Rapid eutectic growth: from rod growth to diffusionless solidification

Numerous experimental data on the rapid solidification of eutectic systems exhibit the formation of metastable solid phases with the initial (nominal) chemical composition. This fact is explained by the suppression of eutectic decomposition due to diffusionless (chemically partitionless) solidification beginning at a high but finite growth velocity of crystals. In the present work, a model is suggested for the diffusionless growth to analyse the atomic diffusion in the rod eutectic couples growing into supercooled liquid. A simplified calculating method for the equation related to the Bessel function in the solution of the growth of rod eutectics is obtained. This method can also be used in the calculation of other rod eutectic growth models. This article is part of the theme issue ‘Transport phenomena in complex systems (part 2)’.

Local solidification thermal parameters affecting the eutectic extent in Sn-Cu and Sn-Bi solder alloys

Purpose Overall, selection maps about the extent of the eutectic growth projects the solidification velocities leading to given microstructures. This is because of limitations of most of the set of results when obtained for single thermal gradients within the experimental spectrum. In these cases, associations only with the solidification velocity could give the false impression that reaching a given velocity would be enough to reproduce a result. However, that velocity must necessarily be accompanied by a specific thermal gradient during transient solidification. Therefore, the purpose of this paper is to not only project velocity but also include the gradients acting for each velocity. Design/methodology/approach Compilation of solidification velocity, v, thermal gradient, G, and cooling rate, Ṫ, data for Sn-Cu and Sn-Bi solder alloys of interest is presented. These data are placed in the form of coupled growth zones according to the correlated microstructures in the literature. In addition, results generated in this work for Sn-(0.5, 0.7, 2.0, 2.8)% Cu and Sn-(34, 52, 58)% Bi alloys solidified under non-stationary conditions are added. Findings When analyzing the cooling rate (Ṫ = G.v) and velocity separately, in or around the eutectic composition, a consensus cannot be reached on the resulting microstructure. The (v vs. G) + cooling rate diagrams allow comprehensive analyzes of the combined v and G effects on the subsequent microstructure of the Sn-Cu and Sn-Bi alloys. Originality/value The present paper is devoted to the establishment of (v vs. G) + cooling rate diagrams. These plots may allow comprehensive analyses of the combined v and G effects on the subsequent microstructure of the Sn-Cu and Sn-Bi alloys. This microstructure-processing mapping approach is promising to predict phase competition and resulting microstructures in soldering of Sn-Cu and Sn-Bi alloys. These two classes of alloys are of interest to the soldering industry, whereas manipulation of their microstructures is considered of utmost importance for the metallurgical quality of the product.