MICROSTRUCTURE DEVELOPMENT AND ULTRAMICROHARDNESS OF ALFECE SOLIDIFIED UNDER FREE-FALL CONDITIONS

Small droplets of Al96Fe3Ce1 were solidified without a crucible under free fall. The microstructure reveals three features: a segregation-fr ee zone (zone A), followed by a cellular structure which subsequently refines (zone B). The cellular structure consists of an aluminium soli d solution matrix containing small (d < 100 nm) spherical AlFeCe preci pitates surrounded by a lamellar eutectic wall of Al-Al3Fe. The ultra microhardness of the segregation-free zone is approximately 2 GPa. Thi s is 15% higher than the corresponding value of the coarse cellular zo ne, which in turn is about 15% higher than the fine structure. While t he hardness of zone A is equal to the values found for this zone in bi nary Al-8at.%Fe alloys, the addition of cerium increases the hardness of zone B by a factor of up to 1.5-1.7 as a result of dispersion harde ning.