Page 198 - Materials Chemistry, Second Edition

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3.2. Metallic Structures and Properties

nucleation/epitaxial growth of graphite. Other common graphitizer dopants are Si
(typically added as metal ferrosilicon compounds), Ni, and Cu; by contrast, Cr, Mo,
V, and W are antigraphitizers, promoting the formation of carbides (Cr 7 C 3 /Cr 23 C 6 ,
Mo 2 C/Fe 3 Mo 3 C, VC–V 4 C 3 ,W 2 C/Fe 3 W 3 C, respectively).

3.2.2. Hardening Mechanisms of Steels

In this section, we will describe the primary techniques that may be used to strengthen
a metal. It should be noted that these methods are applicable to all metal classes, not
just the iron alloys predominantly described herein.

Strain hardening
An external pressure (stress) that is exerted on a material will cause its thickness to
decrease. A shear stress is applied parallel to the surface of a material, and may
cause the sliding of atomic layers over one another. The resultant deformation in the
size/shape of the material is referred to as strain, related to the bonding scheme of
the atoms comprising the solid. For example, a rubbery material will exhibit a
greater strain than a covalently bound solid such as diamond. Since steels contain
similar atoms, most will behave similarly as a result of an applied stress. If a stress
causes a material to bend, the resultant flex is referred to as shear strain. For small
shear stresses, steel deforms elastically, involving no permanent displacement of
atoms. The deformation vanishes when shear stress is removed. However, for a large
shear stress, steel will deform plastically, involving the permanent displacement of
atoms, known as slip.
Dislocation defects and thermal energy assist slip, allowing a sheet of atoms to
slip gradually past one another. To stop slip, one can either lower the temperature, or
spoil the crystal structure. A process referred to as work hardening (bending/
hammering the cold material) is used to break up crystallites, introducing disloca-
tions in the material. The more dislocations exist, the more they will interact with
one another, becoming pinned or entangled with one another. This will impede the
movement of dislocations and strengthening of the material. This is done at low
temperature (T 0.5T m , where T m ¼ melting point), so the metal atoms cannot
rearrange themselves, which would negate the effect. Consequently, low-melting
metals such as Sn and Pb may not be cold-worked at room temperature. In contrast,
hot working is performed at temperatures above the recrystallization temperature of
the metal. Although hot working requires less energy and induces larger deforma-
tions than cold working, most metals exhibit surface oxidation that may deleteri-
ously affect its overall properties and applications.

Grain size hardening
As we have seen, it is not simply the carbon concentration, but rather the microstructure
of Fe–C alloys that affects its physical properties. The size of the individual micro-
crystals (or grains) that comprise these aggregates greatly influences many properties
of the bulk crystal. Both optical microscopy and X-ray diffraction are used to