On the growth morphology of unidirectionally solidified Al-CuAl₂, Al-NiAl₃ and Al-CO₂Al₉ eutectic alloys

Abstract

Al-CuAl₂, Al-NiAl₃ and Al-Co₂Al₉ eutectic alloys were solidified unidirectionally by the thermal-valve technique, and changes of growth morphology with changes of freezing conditions were studied.
The results obtained are as follows:
(1) Each eutectic alloy showed a transition from planar to colony structures with the occurrence of constitutional supercooling which depended on the freezing conditions and the impurity contents.
(2) Growth morphologies of intermetallic compounds in Al-CuAl₂, Al-NiAl₃ and Al-Co₂Al₉ eutectics were also changed from degenerate, blade and ribbon type to lamellar, rod and fibre forms respectively with an increase of freezing rate.
(3) Linear relationships were obtained between the square root of freezing rate, R^1/2 and the reciprocal of interphase spacing, λ^-1 or the interface undercooling, ΔT. Applying these relations to Jackson and Hunt's equation, interphase boundary energy, σ_αβ and diffusion coefficient, D were estimated.
(4) Measured values of ultimate tensile strength of these unidirectionally solidified eutectic alloys were not sufficiently high as a composite material.