Page 221 - Materials Chemistry, Second Edition

208 3 Metals

all proportions. Monel (68% Ni, 32% Cu) is used to handle corrosive materials such
as F 2 or HF. US currency coinage such as quarters and nickels are alloys of Cu/Ni,
containing 91.7% Cu and 75% Cu, respectively. By comparison, Canadian quarters
and nickels are predominantly steel, with only ca. 3.5% Cu and 2% Ni. Pennies are
predominantly nickel, with a thin layer of copper deposited by electroplating. High-
strength Cu/Ni alloys are produced from the addition of 1.5–2.5 wt.% Al, which
causes precipitate hardening through formation of Ni 3 Al crystallites.
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The atomic radii of Cu, Sn (1.45 A), and Zn (1.42 A) are also nearly identical,
allowing for a full gamut of Cu/Sn and Cu/Zn alloy concentrations to be produced,
known as bronze and brass, respectively. Although the use of bronze dates back to at
least 3,000 B.C., there are also early examples of brass artifacts that date back to ca.
2,200 B.C. in India. Most likely, the discovery of bronze resulted from the inquisi-
tive mixing of available metals at the time, only to discover that Au/Sn alloys
possessed a greater strength than iron; steels were not developed until thousands
of years later. Since zinc metal was not available until the mid-eighteenth century,
and tin was readily obtained, the widespread production of bronzes occurred at the
expense of brasses. In the absence of pure zinc, early formulations of Cu/Zn alloys
were most likely made through heating a mixture containing zinc oxide, copper
metal, and a reducing agent such as charcoal in a closed crucible. A temperature in
excess of 950 C was necessary to reduce ZnO, and even a trace amount of oxygen

would preclude the formation of the alloy.
The strength of a bronze increases with the tin content; however, its toughness and
malleability decreases. The maximum strength of bronze occurs at ca. 30% Sn, but
at this concentration the alloy is much too brittle for most applications due to the
formation of Cu 3 Sn particles. Recall that this phenomenon also occurred for the
formation of Fe 3 C in iron–carbon alloys – also involving a transition metal and
Group 14 dopant. If more than 15% Sn is used, the alloy is called “bell metal,” due to
its resonating sound when tolled.
Although the radii of Cu and Zn satisfy the Hume-Rothery constraints for solid
solutions, these metals do not share the same crystal lattice. Whereas the coinage
metals are fcc, zinc crystallizes in a hcp array. Hence, as we introduce more Zn into
the Cu lattice, there will be a shift in the overall structure. We may think of this
change as occurring as a result of the change in electron concentrations of the solid.
For instance, each Cu atom contributes one 4s electron to the valence shell of the
extended lattice; by contrast, each Zn atom contributes two 4s electrons. For small
concentrations of Zn, the fcc a-brass structure is formed. However, as the Zn
concentration reaches 50%, the bcc b-brass (CuZn) phase predominates. The elec-

tron concentration, n, for the b-brass structure is 1.5 (i.e.,le (for Cu) + 2e (for
Zn)/2 atoms). Due to the 1:1 combination of Cu and Zn, and the overpowering 2:1
electronic effect of Zn/Cu, the bcc structure becomes more stable. Other interme-
tallics that exhibit the b-brass structure are AgZn, AuZn, AgCd, Cu 3 Al, Cu 5 Sn,
CoAl, FeAl, and NiAl.
A further increase in the Zn concentration results in the complex g-brass, Cu 5 Zn 8 ,
with n ¼ 1.615 (i.e.,[5 1e (for Cu) + 8 2e (for Zn)]/13 atoms). Other