The age-hardening of copper-aluminium alloys

The crystallographic requirement for the formation of G-P zones is that the material within the zones shall have an epitaxial relationship with the matrix, and thus the eventual precipitate should have a similar unit cell size in one direction as tha matrix. In the Al-Cu system, the f.c.c. structure of aluminium has a lattice parameter of 0.4014 nm, and the tetragonal CuA12 compound has lattice parameters a = 0.4872 and b = 0.6063 nm respectively. 

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