DEVELOPMENT OF SOLIDIFICATION MICROSTRUCTURES IN A FIBER REINFORCED ALLOY

The solidification behaviour of a fibre reinforced Al-6 wt % Cu alloy, containing 30 vol % of 3-mu-m diameter, semi-continuous, aligned alumina fibres has been studied. Results are presented to show the influence of fibres on the microstructural development of and microsegregation in the matrix during freezing. The effect of total solidification time, theta-t, on solidification behaviour was examined for 1 < theta-t < 520 s. By using interrupted solidification experiments microstructural development was studied in detail. It was found that alpha-Al begins to grow within interfibre regions, and grows towards the Al2O3 fibres, avoiding them where possible. Consequently fibres are located in the last regions to solidify. When theta-t > 10 s the final microstructure is non-dendritic, and CuAl2 is located predominantly at the fibre-matrix interface. When theta-t congruent-to 1 s it was observed that the final microstructure is dendritic with a periodic segregation pattern, and the CuAl2 is more dispersed. The matrix composition becomes more uniform, and the minimum matrix composition rises as theta-t increases. The growth and microsegregation are analysed and discussed using simple semi-analytical models. The implications are that fibres significantly influence solidification behaviour if lambda-f/lambda-s < 1, where lambda-f is the average interfibre spacing and lambda-s the secondary dendrite arm spacing which would develop in the unreinforced alloy.