Microstructure, phases morphologies and hardness of a Bi–Ag eutectic alloy for high temperature soldering applications

• Growth of eutectic cells prevailed for lower “v” values.
• The experimental growth formula λ2v = C encompasses eutectic growth.
• Interphase spacing between Ag-rich spheroids occurred in the range 0.4–1.7 μm.
• Hall–Petch type equations are proposed relating to hardness.

The conversion to RoHS-compliant lead-free assembly has been a considerable challenge to the electronics industry. Among several alternative solder alloys, Bi–Ag alloys have been highlighted as a potential candidate to replace high Pb solder alloys for applications in oil and gas, automotive and avionics industries. The typical melting temperatures of Bi–Ag near-eutectic alloys are considered acceptable and excellent mechanical properties may be achieved with appropriate microstructures. Such promising alloys for high temperature soldering remain barely understood especially regarding non-equilibrium solidification features. In this study, a directional solidification experiment was carried out with the Bi–2.5 wt%Ag eutectic so that a large range of cooling rates (Ṫ) could be obtained under unsteady-state conditions. The experimental investigation include: thermal solidification parameters (growth rate, v   and cooling rate, Ṫ), microstructure parameters (eutectic/dendritic spacing, interphase spacings) and phases morphologies analyzed by optical microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and hardness. Experimental interrelations between hardness and microstructure (scale and morphology) of the eutectic Bi–Ag are reported. Solidification parameters are also associated with each configuration observed along the casting, i.e., coexistence of dendrites and eutectic cells for regions very close to the cooled casting surface, eutectic cells prevailing and eutectic cells together with β-Bi primary phase. The cell spacing, λc, is correlated with hardness by Hall–Petch type equations.