Cu-Ni-Sn: A Key System for Lead-Free Soldering

Being the most complex constituent of the quaternary system Ag-Cu-Ni-Sn, the ternary system Cu-Ni-Sn is the key system for the investigation of the interactions of Ag-Cu-Sn solder alloys with Ni as a contact material. Although this system has been thoroughly studied in the literature, there are still many uncertainties left. In the present work, a study of the phase equilibria in four isothermal sections at 220, 400, 500, and 700°C of the Cu-Ni-Sn system was carried out following a comprehensive literature study. The methods employed were x-ray diffraction (XRD), metallography, and scanning electron microscopy including electron probe microanalysis. The ternary solubilities of the Ni₃Sn₂-Cu₆Sn₅ and Ni₃Sn-Cu₃Sn fields were characterized in detail. So far no continuous solubility between the respective phases has been found. At 25 at.% Sn the existence of two ternary compounds formed from the BiF₃-type (Cu,Ni)₃Sn phase and reported in literature could be confirmed. On the other hand, our results differ significantly from the very recent literature related to lead-free soldering.     

Microstructology of solid-state reactions

The use of a combined thermodynamic and diffusion kinetic approach in predicting the product morphology developed during solid-state displacement reactions is exemplified by the interactions in the GaSb/Co and SiC/Me (where Me=Cr,Pt,Co) systems. The influence of mechanical stresses induced during the interaction on the formation of microstructures is demonstrated. It was shown that the manifestation of the effects accompanying reactive-phase formation in inorganic solids (like the Kirkendall effect and the net volume change during internal precipitation) causes the generation and relaxation of the stresses. This significantly contributes to the morphological evolution of the reaction zone. Different micro- and macrodefects (vacancies, dislocations, pores, cracks, etc.) that can be generated inside the diffusion zone can also affect (or even control) the course of the reaction and determine the topological arrangement of the product phases within the resulting microstructure.     

Phase relations and interfacial reactions in the Ga-P-Co system at 500 °C

The isothermal cross section through the ternary phase diagram Ga-P-Co at 500 °C was constructed. No ternary phases exist in the system at this temperature. All binary Co-P and Co-Ga intermetallics are in equilibrium with GaP, which, under these conditions, dissolves up to 4 at.% of cobalt. The use of equilibrium phase diagram information in predicting the variation of chemical composition in the diffusion zone during solid-state reaction between GaP and Co is demonstrated.     

Phase relations in the Bi-Ni-Pd system at 235 °C

The Bi-Pd and Ni-Bi binary phase equilibria at 235 °C were investigated. It was concluded that Bi₂Pd, BiPd, Bi₃Pd₅, and BiPd₃ are stable intermetallics in the Bi-Pd system and NiBi and NiBi₃ are stable in the Ni-Bi system at this temperature. An isothermal cross section of the ternary Ni-Bi-Pd system at the same temperature was established experimentally. No ternary compound has been found. However, the Pd-rich corner of the phase diagram cannot be determined due to the sluggishness of the kinetics involved.     

Wet adhesion of epoxy–amine coatings on 2024-T3 aluminum alloy

To explore the influence of the molar epoxide-to-amine ratio E/NH on the coating adhesion strength, samples with E/NH ratios ranging from 0.6 to 1.4 were prepared and fully cured. Coatings with an E/NH value of 1.0 appeared to have the highest T_g and cross-link density, and the lowest water absorption ability, which was proved to be independent from solubility or free volume effects. Being verified by the classic “cross-hatch” and the more quantitative pull-off tests, the wet adhesion of coatings was found to be significantly influenced by the epoxide content. Excess epoxide groups highly likely to favor the formation of stronger adhesive bonding at coating–metal interface. No clear correlation could be established between the coating adhesion strength and their cross-link density and water-uptake. In addition, the measurements of the thermal expansion coefficients of substrate and coatings revealed the presence of a tensile internal stress, which is more pronounced for stoichiometric samples. The existence of compressive internal stress induced by water uptake was also confirmed by the adhesion tests on samples which were immersed in water for two days. In these samples, a partial recovery of adhesion strength was found, after the samples were dried for two days, indicating that the compressive internal stress deteriorated the load-bearing properties of adhesive bonds and contributed to debonding. This influence becomes larger as the coating thickness increases as well as the E/NH off-stoichiometry increases. Interestingly, for the samples possessing excess epoxide functional groups, a higher degree of recovery was obtained. This was not the case for coatings with excess amine, confirming that specifically the epoxide groups facilitate the formation of strong bonds against water at the polymer–metal interface.     

Physico-chemical analysis of compound growth in a diffusion couple with two-phase end-members

A general treatment of a diffusion-controlled growth of a stoichiometric intermetallic in reaction between two two-phase alloys is introduced. A reaction couple, in which a layer of Co₂Si is formed during interdiffusion from its adjacent saturated phases is used as a model system. On the basis of chemical reaction equations occurring at the interphase interfaces, data on relative mobilities of diffusing species and the integrated diffusion coefficient in the product phase are deduced. The analysis yields numerical results identical to those calculated from classical Wagner's theory for the case in which the terminal phases of the diffusion couple are initially saturated.     

The diffusion couple technique in phase diagram determination

The diffusion couple technique is a valuable experimental approach in studying phase relations in multicomponent systems. The use of different modifications of the method is illustrated by examples of phase diagram determination in various ternary systems. It is also shown that a number of error sources may appear when multiphase diffusion experiments are employed for constructing isothermal cross-sections. The difficulties connected with the concentration measurement at the interfaces and problems associated with the formation of a quasi-equilibrated diffusion zone are discussed. It is demonstrated that the efficiency of the diffusion couple technique is very high. However, in order to increase the reliability of the information obtained about a ternary diagram, a combination of the diffusion methods with an investigation of selected alloys is desirable.     

Thermodynamic and kinetic study of diffusion paths in the system Cu-Fe-Ni

An understanding and modeling of diffusion paths in ternary systems requires a combined thermodynamic and diffusion kinetic approach. The driving force for the intrinsic diffusion of each component is the gradient of the chemical potential (or activity) which can be calculated from the concentration profile if the thermodynamic properties of the system are known. For studying the diffusion behavior in ternary metallic systems, the Cu-Fe-Ni-system was chosen because of its experimental and thermodynamic simplicity. Concentration profiles and diffusion paths in single-phase areas and across an α/β interface were studied experimentally at 1000 °C using the diffusion couple technique. Coefficients for interdiffusion and tracer diffusion have been calculated at the intersection points of two independent diffusion paths with a common composition. A concentration dependence for the tracer diffusion coefficients for each component was calculated and found to be consistent with the literature data in the binary Cu-Ni and Fe-Ni systems. The calculated vacancy flux in the couples was consistent with the experimentally observed marker shift.