Copper Aluminide, CuAl2
Copper Aluminide, CuAl2, is an intermetallic compound between the two elements. It is the thermodynamically stable precipitate formed when an alloy such as Al- 4wt% Cu is slowly cooled from the single solid solution a-phase to room temperature. The compound can exist over a small range of compositions about stoichiometric and exhibits congruent melting at about 600 C. 

In overaged precipitation hardened Cu-Al alloys, CuAl2 is the precipitate that provides the material hardening. The precipitate has incoherent boundaries with the (Al) matrix and consequently the interaction with dislocations is reduced compared to non-equilibrium precipitates with coherent boundaries with the matrix.

From: Callister, 
"Materials Science and Engineering," 
Wiley (1994)