The Growth Pattern of Co₃Sn₂ in Directional Solidification of Co-Sn Hypereutectic Alloy Melts

Abstract

Co₇₀Sn₃₀ hypereutectic alloy was directionally solidified at various velocities at temperature gradients of 200 and 300 K/cm. At a given temperature gradient, α-Co/β-Co₃Sn₂ lamellar eutectic grows with a planar interface when the withdrawal velocity is small. The increases of withdrawal velocity result in primary β-Co₃Sn₂ phase growing ahead of the eutectic, and its morphology changes from dendritic pattern to fractal seaweed pattern above a critical withdrawal velocity. The rise of the temperature gradient narrows the withdrawal velocity range within which primary β-Co₃Sn₂ phase can grow in the dendritic pattern. The tip-splitting frequency of the β-Co₃Sn₂ seaweed vs growth velocity shows a power law $f \propto V^{{{3 \mathord{\left/ {\vphantom {3 2}} \right. \kern-0pt} 2}}}$ The change of the primary arm spacing of β-Co₃Sn₂ with the growth velocity follows $d_{1} = k_{1} V^{{ - e_{1} }} ,$ where the exponent e₁ is larger for seaweed growth than for dendritic growth.