Page 70 - Mechanical Behavior of Materials

Section 3.2 Alloying and Processing of Metals 69

Figure 3.2 Microstructures of 70% Cu, 30% Zn brass in three conditions: cold worked (top);
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annealed one hour at 375 C (bottom left); and annealed one hour at 500 C (bottom right).
(Photos courtesy of Olin Corp., New Haven, CT.)

The effects of cold work can be partially or completely reversed by heating the metal to such
a high temperature that new crystals form within the solid material, a process called annealing.If
this is done following severe cold work, the recrystallized grains are at first quite small. Cooling the
material at this stage creates a situation where strengthening is said to be due to grain refinement,
because the grain boundaries impede dislocation motion. A long annealing time, or annealing at a
higher temperature, causes the grains to coalesce into larger sizes, resulting in a loss of strength,
but a gain in ductility. The microstructural changes involved in cold working and annealing are
illustrated in Fig. 3.2.

3.2.2 Solid Solution Strengthening

Solid solution strengthening occurs as a result of impurity atoms distorting the crystal lattice and
thus making dislocation motion more difficult. Note that alloying elements are said to form a solid
solution with the major constituent if their atoms are incorporated into the crystal structure in
an orderly manner. The atoms providing the strengthening may be located at either interstitial or
substitutional lattice positions. Atoms of much smaller size than those of the major constituent
usually form interstitial alloys, as for hydrogen, boron, carbon, nitrogen, and oxygen in metals.
Substitutional alloys may be formed by combinations of two or more metals, especially if the atomic
sizes are similar and the preferred crystal structures are the same.