Two important binary systems
Copper-Zinc (‘Brass’) [2.1]
Fig. 6.1 shows the phase diagram, which is characterized by a series of five peritectic reactions from the copper to the zinc-rich side. At 73 at.% zinc the δ-phase undergoes eutectoid decomposition into γ and ε on cooling.
The α-phase is close packed cubic, i.e. fee like copper; the β-phase is body centered cubic (bcc). Below 468 to 454 °C an ordered arrangement β′ of Cu and Zn atoms (present in the ratio of approximately 1:1) forms on the bcc lattice, as shown in fig. 6.2. It is interesting that the β field widens with increasing temperature above the β′β transition temperature and that consequently the solubility of Zn and Cu is reduced, contrary to (5-22). The gain in entropy on disordering apparently stabilizes β, i.e. lowers the F β parabola with increasing temperature by more than F α and F γ are lowered. The γ-phase is complex cubic with 52 atoms per unit cell. It can be imagined as consisting of 3 × 3 × 3 unit cells of the bcc β′-brass from which 2 atoms have been removed and the remainder slightly displaced. δ also has a very large and complicated cubic unit cell, ε and η, on the other hand, are hexagonal structures, described as close-packed hexagonal or cph.