Page 84 - Mechanical Behavior of Materials

Section 3.4 Nonferrous Metals 83

or nickel improve corrosion resistance; and aluminum, tin, and zirconium improve creep resistance
of the alpha phase.
Alpha alloys are strengthened mainly by solid-solution effects and do not respond to heat
treatment. The other alloys can be strengthened by heat treatment. As in steels, a martensitic
transformation occurs upon quenching, but the effect is less. Precipitation hardening and the
effects of complex multiple phases are the principal means of strengthening alpha–beta and
beta alloys.

3.4.3 Other Nonferrous Metals
A wide range of copper alloys are employed in diverse applications as a result of their electrical
conductivity, corrosion resistance, and attractiveness. Copper is easily alloyed with various other
metals, and copper alloys are generally easy to deform or to cast into useful shapes. Strengths are
typically lower than for the metals already discussed, but still sufficiently high that copper alloys
are often useful as engineering metals.
Percentages of alloying elements range from relatively small to quite substantial, such as 35%
zinc in common yellow brass. Copper with approximately 10% tin is called bronze, although this
term is also used to describe various alloys with aluminum, silicon, zinc, and other elements.
Copper alloys with zinc, aluminum, or nickel are strengthened by solid-solution effects. Beryllium
additions permit precipitation hardening and produce the highest strength copper alloys. Cold
work is also frequently used for strengthening, often in combination with the other methods. A
variety of common names are in use for various copper alloys, such as beryllium copper, naval
brass, and aluminum bronze. The UNS numbering system with a prefix letter C is used for
copper alloys.
Magnesium has a melting temperature near that of aluminum, but a density only 65% as great,
making it only 22% as dense as steel and the lightest engineering metal. This silvery-white metal is
most commonly produced in cast form, but is also extruded, forged, and rolled. Alloying elements
do not generally exceed 10% total for all additions, the most common being aluminum, manganese,
zinc, and zirconium. Strengthening methods are roughly similar to those for aluminum alloys. The
highest strengths are about 60% as large, resulting in comparable strength-to-weight ratios. The
naming system in common use is generally similar to that for aluminum alloys, but differs as to
the details. A combination of letters and numbers that identifies the specific alloy is followed by a
processing designation, such as AZ91C-T6.
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Superalloys are special heat-resisting alloys that are used primarily above 550 C. The major
constituent is either nickel or cobalt, or a combination of iron and nickel, and percentages of alloying
elements are often quite large. For example, the Ni-base alloy Udimet 500 contains 48% Ni, 19%
Cr, and 19% Co, and the Co-base alloy Haynes 188 has 37% Co, 22% Cr, 22% Ni, and 14% W, with
both also containing small percentages of other elements. Nonstandard combinations of trade names
and letters and numerals are commonly used to identify the relatively small number of superalloys
that are in common use. Some examples, in addition to the two just described, are Waspaloy, MAR-
M302, A286, and Inconel 718.
Although nickel and cobalt have melting temperatures just below that of iron, superalloys have
superior resistance to corrosion, oxidation, and creep compared with steels. Many have substantial
strengths even above 750 C, which is beyond the useful range for low-alloy and stainless steels.
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