What Is Ductility in Metallurgy?

Ductility in metals explained

piano wire ductility
Piano wires are pulled taught, a test of their ductility. Image author: Matt Billings. Licensing: Creative Commons Attribution 2.0 Generic

Ductility is a measure of a metal's ability to withstand tensile stress.

Tensile stress is a force pulling the two ends of a material away from each other. The game tug-of-war provides a good example of tensile strength being applied to a rope. 

Ductility, then, is the plastic deformation that occurs as a result of such types of strain.

Metals with high ductility (e.g. copper) are able to be drawn into long, thin wires without breaking.

While metals with low ductility (e.g. bismuth) will, instead, rupture when put under tensile stress. 

By contrast, malleability is the measure of a metal's ability to withstand compression, such as hammering, rolling or pressing. 

While similar, metals that are ductile are not necessarily malleable. A common example of the difference between these two properties is lead, which is highly malleable but not highly ductile. 

This is due to the crystal structure of metals, which dictates how they will deform under stress. 

The atomic particles that make up metals can deform under stress either by slipping over each other or stretching away from each other. The crystal structure of more ductile metals allows for the metal's atoms be stretched farther apart (a process called 'twinning').  

Metals that more readily twin - more ductile metals - also more readily deform in other directions. 

Ductility in metals is also related to temperature.

As metals are heated, they generally become less brittle, allowing for plastic deformation. In other words, when heated, most metals become more ductile and can be more easily drawn into wires without breaking. 

Again, lead proves to be an exception to the rule, as it becomes more brittle as it is heated.

While it is difficult to directly compare ductility between metals, gold and platinum are considered to be the most ductile. Gold, it is said, can be drawn into wires so fine that one ounce of the metal could reach up to fifty miles.