On the crystal structure of AlaCuMg, S′, in Al-Cu-Mg alloys

The major precipitate phase in Al-Cu-Mg alloys is S', Al2CuMg, which is believed to be a distorted form of the equilibrium S phase (orthorhombic structure, space group Cmcm, 16 atoms in the unit cell). Three additional space groups have been proposed for S', namely, Bm2m, Pmm2, and Cmcm (with different atom positions to S). HREM, nanodiffraction and image simulation has been used to determine the crystal structure of S' in Al-4.11Cu-1.55Mg alloys, heat treated at 200 degrees C for times of 10-10,000h. It is proposed that S' has the same crystal structure as the equilibrium S phase.