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530 • Chapter 13 / Types and Applications of Materials(Figure 13.3a) and ductile (nodular) iron (Figure 13.3b), and, in fact, some of the
graphite (less than 20%) may be as nodules. However, sharp edges (characteristic of
graphite flakes) should be avoided; the presence of this feature leads to a reduction
in fracture and fatigue resistance of the material. Magnesium and/or cerium is also
added, but concentrations are lower than for ductile iron. The chemistries of CGIs
are more complex than for the other cast iron types; compositions of magnesium,
cerium, and other additives must be controlled so as to produce a microstructure that
consists of the worm-like graphite particles, while at the same time limiting the degree
of graphite nodularity, and preventing the formation of graphite flakes. Furthermore,
depending on heat treatment, the matrix phase will be pearlite and/or ferrite.
As with the other types of cast irons, the mechanical properties of CGIs are
related to microstructure: graphite particle shape as well as the matrix phase/
microconstituent. An increase in degree of nodularity of the graphite particles leads
to enhancements of both strength and ductility. Furthermore, CGIs with ferritic ma-
trices have lower strengths and higher ductilies than those with pearlitic matrices.
Tensile and yield strengths for compacted graphite irons are comparable to values
for ductile and malleable irons, yet are greater than those observed for the higher-
strength gray irons (Table 13.5). In addition, ductilities for CGIs are intermediate
between values for gray and ductile irons; also, moduli of elasticity range between
140 and 165 GPa (20× 106 and 24× 106 psi).
Compared to the other cast iron types, desirable characteristics of CGIs include
the following:- Higher thermal conductivity
- Better resistance to thermal shock (i.e., fracture resulting from rapid temperature
changes) - Lower oxidation at elevated temperatures
Compacted graphite irons are now being used in a number of important
applications—these include: diesel engine blocks, exhaust manifolds, gearbox hous-
ings, brake discs for high-speed trains, and flywheels.
13.3 NONFERROUS ALLOYS
Steel and other ferrous alloys are consumed in exceedingly large quantities because
they have such a wide range of mechanical properties, may be fabricated with relative
ease, and are economical to produce. However, they have some distinct limitations,
chiefly: (1) a relatively high density, (2) a comparatively low electrical conductivity,
and (3) an inherent susceptibility to corrosion in some common environments. Thus,
for many applications it is advantageous or even necessary to utilize other alloys
having more suitable property combinations. Alloy systems are classified either ac-
cording to the base metal or according to some specific characteristic that a group
of alloys share. This section discusses the following metal and alloy systems: copper,
aluminum, magnesium, and titanium alloys, the refractory metals, the superalloys,
the noble metals, and miscellaneous alloys, including those that have nickel, lead,
tin, zirconium, and zinc as base metals.
On occasion, a distinction is made between cast and wrought alloys. Alloys that
are so brittle that forming or shaping by appreciable deformation is not possible
ordinarily are cast; these are classified ascast alloys. On the other hand, those that
wrought alloy are amenable to mechanical deformation are termedwrought alloys.