Materials for engineering, 3rd Edition - (Malestrom)

98
Materials for engineering
Tensile strength (N mm –2 )
400
200
Elongation
Tensile strength
60
40
20
Elongation
0
0 10 20 30 40 50 60
Wt% zinc
3.16 Showing the change in tensile strength and ductility with Zn
content of brasses.
mechanical strength and corrosion resistance. The addition of 1–2% Fe increases
their resistance to impingement attack in moving sea-water.
Copper–tin alloys or bronzes are again essentially α-solid solutions of Sn
in Cu. The relevant phase diagram is shown in Fig. 3.17 and cast alloys may
have tin contents in the range 5–19%. Tin is a strong solution-hardening
element, but cast alloys are far from equilibrium and show cored microstructures
and increasing volume fractions of the hard (α + δ) eutectoid as the Sn
content increases above about 7%. Typical applications include cast bearings
and bushings, and phosphor-bronzes (containing 0.3–1% P) are employed
when a bearing surface is required to bear heavy loads with a low coefficient
of friction.
Copper-rich aluminium alloys are known as aluminium bronzes, the
composition of the commercial alloys ranges from 5–11% Al, and the phase
diagram of Fig. 3.18 enables the microstructures to be understood. The alloys
are characterized by high strength coupled with a high resistance to corrosion
and wear. Up to 7% Al, the alloys consist of single-phase α, which is easily
worked. They find widespread application in heat-exchanger tubing. The
α/β alloys contain ~10% Al, and are either cast or hot-worked in the β-phase
field. Slow cooling from β produces a eutectoid (α + γ 2 ) mixture, and quenching
produces a hardened martensitic structure of β′. Although these changes are
analogous to those found in steel (see later), such heat treatments are not
widely applied to aluminium bronzes in practice.
The machining properties of brasses and bronzes are enhanced by the
addition of a few % of lead which, being insoluble in both solid and liquid
phases, appears as globules in the microstructure. These allow turnings to
break up during machining.
Table 3.6 gives the properties of some copper alloys.