Ductility is a mechanical property used to describe the extent to which materials can be deformed plastically without fracture.

In material science, ductility specifically refers to a material's ability to deform under tensile stress; this is often characterized by the material's ability to be stretched into a wire. Malleability, a similar concept, refers to a material's ability to deform under compressive stress; this is often characterized by the material's ability to form a thin sheet by hammering or rolling. Ductility and malleability do not always correlate with each other; for instance, gold is both ductile and malleable, but lead is only malleable. Commonly, the term "ductility" is used to refer to both concepts, as they are very similar.

Geology

In Earth science the brittle-ductile transition zone is a zone, at an approximate depth of in continental crust, at which rock becomes less likely to fracture and more likely to deform ductilely. In glacial ice this zone is at approximately depth. It is not impossible for material above a brittle-ductile transition zone to deform ductilely, nor for material below to deform brittly. The zone exists because as depth increases confining pressure increases, and brittle strength increases with confining pressure whilst ductile strength decreases with increasing temperature. The transition zone occurs at the point where brittle strength exceeds ductile strength.

Materials science

Ductility is especially important in metalworking, as materials that crack or break under stress cannot be manipulated using metal forming processes, such as hammering, rolling, and drawing. Malleable materials can be formed using stamping or pressing, whereas brittle metals and plastics must be molded.

High degrees of ductility occur due to metallic bonds, which are found predominantly in metals and leads to the common perception that metals are ductile in general. In metallic bonds valence shell electrons are delocalized and shared between many atoms. The delocalized electrons allow metal atoms to slide past one another without being subjected to strong repulsive forces that would cause other materials to shatter.

Ductility can be quantified by the fracture strain $\backslash varepsilon\_f$, which is the engineering strain at which a test specimen fractures during a uniaxial tensile test. Another commonly used measure is the reduction of area at fracture $q$.G. Dieter, Mechanical Metallurgy, McGraw-Hill, 1986

The following list ranks metals from the greatest ductility to least: gold, silver, platinum, iron, nickel, copper, aluminium, zinc, tin, and lead. . The malleability of the same metals are then ranked from greatest to least: gold, silver, lead, copper, aluminium, tin, platinum, zinc, iron, and nickel. The ductility of steel varies depending on the alloying constituents. Increasing levels of carbon decreases ductility. Many plastics and amorphous solids, such as Play-Doh, are also malleable.