When mechanical stress is applied to a solid object, it will depend on the structure of the solid whether it deforms into various shapes without breaking or not. Materials that are easily deformed without breaking when put under mechanical pressure are considered to be malleable. Materials that are easily deformed when put under tensile stress are considered to be ductile.
Definition of Malleable
The word malleable comes from the Medieval Latin malleabilis, which itself came from the original Latin malleare, meaning "to hammer."
Malleable materials can be easily deformed without breaking under mechanical pressure, or "compressive stress." Since these materials do not break while being deformed, they can be forced into different shapes or thin sheets. This can be done by hammering, pressing or rolling.
A common example of a malleable material is gold, which is often compressed into gold leaf for use in art, architecture, jewelry and even food. Other malleable metals include iron, copper, aluminum, silver and lead, as well as the transition metal zinc at certain temperatures. Many materials that are very malleable are also very ductile; lead is an exception, with low ductility and high malleability.
Definition of Ductile
Closely related to the concept of malleability is ductility. While malleability has to do with compressive stress, or mechanical pressure, ductility relates to tensile stress, or mechanical stretching.
"Ductile" originates from the Latin word ductilis, which means "that may be led or drawn."
Something that is ductile (sometimes also called tractile) can be easily stretched or drawn out into a thin wire. Ductile copper is a good example of both malleability and ductility, being able to be pressed and rolled into sheets as well as stretched into wires.
Metals are often mixed as alloys to improve their physical properties. High-tensile steel is an example of an alloy that has higher ductility than any of its component metals, and it is often used in airplanes, cars and other engineering applications.
How Metals Deform
Layers of ions in a metal can move and slide over one another without breaking their metallic bonds; this is what allows a metal to bend or stretch without breaking. However, some harder metals don't have clear layers and instead have a crystal structure with smaller component units of atoms.
These unit clumps of atoms, called grains, have boundaries between them called grain boundaries. The more grain boundaries per unit volume a metal has, the less malleableness or ductility it will have. The metal will instead be more brittle and tend to break along these grain boundaries.
Materials are more malleable and more ductile when they have dislocations, or missing ions in the layer structure. These defects can move through the crystal structure of the metal as it deforms, increasing its ability to deform without breaking.
When most metals are heated, their grains become larger. The atoms are then in a more regular structure and can more easily slip over each other without breaking their bonds. This allows the metals to be more easily deformed. "Cold working" does the opposite: Deforming the metal when it's cold creates more grain boundaries, making the metal stiff and brittle.
Interestingly, some metals also show elasticity. When a very small amount of stress is put on a metal, the atoms start to roll over each other. But then, when the stress is released, the atoms roll back to their original positions. Larger amounts of stress change the atoms' positions permanently.