Oxidation of Ag–Sn–La Alloy Powders

The gas atomized Ag-9.26wt%Sn-0.44wt%La alloy powders were oxidized in air between 400 and 900 °C. The oxidation thermodynamics, kinetics and microstructure of the alloy were investigated. We suggested that the addition of La may accelerate oxidation of Sn and prevent the formation of the dense SnO₂ film. The suitable oxidation temperature of the alloy powders is 800 °C in air. After internal oxidation, many cracks were observed on the surface of the alloy powders. In addition, the whole oxidation process of the alloy powders is controlled by the oxygen diffusion. The diffusion coefficient of oxygen in the alloy powders at 800 °C is about 1.5 times larger than that at 700 °C on the initial stage of internal oxidation, while that is 4.5 times on the subsequent stage.     

Effects of Sn additions on microstructure and corrosion resistance of heat-treated Ti–Cu–Sn titanium alloys

The microstructure and corrosion properties of Ti7CuxSn (x?=?0–5?wt-%) alloys after solution treatment have been investigated. The alloys were solution-treated (ST) at 1000°C for 2?h, followed by quenching in water to room temperature. It was found that the microstructure of the ST Ti7Cu alloy had only a martensite structure, and that addition of Sn could refine the microstructure of Ti7CuxSn alloy. Notably, the pseudo dendritic α-Ti phase was formed in ST Ti7Cu5Sn alloys. Potentiodynamic polarisation curves and electrochemical impedance spectroscopy data demonstrated that adding Sn improved the electrochemical corrosion behaviour of the Ti7CuxSn alloy.     

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.     

Contribution to the investigation of ternary Pr–Cu–Sn alloys

With regard to the work in progress on the ternary R–Cu–Sn alloys formed by the rare earths, the data obtained in a partial revision of the Pr–Cu–Sn system are reported. The isothermal section at 400°C has been studied in the composition range: PrCu₅–Pr–Pr₅Sn₃–PrCuSn. The general characteristics of the systems formed by the light R are compared and discussed.     

The effect of Sn on autoclave corrosion performance and corrosion mechanisms in Zr–Sn–Nb alloys

The desire to improve the corrosion resistance of Zr cladding material for high burn-up has resulted in a general trend among fuel manufacturers to develop alloys with reduced levels of Sn. While commonly accepted, the reason for the improved corrosion performance observed for low-tin zirconium alloys in high-temperature aqueous environments remains unclear. High-energy synchrotron X-ray diffraction was used to characterize the oxides formed by autoclave exposure on Zr–Sn–Nb alloys with tin concentration ranging from 0.01 to 0.92 wt.%. The alloys studied included the commercial alloy ZIRLO® (ZIRLO® is a registered trademark of Westinghouse Electric Company LLC in the USA and may be registered in other countries throughout the world. All rights reserved. Unauthorized use is strictly prohibited.) and two variants of ZIRLO with significantly lower tin levels, referred to here as A-0.6Sn and A-0.0Sn. The nature of the oxide grown on tube samples from each alloy was investigated via cross-sectional scanning electron microscopy. Atom probe analysis of ZIRLO demonstrated that the tin present in the alloy passes into the oxide as it forms, with no significant difference in the Sn/Zr ratio between the two. Synchrotron X-ray diffraction measurements on the oxides formed on each alloy revealed that the monoclinic and tetragonal oxide phases display highly compressive in-plane residual stresses with the magnitudes dependent on the phase and alloy. The amount of tetragonal phase present and, more importantly, the level of tetragonal-to-monoclinic phase transformation both decrease with decreasing tin levels, suggesting that tin is a tetragonal oxide phase stabilizing element. It is proposed that in Zr–Nb–Sn alloys with low Sn, the tetragonal phase is mainly stabilized by very small grain size and therefore remains stable throughout the corrosion process. In contrast, alloys with higher tin levels can in addition grow larger, stress stabilized, tetragonal grains that become unstable as the corrosion front continues to grow further inwards and stresses in the existing oxide relax.     

Microstructure and mechanical property of Sn–Ag–Cu solder material

Among the lead-free solder materials,Sn-AgCu alloys have many advantages,such as good wetting property,superior interfacial properties and high creep resistance.In this article,the organization and welding performance of Sn-Ag-Cu material were investigated.The surface morphology of the two alloys was observed by stereoscopic microscope and scanning electron microscope(SEM).Chemical constitution was examined by X-ray energy-dispersive spectroscopy(EDS).The mechanical properties of Sn-Ag-Cu solder were evaluated systematically compared with those of Sn-Cu solder.The results show that Sn-Ag-Cu solder based on different solder pads has different welding properties.The thickness of intermetallic compound(IMC) at the interface increases with aging time.For the gold-plated pads,there are a large number of IMC graphic,and in the welding interface,it can reduce the reliability of electrical connection.The Sn-AgCu solder joints show a superior mechanical property over the traditional Sn-Cu solder.The number of dimples decreases and that of cavities increases for Sn-Cu0.7 alloy and the fracture surfaces of Sn-Ag3.0-Cu0.5 alloy have many small size dimples which are homogeneously distributed.     

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.     

Magnetic and crystal structure of Th–Fe–Sn intermetallics: ThFe0.22Sn2 and Th4Fe13Sn5

The crystal and magnetic structures of two new ternary phases in the Th–Fe–Sn system: ThFe_0.22Sn₂ and Th₄Fe₁₃Sn₅ have been investigated by high-resolution neutron powder diffraction. The ThFe_0.22Sn₂ phase crystallizes in an orthorhombic crystal structure, space group Cmcm, whilst Th₄Fe₁₃Sn₅ crystallizes in the tetragonal space group P4/mbm. In agreement with bulk magnetization measurements, the compound ThFe_0.22Sn₂ does not order magnetically in the temperature range investigated, while Th₄Fe₁₃Sn₅ orders below 375 K, with the easy magnetization direction of the Fe moments aligned along the tetragonal c-axis.     

Reexamination of the solidification behavior of undercooled Ni–Sn eutectic melts

Employing a high-speed video, in situ recalescence behaviors of Ni–Sn eutectic melts under different undercooling conditions have been investigated. Based on the surface morphologies and cross-sectional microstructures that consist of independent eutectic colonies, copious nucleation is proposed to take place in Ni–Sn eutectic melt regardless of melt undercooling in the unconstrained solidification. The boundary with sharp contrast between the crystallized bright solid and undercooled dark liquid during recalescence is not the solid/liquid interface but the simultaneous thermal release of concentrated crystallizing eutectic colonies. It may not be feasible to measure growth velocities in free solidification of the system due to copious nucleation. The fluctuations in undercooled melts are proposed to activate a chain-like successive nucleation reaction once an effective nucleus is formed. The current observation urges that we should deliberate further the conventional concept that once an effective nucleus is formed in a deeply undercooled melt, the growth will be initiated promptly and the solid/liquid interface will sweep across the entire sample rapidly. The origin of anomalous eutectic formation and the growth have been discussed when the current nucleation phenomenon is taken into account. Based on the copious nucleation theory and the improved driving force for growth, the present recalescence frames and some other experimental phenomena concerning the free growth of the undercooled eutectic system can be well clarified.     

Improving the shear strength of Sn–Ag–Cu–Ni/Cu–Zn solder joints via modifying the microstructure and phase stability of Cu–Sn intermetallic compounds

This study focuses on the correlation between high-speed impact tests and the interfacial reaction in Sn-3.0Ag-0.5Cu-0.1Ni/Cu (wt%) and Sn-3.0Ag-0.5Cu-0.1Ni/Cu-15Zn solder joints. Adding Ni into the Sn–Ag–Cu solder alters the interfacial morphology from scallop type to layer type and exhibits high shear strength after reflow in both solder joints. However, the shear strength of Sn-3.0Ag-0.5Cu-0.1Ni/Cu solder joints degrades significantly after thermal aging at 150 °C for 500 h. It is notable that Sn-3.0Ag-0.5Cu-0.1Ni/Cu-15Zn solder joints still present higher shear strength after aging at 150 °C. The weakened shear strength in Sn-3.0Ag-0.5Cu-0.1Ni/Cu solder joints is due to stress accumulation in the interfacial (Cu,Ni)₆Sn₅ compound induced by the phase transformation from a high-temperature hexagonal structure (η-Cu₆Sn₅) to a low-temperature monoclinic structure (η'-Cu₆Sn₅). However, doping small amounts of Zn into (Cu,Ni)₆(Sn,Zn)₅ can inhibit the phase transformation during thermal aging and maintain strong shear strength. These experiments demonstrate that Sn-3.0Ag-0.5Cu-0.1Ni/Cu-15Zn solder joints can act as a stable connection in the micro-electronic packaging of most electronic products at their average working temperatures.     

Phase equilibria of the Sn–Ag–Ni ternary system and interfacial reactions at the Sn–Ag/Ni joints

There are no previous phase equilibria studies of the Sn–Ag–Ni ternary system, even though the phase equilibria information is important for the electronic industry. The isothermal section of the Sn–Ag–Ni ternary system at 240 °C has been determined in this study both by experimental examination and thermodynamic calculation. Experimental results show no existence of ternary compounds in the Sn–Ag–Ni system, and all the constituent binary compounds have very limited solubilities of the ternary elements. The binary Ni₃Sn₂ phase is very stable and is in equilibrium with most of the phases, Ag₃Sn, ζ-Ag₄Sn, Ag, Ni₃Sn₄ and Ni₃Sn phases. A preliminary thermodynamic model of the ternary system is developed based on the models of the three binary constituent systems without introducing any ternary interaction parameters. This ternary thermodynamic model is used with a commercial software Pandat to calculate the Sn–Ag–Ni 240 °C isothermal section. The phase relationships determined by calculation are consistent with those determined experimentally. Besides phase equilibria determination, the interfacial reactions between the Sn–Ag alloys with Ni substrate are investigated at 240, 300 and 400 °C, respectively. It is found that the phase formations in the Sn–3.5wt%Ag/Ni couples are very similar to those in the Sn/Ni couples.     

Strengthening of an Al–Cu–Sn alloy by deformation-resistant precipitate plates

A variation of the Orowan equation is developed for aluminium alloys strengthened by dispersed, deformation-resistant {1 0 0}_α precipitate plates, and the quantitative effects of precipitate volume fraction, number density and aspect ratio on increments in Orowan strengthening are examined. The validity of the model is examined by a combination of mechanical property measurements and rigorous quantitative stereology of the size and distribution of {1 0 0}_α plates of θ′ phase in an Al–4Cu–0.05Sn (wt.%) alloy aged isothermally at 200 °C. For the ageing conditions selected, the predictions of the model for Orowan strengthening increments are in good quantitative agreements with those observed experimentally. The model predicts that, for a given volume fraction and number density of precipitate plates, an increase in plate aspect ratio can lead to a substantial increase in strength. In addition, it is not necessary to invoke a transition from precipitate deformation to Orowan strengthening to account for the form of the precipitation-strengthening response of the alloy. The maximum strengthening increment is associated with the minimum effective inter-particle spacing, rather than a critical thickness of the precipitate plates.     

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.     

Kinetics of Ag3Sn growth in Ag–Sn–Ag system during transient liquid phase soldering process

The kinetics of the interfacial reaction of a thin layer of Sn sandwiched between two pieces of Ag foil has been investigated at temperatures of 260 °C, 300 °C and 340 °C. A time dependence of the form t^1/n with n = 3 was obtained for the kinetics of both the consumption of the Sn remaining and the thickening growth of the Ag₃Sn scallops formed between Sn and Ag. Such a result can be explained well using the model of grain boundary/molten channel-controlled growth of intermetallic compounds. In this case, the diffusion of Ag atoms through the molten channels existing between the previously formed Ag₃Sn scallops is the controlling mechanism for the kinetics. We also report here the derived kinetic constants including reaction constants and the associated activation energy for guiding the practical transient liquid phase soldering of the Ag–Sn–Ag system.     

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.     

Modification of Sn–1.0Ag–0.5Cu solder using nickel and boron

Effects of Ni and B additions on the microstructure and growth behavior of the intermetallic compound(IMC) of Sn–1.0Ag–0.5Cu alloys(SAC105) were investigated in this study. Results show that microadditions of Ni and B result in volume fraction of primary Sn increasing and the grain size decreasing observably. It is found that a large number of fine reinforcement particles with network-like shape are found in the solder, and the thickness of interfacial IMC layer in the solder joint is grew less than that of SAC105 with longer aging time. Shear test results reveal that as-soldered solder joints of microalloyed SAC105 have better shear strength than that of SAC105 solder alloy.     

Microstructural,mechanical and shape memory characterizations of Ti–Mo–Sn alloys

As a β stabilizing element in Ti-based alloys, the effect of Mo on phase constitution, microstructure, mechanical and shape memory properties was investigated. Different compositions of Ti–xMo–3Sn alloys (where x=2, 4, 6, at.%) were prepared by arc melting. A binary composition of Ti–6Mo alloy was also prepared for comparison. Ti–xMo–3Sn alloys show low hardness and high ductility with 90% reduction in thickness while Ti–6Mo alloy shows high hardness, brittle behavior, and poor ductility. Field emission scanning electron microscopy (FESEM) reveals round morphology of athermal ω (ω_ath) precipitates. The presence of ω_ath phase is also confirmed by X-ray diffraction (XRD) in both as-cast and solution-treated and quenched conditions. The optical microscopy (OM) and FESEM show that the amount of martensite forming during quenching decreases with an increase in Mo content, which is also due to β→ω transformation. The hardness trends reinforce the presence of ω_ath too. The shape memory effect (SME) of 9% is the highest for Ti–6Mo–3Sn alloy. The SME is trivial due to ω_ath phase formation; however, the increase in SME is observed with an increase in Mo content, which is due to the reverse transformation from ω_ath and the stress-induced martensitic transformation. In addition, a new and very simple method was designed and used for shape memory effect measurement.     

Effect of predeformation on globularization of Ti–5Al–2Sn–2Zr–4Mo–4Cr during annealing

The microstructure evolution during annealing of Ti–5Al–2Sn–2Zr–4Mo–4Cr alloy was investigated. The results show that for the alloy compressed at 810 °C and 1.0 s^?1 deformation amount (height reduction) 20% and 50% and annealed at 810 °C, thermal grooving by penetration of β phase is sufficient during the first 20 min annealing, resulting in a sharp increase in globularization fraction. The globularization fraction continuously increases with the increase of annealing time, and a height reduction of 50% leads to a near globular microstructure after annealing for 4 h. For the alloy with deformation amount of 50% by compressing at 810 °C, 0.01 s^?1 and then annealed at 810 °C, thermal grooving is limited during the first 20 min of annealing and large quantities of high-angle grain boundaries (HABs) remain. With long time annealing, the chain-like α grains are developed due to the HABs, termination migration and Ostwald ripening. The present results suggest that a higher strain rate and a larger height reduction are necessary before annealing to achieve a globular microstructure of Ti–5Al–2Sn–2Zr–4Mo–4Cr.