The structure of the unidirectionally solidified Al-Al₂Au eutectic

Abstract

The structure of a number of unidirectionally solidified Al-Al₂Au alloys of eutectic and off-eutectic compositions has been investigated over a wide range of growth rates (1.6×10⁻⁴ to 1.66×10⁻²cm sec⁻¹) using a thermal gradient of approximately 80 to 100‡ C cm⁻¹. The system exhibits an asymmetric coupled growth zone, which widens at high thermal gradient-growth rate ratios. At high solidification rates a broken lamellar structure is obtained, while at low rates a mixed structure with broken lamellae and rods is present. The crystallographic orientation of the phases as determined on different eutectic grains is as follows:

$$\begin{gathered} lamellar interface \left\| {(001)_{Al_{ 2} Au} } \right.\left\| {(01 1)} \right._{Al} \hfill \\ \left[ {1 1 0} \right]_{Al_{ 2} Au} \left\| {\left[ {1 0 0} \right]} \right._{Al} \hfill \\ growth direction of \hfill \\ lamellae and rods \left\| {\left[ {1 1 0} \right]_{Al_2 Au} \left\| {\left[ {1 0 0} \right]_{Al} } \right.} \right. \hfill \\ \end{gathered} $$

while the preferred growth direction for the Al dendrites as well as for the Al₂Au dendrites has been found to be the [100] direction. The interlamellar spacing varies according to the well known relationship

$$\lambda = A \cdot R^{ - n} $$

where R is the growth rate, with n-0.37 for the free-dendrites eutectic structures and n-0.49 for the inter-dendrite eutectic structures.