Page 191 - Materials Chemistry, Second Edition

178 3 Metals

growth in areas of electronics, building materials, homeland security devices, and
future “smart” materials, it is essential that we become familiar with the properties
of individual classes of materials and current applications. Only then will we be able
to extrapolate these properties into new and exciting applications for the future.
In Section 3.1, we saw that a wide variety of applications employ metallic
substances (Table 3.1). In this section, we will examine the various classes of metals
and alloys in more detail, focusing on phase transitions, changes in the microstruc-
ture, and atomic packing of the materials. With this insight, you will be in a good
position to evaluate why a particular metal is more suited than others for an existing
or future application. It should be noted that certain organic polymers may also
exhibit high electrical conductivities. However, this chapter will only discuss inor-
ganic-based metallic classes; organic-based electrical conductors will be detailed in
Chapter 5.

3.2.1. Phase Behavior of Iron–Carbon Alloys
In general, for a mixture of two or more pure elements, there are two types of solid-
solution alloys that may be obtained. Type I alloys are completely miscible with one
another in both liquid and solid states. As long as the Hume-Rothery rules are
satisfied, a random or ordered substitutional alloy will be produced. We will see
many examples of these alloys for a variety of metal dopants in stainless steels. By
comparison, type II alloys are only miscible in the molten state, and will separate
from one another upon cooling. These alloys are usually associated with compound
formation from the alloying of metals or metals/nonmetals that are too dissimilar in
their reactivities (e.g., Cu and Al to form CuAl 2 precipitates). The eutectic compo-
sition represents the lowest melting point of type II alloys.
Type I alloys contain two types of atoms that are arranged within a single lattice.
When solidification of the solution begins, the temperature may be higher or lower
than the freezing point of the pure solvent. Unlike a pure molten metal, most solid
solutions will solidify over a temperature range due to differing diffusion rates of the
metals en route toward their preferred crystal arrangement (Figure 3.15).
Pure iron exists as a variety of allotropes depending on the external temperature or
pressure. As the temperature is increased, iron undergoes allotropic transformations
from a-Fe (ferrite, bcc) to g-Fe (austenite, fcc), and finally to a narrow region of d-Fe
(bcc) before melting. As the temperature of the standard bcc crystal lattice is
increased,thermallyinduced atomic motionincreases,andit becomesmore energetically
favorableforatomsinthecenteroflatticeunitcellstomigrateintoface-centeredpositions
of neighboring unit cells (Figure 3.16). However, as the magnitude of lattice vibrations
continue to increase toward the melting point, the bcc structure is favored. This is due to
the more open bcc structure being able to accommodate a larger range of vibrational
motion than a relatively dense fcc array.
As seen earlier, the steps used to purify iron involves carbonaceous material in
order to remove the oxide-based impurities via exothermic formation of CO and
CO 2 . Hence, carbon will be pervasive in a variety of concentrations throughout all