The interphase spacings lambda in the microstructures of most binary and pseudobinary ''normal'' metallic eutectics can roughly be quantified with regard to their respective solidification rates v according to a general relationshiplambda(2).v = constantthe constant being commonly between 10(-10) and 10(-11) cm(3) s(-1). The designation ''normal'' relates to similar degrees of undercooling of either phase on a plain-front solidification process at variance with anomaleous eutectics which solidify at different degrees of undercooling in an uncoupled or weakly-coupled manner. The ''constant'', however, is an individual value for each eutectic system. It is lowest for ''simple'' eutectics forming no intermetallic compounds and having low terminal solid solutions. The constant is raised for systems by 3 orders of magnitude or more if extensive terminal solid solutions are present. A closer fit for deliberate normal eutectics is presented in this pragmatic approach if the concentration differences between the terminal phases in eutectics Delta C-E [at%] are attributed to them as a square term according to lambda(2).v.Delta C-E(2) = constant. The data based on this relation still show some scatter but they are grouping to distinct material families e.g. to Pb-, Ag- or Al-based eutectics. The remanent differences are estimated to disappear as soon as the relation is extended by the interdiffusion coefficient D according tolambda(2).v.Delta C-E(2).D congruent to 1.9.10(-12) {cm(5).s(-2).at%)as is confirmed in cases where reliable D values are available. These findings are in contrast to the current published theories on eutectic solidification. Suggestions are given where the boundary conditions have to be altered in order to attain full accord between experiment and theory.
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