In situ observation of microstructure evolution in low-melting Bi–In–Sn alloys by light microscopy

An experimental method is presented for the in situ observation of microstructure evolution during solidification or solid phase transformations in metallic alloys with low melting temperature. It is based on the direct observation of a thin metallic sample embedded between polished glass plates using a light microscope. The sample is placed in a Bridgman type furnace that enables unidirectional solidification under well defined conditions. As examples we show microstructures obtained during coupled, lamellar eutectic growth in two alloys with compositions close to the non-variant eutectic TE = 332 K from the Bi–In–Sn system for different solidification conditions. Stable lamellar patterns with an ABCB stacking sequence are found to be dominant for 2D-samples (∼10 μm in thickness), where A ≡ γ-Sn, B ≡ BiIn2 and C ≡ β-In phase. The spacing of this stacking sequence was determined as a function of growth rate. The results are compared with the data for bulk (3D) samples of the same composition, taken from the literature. The direct observation of the microstructures at the solid/liquid interface allows the verification of the validity of two different descriptions of the Bi–In–Sn phase diagram proposed earlier, namely a description with eutectic and a second with peritectic reactions involving BiIn2, γ-Sn, β-In and liquid. All observed solidification patterns are consistent with the eutectic description.