Page 69 - Mechanical Behavior of Materials

68 Chapter 3 A Survey of Engineering Materials

Figure 3.1 Some methods of forming metals into useful shapes are (a) forging, which
employs compression or hammering; (b) rolling; (c) extrusion; and (d) drawing.

Table 3.2 Strengthening Methods for Metals and Alloys
Method Features That Impede Dislocation Motion
Cold work High dislocation density causing tangles
Grain refinement Changes in crystal orientation and other irregularities
at grain boundaries
Solid solution Interstitial or substitutional impurities
strengthening distorting the crystal lattice
Precipitation hardening Fine particles of a hard material precipitating
out of solution upon cooling
Multiple phases Discontinuities in crystal structure
at phase boundaries
Quenching and tempering Multiphase structure of martensite and Fe 3 C
precipitates in BCC iron

3.2.1 Cold Work and Annealing

Cold work is the severe deforming of a metal at ambient temperature, often by rolling or drawing.
This causes a dense array of dislocations and disorders the crystal structure, resulting in an increase
in yield strength and a decrease in ductility. Strengthening occurs because the large number of
dislocations form dense tangles that act as obstacles to further deformation. Hence, controlled
amounts of cold work can be used to vary the properties. For example, this is done for copper
and its alloys.