STRUCTURAL EVOLUTION IN MECHANICALLY ALLOYED AL-FE POWDERS

The structural evolution in mechanically alloyed binary aluminum-iron powder mixtures containing 1, 4, 7.3, 10.7, and 25 at. pct Fe was inve stigated using X-ray diffraction (XRD) and electron microscopic techni ques. The constitution (number and identity of phases present), micros tructure (crystal size, particle size), and transformation behavior of the powders on annealing were studied. The solid solubility of Fe in Al has been extended up to at least 4.5 at. pct, which is close to tha t observed using rapid solidification (RS) (4.4 at. pct), compared wit h the equilibrium value of 0.025 at. pct Fe at room temperature. Nanom eter-sized grains were observed in as-milled crystalline powders in al l compositions. Increasing the ball-to-powder weight ratio (BPR) resul ted in a faster rate of decrease of crystal size. A fully amorphous ph ase was obtained in the Al-25 at. pct Fe composition, and a mixed amor phous phase plus solid solution of Fe in Al was developed in the Al-10 .7 at. pct Fe alloy, agreeing well with the predictions made using the semiempirical Miedema model. Heat treatment of the mechanically alloy ed powders containing the supersaturated solid solution or the amorpho us phase resulted in the formation of the Al3Fe intermetallic in all b ut the Al-25 at. pct Fe powders. In the Al-25 at. pet Fe powder, forma tion of nanocrystalline Al5Fe2 was observed directly by milling. Elect ron microscope studies of the shock-consolidated mechanically alloyed Al-10.7 and 25 at. pct Fe powders indicated that nanometer-sized grain s were retained after compaction.