Brass, Cartridge Brass, Muntz Metal
Brass is a series of binary alloys between copper and zinc, with an upper zinc content of about 50 wt %. The phase diagram is shown, and two brasses are identified. Cartridge brass has the composition Cu-30 wt % Zn (green line) and Muntz metal Cu-40 wt % Zn (red line). At room temperature cartridge brass is a single phase substitutional solid solution of zinc in the fcc copper structure. Muntz metal is a two phase (a and g) material at room temperature, both of which are solid solutions of zinc in copper but with different compositions. When heated above about 800 C, Muntz metal becomes a single phase b-solid solution.  The presence of two phases at room temperature makes Muntz metal less ductile that cartridge brass, but with a higher tensile strength. Hot working of the material in the b-phase takes advantage of the higher ductility of this single phase.
From: Guy and Hren, "Elements of Physical Metallurgy," Addison Wesley (1974)