Mechanisms of formation of Al4Cu9 during mechanical alloying: An experimental study

In spite of its complex structure, the Cu-rich γ1-Al4Cu9 compound is frequently observed in Al-Cu processing. In order to better understand the intriguing formation of this compound, a thorough experimental study of Al-25 at.% Cu mixtures produced by mechanical alloying (MA) was performed. Strong morphological changes are found to occur for moderate milling times. γ1-Al4Cu9 is formed during the early milling steps (5.5 h), its volume fraction increasing continuously with milling time, while the equilibrium θ-Al2Cu expected for this composition is detected only for longer times (90 h). Whereas the early formation of γ1 suggests that its nucleation should occur at Al/Cu interfaces, direct visualization reveals the presence of γ1 crystallites within Cu-rich zones. The thermal instability of MA-formed γ1-Al4Cu9 is evidenced, this phase evolving towards θ-Al2Cu at temperatures lower than those predicted for equilibrium conditions. These observations indicate that γ1-Al4Cu9 may be able to form through non-thermally activated mechanisms.