the Al/Pb–0.8%Ag anode surface is characterized by granular crystals,and the PbSO4–PbO2layer under the MnO2–PbO2layer is characterized by well-organized orientation crystallites of PbSO4,which are concentrated in certain zones.     

Atomistic characterization of the chemically heterogeneous Al–Pb solid–liquid interface

The chemically heterogeneous interface between solid Al and liquid Pb at 625 K is examined by using molecular-dynamics simulation. For the interfacial orientations (1 0 0), (1 1 0) and (1 1 1), we characterize the interface by calculating the density, potential energy, stress and diffusion constant profiles as well as a two-dimensional Fourier analysis of the interfacial layers. Our results are consistent with experimental observations [Acta Mater 2001;49:4259], based on the equilibrium shape of liquid Pb inclusions in solid Al just above the melting temperature of Pb, that the (1 1 1) interface is faceted, while the (1 0 0) and (1 1 0) interfaces are rough. We found that Al and Pb form immiscible two-dimensional domains within the interfacial layers, rather than an intermixed interfacial alloy, as was observed in recent simulations of the Cu–Pb interface [Acta Mater 2011;59:3137]. In addition, in contrast to earlier observations on the (1 1 1) Cu–Pb interface at this temperature, no prefreezing layers are found in Al–Pb interfaces for any of the orientations studied.     

Electromigration in flip chip solder bump of 97Pb–3Sn/37Pb–63Sn combination structure

Electromigration damage in the flip chip solder bump of 97Pb–3Sn/37Pb–63Sn (numbers are all in wt% unless specified otherwise) combination structure was studied after current stressing at 140 °C with a density of 2.55 × 10⁴ A/cm² for up to 20 h. The under bump metallurgy for the 97Pb–3Sn solder on the chip side was TiW/Cu/electroplated Cu while the bond-pad for the 37Pb–63Sn solder on the printed circuit board (PCB) side was electroless Ni/Au. We observed in the thermo-electromigration test that failure occurred at the top of the bump with a downward electrical current flow while there was no failure in the opposite current polarity. The Pb atoms were found to move in the same direction as with the electron current flow. Therefore, in the case of the downward electron flow, the composition of the upper solder bump changed from 97Pb–3Sn to 83Pb–17Sn and it enabled the Cu₆Sn₅ phase to precipitate onto the chip side. Due to the precipitation and growth of the Cu₆Sn₅ intermetallic compound, the Cu under bump metallurgy was quickly consumed and the subsequent void formation induced failure.     

Solidification of Pb–Al Alloys Under the Influence of Electric Current Pulses

Continuous solidification experiments are carried out with Pb–Al alloys under the influence of the electric current pulses(ECPs). The results demonstrate that the ECPs mainly affect the microstructure formation through changing the energy barrier for the nucleation of the minority phase droplets(MPDs) and minority phase particles(MPPs) during cooling Pb–Al alloys in the liquid–liquid and liquid–solid phase transformation temperature ranges in advance of the solidification of the matrix liquid. For Pb–Al alloys with Al-rich droplets/particles as the minority phase, the ECPs lower the energy barriers for the nucleation of the MPDs/MPPs and cause a significant increase in the nucleation rate of the MPDs/MPPs and, thus,promote the formation of Pb–Al alloys with a well-dispersed or even nanoparticles dispersed microstructure. The ECPs parameters show an important influence on the microstructure formation of Pb–Al alloys. The refinement extent of the MPDs/MPPs increases with the increase in the peak current density. For a given peak current density, the refinement extent of the MPDs/MPPs increases with the increases in the pulse frequency and pulse width first, and then level off and become asymptotic.     

Electrochemical impedance spectroscopy evaluation of behaviour of Pb–Ag anodes for zinc electrowinning

Abstract

Pb–0·5–0·7Ag anodes are widely used in the industry of zinc electrowinning. Two commercial lead anodes containing 0·56 and 0·69%Ag were studied by electrochemical impedance spectroscopy to evaluate their electrochemical activity. An industrial acid zinc sulphate electrolyte containing glue and chloride ion, but without manganese addition, was considered. In this study, 5 h of electrolysis at a density of 50 mA cm^?2 (currently used in practice) and also 6 h of potential decay were made to represent electrowinning periods of maintenance both at 38°C. During the 5 h polarisation, the average double layer capacity of Pb–0·56Ag alloy was higher (～9%) than that of Pb–0·69Ag alloy. During the first hour of potential decay, the Warburg impedance controls the electrochemical reaction. For the period from the second to sixth hour, the double layer capacity decreased with immersion time, and the charge transfer resistance increased with time. During the potential decay, the average charge transfer resistance of Pb–0·69Ag anode was higher (～52%) than that of Pb–0·56Ag anode.     

Gravity-induced sedimentation during melting and liquid phase sintering of Pb–Sn alloys

Gravity-induced phase separation during melting of Pb–Sn alloys results in a macroscopic concentration gradient that must be eliminated before phase equilibrium is established. When Pb–Sn alloys are heat treated in solid-plus-liquid regions (akin to liquid phase sintering), recession of the mushy (solid-plus-liquid) region accompanies elimination of the concentration gradient. Sedimentation apparently ceases as phase equilibrium is approached. Some solid-plus-liquid alloys do not sediment. These have a solid volume fraction sufficient to sterically hinder the gravity-induced phase separation. This critical volume fraction is about 0.50 for hypoeutectic (Pb-rich) alloys and about 0.75 for hypereutectic (Sn-rich) alloys.     

Anomalous temperature-dependent Young’s modulus of a cast LAST (Pb–Sb–Ag–Te) thermoelectric material

Thermomechanical characterization is important to material evaluation and device design in the development of thermoelectric technology. In this study, we utilize the resonant ultrasound spectroscopy (RUS) technique to examine the elastic behavior of a cast LAST (Pb–Sb–Ag–Te) material with a composition of Ag_0.86Pb₁₉Sb_1.0Te₂₀ between room temperature and 823 K. The temperature-dependent Young’s modulus exhibits a monotonically decreasing trend with increasing temperature. However, an abnormal slope change in the Young’s modulus–temperature curve around 500 K is observed. In addition, hysteresis between heating and cooling data in the temperature range of 450–550 K is observed, which appears to be dependent on the heating/cooling rate during the RUS experiments such that the hysteresis disappears when the heating/cooling rate was decreased from 5 to 2 K min^–1. In this study we propose an order–disorder transition model for the anomalous temperature-dependent Young’s modulus behavior observed in this study.     

Development of glue-type potentials for the Al–Pb system: phase diagram calculation

Empirical many-body potentials of the glue-type have been constructed for the Al–Pb system using the “force matching” method. The potentials are fitted to experimental data, physical quantities derived from ab initio linear muffin-tin orbitals calculations and a massive quantum mechanical database of atomic forces generated using ultrasoft pseudopotentials in conjunction with ab initio molecular statics simulations. Monte Carlo simulations using these potentials have been employed to compute an Al–Pb phase diagram which is in fair agreement with experimental data.     

Development of Finnis–Sinclair type potentials for the Pb–Bi–Ni system—II. Application to surface co-segregation

Empirical many-body central-force potentials of a Finnis–Sinclair type have been constructed for the Pb–Bi–Ni system using both experimental data and physical quantities derived from ab initio linear muffin-tin orbitals calculations. Monte Carlo simulations using these potentials have been employed to compute the segregation profiles at the (111) surface of Pb–Bi–Ni alloy containing 5 at.% Bi and 0.04 at.% Ni at 400 and 500 K. These calculations indicate strong segregation of Ni to the subsurface atom layer due to its strong interaction with Bi. This is in reasonable agreement with the results of the evaluation of segregation energies of Ni for different equilibrium segregation profiles in Pb₉₅Bi₀₅ by the ab initio linear muffin-tin orbitals method. The strong interaction between Ni and Bi leads to co-segregation of Bi to several of the outermost atom layers, which, in turn, drives further segregation of Ni to the surface layer.     

Improving gold recovery of Pb–Zn sulfide ore by selective activation with organic acid

Free gold and gold wrapped in sulfides are considered as the object of gold floatation. However,floatation of free gold exhibits more variables in practice.In this study, improving gold recovery of a Pb–Zn sulfide ore from Yunnan Province, China, was investigated. The results show that free gold and auriferous sulfides account for 94.99 % of total gold. Without adding organic acid in floatation, only 82 % recovery of gold could be obtained.Gold recovery in Au/Pb concentrates increases by 9.29 %with oxalate added and by 7.35 % with citric acid added,respectively, while performances of lead and arsenic nearly keep a constant. A possible reason is that free gold is of wonderful selectivity against pyrite with organic activators.A new method to enhance gold recovery is proposed.     

Magnetic behavior of atomically engineered NiO–Co–Cu-based giant magnetoresistance spin valves using Pb as a surface modifier

The present study illustrates the efficacy of using surface modifiers, or simply surfactants, to alter, modify, or control the nature of interfaces and film morphology in order to control physical properties in magnetic multilayers. In particular, the magnetic properties of giant magnetoresistive NiO–Co–Cu-based symmetric spin valves are discussed in relation to the modification of interfaces and nanostructure using surface modifier Pb. Results show that a ML of Pb deposited on the first Co/Cu bilayer of the symmetric spin valve leads to reduced in-plane magnetic anisotropy and coupling. In the presence of Pb, the coherent growth mode of Co–Cu layers is disrupted, leading to a fine grain size (≈1–5 nm) in the metal layers. Although this grain size corresponds to the critical wavelength for maximum coupling due to Néel’s so-called “orange-peel” effect, the absence of topographical correlation between various interfaces prevent the occurrence of this form of coupling. At the same time, averaging of exchange interactions over such a fine grain size, which is much lower than the characteristic exchange length for Co (≈25–45 nm), precludes the display of local magnetocrystalline easy axes, thereby leading to low switching fields. The nature of magnetization reversal in the “free” Co layer of the Pb-free spin valve is highly local in nature and occurs by nucleation, growth and coalescence together of irregular micron or sub-micron sized domains.     

Effects of Pb and Cu additives on microstructure and wear corrosion resistance behaviour of PM Al–Si alloys

Abstract

The wear and wear corrosion resistance behaviour of Al–20Si–XPb–YCu (X=0–10 wt-%, Y=0–3 wt-%) alloys fabricated by a powder metallurgy (PM) technique and subsequent heat treatments were evaluated by a block on ring tribotest. The microstructure of all aluminium alloys was observed by optical microscopy and scanning electron microscopy with X-ray energy dispersive spectroscopy. The effects of applied potentials and environments including dry air and 3.5 wt-%NaCl aqueous solution were studied. The results of microstructure analysis indicated that Pb exhibited a bimodal distribution in the Pb containing alloys, and Cu particles become to form the intermetallic phase CuAl₂. Furthermore, the hardness rises significantly for both Pb and Cu containing alloys only after solid solution quenching treatment. The wear and corrosion results showed that the addition of both lead and copper would improve the wear resistance but lead to a higher corrosion rate whereas heat treatment had a beneficial effect of reducing the corrosion rate of most alloys with the exception of Al–Si alloy. Furthermore, by comparison of all alloys after heat treatment, the wear corrosion resistance of Al–Si alloy was inferior to the other alloys; consequent additions of Pb and Cu further improved its wear corrosion resistance. Moreover, at an anodic potential, the wear corrosion rate and current density of both Al–Si and Al–Si–Cu alloys containing particle Pb decrease significantly owing to a corrosion product layer composed of Al, O and Pb elements.     

Proximity effect controlled superconducting behavior of novel biphasic Pb–Sn nanoparticles embedded in an Al matrix

We report the synthesis of biphasic Pb (46 at.%)–Sn (54 at.%) nanoparticles dispersed in an aluminum matrix and explore the nature of the superconducting transition in these particles. The nanoscaled Pb–Sn alloy particles were dispersed in Al by rapid solidification and the two-phase nature of these particles was characterized by transmission electron imaging, diffraction and composition mapping. A weak superconducting transition occurs at 3.1 K in these alloys, which is much lower than the T_C expected for a Pb₄₆Sn₅₄ alloy or that due to the proximity effect between Pb and Sn. We show that it is the superconducting Al matrix with T_C = 1.2 K that plays a major role in determining the effective transition temperature of the system.     

Effect of MnO4− and silver content on electrochemical behaviour of Pb–Ag alloy anodes during potential decay periods

The effect of MnO₄^? and silver content on electrochemical behaviour of five commercial Pb–Ag alloy anodes was studied in acid zinc sulphate electrolyte with and without MnO₄^? ions at 38 °C during potential decay periods. When the anodes were immersed into acid zinc sulphate electrolyte without MnO₄^? ions, the Pb–0.72%Ag anode entered complete passivation state in the shortest time among the five anodes, followed by anodes Pb–0.67%Ag, Pb–0.60%Ag, Pb–0.58%Ag and Pb–0.29%Ag– 0.1%Ca by measurement of open circuit potential. During immersion of the anodes, MnO₄^? ions accelerated the passivation and increased the corrosion current density of the anodes. After immersion in zinc electrolyte with MnO₄^?, the anode Pb–0.72%Ag had the best corrosion resistance, followed by anodes Pb–0.67%Ag, Pb–0.60%Ag, then the close anodes Pb–0.58%Ag and Pb–0.29%Ag– 0.1%Ca by the electrochemical impedance spectroscopy (EIS) analysis.     

Microstructure of Rapidly Solidified Alloys of the Sn–Zn–Bi–In System

The results of a study of the phase composition and microstructure of foils of Sn–8.0 Zn–3.0 Bi–X In (X = 1.5, 2.5, 4.5, 9.0) (wt %) alloys formed by rapidly quenching from the melt at a cooling rate of up to 5 × 10⁵ K/s have been presented. The dependence of the phase composition of the rapidly quenched foils on the concentration of In has been determined. It has been shown that, in rapidly quenched foils, crystallization occurs with the formation of supersaturated solid solutions based on β-Sn and γ phase (Sn₄In). The mechanisms and rates of decomposition of the supersaturated solid solutions at room temperature have been established. The specific features of the formation of the microstructure of the foils have been discussed. The grain structure has been studied by the electron back-scatter diffraction (EBSD) method; the formation of an elongated shape of grains and the high specific surface area of small-angle boundaries has been explained.     

Influence of Mo in the structure of rapidly solidified Fe–Mo–Cr–Mn–Si–C alloy

Abstract

An Fe–Mo–Cr–Mn–Si–C alloy was prepared in an induction furnace and was cast into cylindrical rod in a copper mould in castmatic equipment (low pressure casting). A single phase non-equilibrium featureless (no visible microstructures after deep etching) phase was observed over a certain range of thickness of the rod. In this present work, the extent of the featureless phase was studied with different concentrations of Mo (5–25 wt-%) for 5·5 mm diameter of cylindrical rod at a cooling rate of 1100 K s^–1. Light optical microscopy, scanning electron Microscopy and Vickers hardness tests were used to analyse the samples. The amount of the featureless area varies as the Mo content changes and the maximum featureless area was obtained for 7 wt-% of Mo. This single phase featureless structure exhibits very high hardness (>1350 HV) which can be used in many interesting applications with or without suitable heat treatments.