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'Ductility' is the mechanical property of being capable of sustaining large plastic deformations due to tensile stress without fracture (in metals, such as being drawn into a wire). It is characterized by the material flowing under shear stress. It is contrasted with brittleness.

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Definition

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Definition

The property of a material by which it can withstand extensive deformation without failure under high tensile stresses is said to be its ductility. Gold, copper, aluminum, and steel express high ductility. Ductility can be quantified by the fracture strain, which is the strain at which a test specimen breaks during a uniaxial tensile testG. Dieter, ''Mechanical Metallurgy'', McGraw-Hill, 1986.
In Earth science, the brittle-ductile transition zone is a zone at an approximate depth of 10 km in the Earth, at which rock becomes less likely to fracture, and more likely to deform ductilely. In glacial ice this zone is at approximately 30 metres 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.
In physics/materials science the ductile-brittle transition temperature (DBTT), nil ductility temperature (NDT), or nil ductility transition temperature of a material represents the point at which the fracture energy passes below a pre-determined point (for steels typically 40 J^[1] for a standard Charpy impact test). DBTT is important since once a material is cooled below the DBTT, it has a much greater tendency to shatter on impact instead of bending or deforming. For example, ZAMAK 3, a zinc die casting alloy exhibits good ductility at room temperature but shatters at sub zero temperatures when impacted. DBTT is a very important consideration in materials selection when the material in question is subject to mechanical stresses. See the section on Glass transition temperature for a related discussion.
In some materials this transition is sharper than others. For example, the transition is generally sharper in materials with a body-centered cubic (BCC) lattice than those with a face-centered cubic (FCC) lattice. DBTT can also be influenced by external factors such as neutron radiation which leads to an increase in internal lattice defects and a corresponding decrease in ductility and increase in DBTT.

Notes

1. John, Vernon. ''Introduction to Engineering Materials'', 3rd ed.(?) New York: Industrial Press, 1992. ISBN 0831130431.