Ductility Explained: Tensile Stress and Metals

Ductility in Metals Explained

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Wikimedia Commons/Matt Billings

Ductility is a measure of a metal's ability to withstand tensile stress—any force that pulls the two ends of a material away from each other. The game of tug-of-war provides a good example of tensile strength being applied to a rope. Ductility is the plastic deformation that occurs as a result of such types of strain. The term "ductile" literally means that a metal substance is capable of being stretched out into a thin wire, and it does not become weaker or become brittle in the process.

Metals With High or Low Ductility 

Metals with high ductility such as such as copper can be drawn into long, thin wires without breaking. Copper has historically served as an excellent conductor of electricity, but this metal can conduct just about anything. Metals with low ductilities, such as bismuth, will instead rupture when they're put under tensile stress.

Ductility vs. Malleability

By contrast, malleability is the measure of a metal's ability to withstand compression, such as hammering, rolling, or pressing. While these two concepts may seem similar on the surface, 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 due to its crystal structure. The crystal structure of metals dictates how they will deform under stress.

The atomic particles that makeup 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 the metal's atoms to be stretched farther apart, a process called "twinning." More ductile metals are those that more readily twin, and they also more readily deform in other directions.

The Effect of Temperature

Ductility in metals is also related to temperature.

As metals are heated, they generally become less brittle, allowing for plastic deformation. In other words, most metals become more ductile when they're heated and can be more easily drawn into wires without breaking. Lead proves to be an exception to this rule, as it becomes more brittle as it is heated.

What Are the Most Ductile Metals?

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