Fibrous eutectic growth in succinonitrile–neopentylglycol–(D)camphor–aminomethylpropanediol alloys for thin and bulk sample geometry

The eutectic spacing (λ) was investigated in three alloys of succinonitrile–neopentylglycol–(D)camphor–aminomethylpropanediol (SCN–NPG–DC–AMPD) system over a wide range of temperature gradients (G) and growth velocities (V), both in thin ⩽5 μm (two-dimensional (2-D)) samples and in bulk (three-dimensional (3-D)) samples. The compositions were selected such that fraction of rod-forming phase (f) was equal to 11.5 vol.% for the invariant SCN–12.2 wt.% NPG alloy, 23.3 vol.% for the univariant SCN–24.2 wt.% DC–0.5 wt.% NPG alloy and 29.8 vol.% for the bivariant SCN–33.0 wt.% DC–6.0 wt.% NPG–3.0 wt.% AMPD alloy. For high G/V   conditions significant deviations from growth at minimum undercooling were found with spacing selection closer to the pinch-off limit. The experimental results show that confining the 3-D eutectics to 2-D growth conditions invariably leads to larger values of the mean rod spacing, but to smaller values of the mean rod diameter. The 2-D/3-D ratio of the square of mean rod diameters η=d2DR/d3DR2 was found to be independent of the phase fraction of the rod-forming phase f and equal to η = 0.80 ± 0.02. The 2-D/3-D ratio of square of rod spacing k=λ2DR/λ3DR2 was found to dependent on f   following k=(2f)-1k=(2f)-1 or else k=η38πf-1. These findings are discussed in terms of the Jackson–Hunt model of eutectic growth and simple geometrical considerations.