Coarsening of eutectic fiber composites during cooling from the eutectic temperature

Abstract

Coarsening and microstructural changes, which occur on cooling from the eutectic temperature, were experimentally studied in Al-Al₃Ni eutectic fiber composites solidified at rates of 0.8, 5.0 and 32.5 cm per h. At the two higher growth rates, changes in fiber density on the order of 10 pct were observed between the solid-liquid interface and positions 2.0 cm from the interface. At the low growth rate the surface roughness of the as-grown fiber was noticeably decreased during cooling from the eutectic temperature. Theoretical expressions relating the change in average fiber radius or fiber density as a function of position were developed in terms of the solidification conditions, growth rate and temperature gradient, as well as the usual kinetic and thermodynamic factors influencing coarsening. The model was combined with independently obtained isothermal coarsening data to compare the experimentally observed extent of coarsening with that predicted. The pre-dictions of the model agreed qualitatively with the data on the effect of growth rate on coarsening, but the absolute extent of coarsening was greater than that predicted from the isothermal studies.