Overstability of lamellar eutectic growth below the minimum-undercooling spacing

We investigate the stability of lamellar eutectic growth by thin-sample directional solidification experiments and two-dimensional phase-field simulations. We find that lamellar patterns can be morphologically stable for spacings smaller than the minimum undercooling spacing $\lambda_m$. Key to this finding is the direct experimental measurement of the relationship between the front undercooling and spacing, which identifies $\lambda_m$ independently of the Jackson-Hunt theory and of uncertainties of alloy parameters. This finding conflicts with the common belief that patterns with $\lambda<\lambda_m$ should be unstable, which is based on the Jackson-Hunt-Cahn assumption that lamellae grow normal to the envelope of the front. Our simulation results reveals that lamellae also move parallel to this envelope to reduce spacing gradients, thereby weakly violating this assumption but strongly overstabilizing patterns for a range of spacing below $\lambda_m$ that increases with $G/V$ (temperature gradient to growth rate ratio). This range is much larger than predicted by previous stability analyses and can be significant for standard experimental conditions. An analytical expression is obtained phenomenologically that predicts well the variation of the smallest stable spacing with $G/V$. We present also results that shed light on the history-dependent selection and long-time evolution of the experimentally observed range of spacings.