Elasticity - Crystalline Materials

Metals are examples of crystalline materials. Solid pieces of metal contain millions of microscopically small crystals stuck together, often in random orientations. Within a single crystal, atoms are arranged in orderly rows. They are held by attractive forces on all sides. Scientists model the attractive force as a sort of a spring. When a spring is stretched, a restoring force tries to return it to its original length. When a metal rod is stretched in tension, its atoms are pulled apart slightly. The attractive force between the atoms tries to restore the original distance. The stronger the attraction, the more force must be applied to pull the atoms apart. Thus, stronger atomic forces result in larger elastic modulus.

Stresses greater than the elastic limit overcome the forces holding atoms in place. The atoms move to new positions. If they can form new bonds there, the material deforms plastically; that is, it remains in one piece but assumes a new shape. If new bonds cannot form, the material fractures.

The ball and spring model also explains why metals and other crystalline materials soften at higher temperatures. Heat energy causes atoms to vibrate. Their vibrations move them back and forth, stretching and compressing the spring. The higher the temperature, the larger the vibrations, and the greater the average distance between atoms. Less applied force is needed to separate the atoms because some of the stretching energy has been provided by the heat. The result is that the elastic modulus of metals decreases as temperature increases.