Infiltration of fiber preforms by a binary alloy: Part II. Further theory and experiments

In a previous article,[1] a theoretical analysis was developed to describe the infiltration of fiber preforms by a binary alloy, and its solution was given for unidirectional adiabatic infiltration under constant applied pressure. This article further develops the analysis by proposing a model to predict the permeability of fibrous preforms containing solidified primary metal, by deducing the final composite microstructure from processing parameters, and by addressing the influence of external cooling on macrosegregation within the composite. Experimental procedures established for the infiltration of fiber preforms by pure aluminum are modified to produce samples infiltrated under nearly adiabatic conditions. Samples of SAFFIL alumina fiber preforms infiltrated adiabatically under constant applied pressure with Al-4.5 wt pct Cu show longitudinal variations in copper concentration, which are well predicted by theory presented in Part 1.[1] The microstructures in the infiltrated composite samples also agree with analysis: the grain size is small where solid and liquid matrix coexisted during infiltration, whereas it is large where remelting occurred, indicating that SAFFIL fibers do not promote nucleation of Al-Cu. The model proposed here for permeability of the preform in the presence of solidified metal yields infiltration rates in agreement with experimental data. Finally, samples produced under non-adiabatic conditions exhibit transverse macrosegregation; this is explained for simple limiting cases of heat transfer at the die wall.