Definitions

ductility

ductility

[duhk-tl, -til]
ductility, ability of a metal to plastically deform without breaking or fracturing, with the cohesion between the molecules remaining sufficient to hold them together (see adhesion and cohesion). Ductility is important in wire drawing and sheet stamping. The metal must neither break nor be scraped off during these processes. Platinum, steel, copper, and tungsten have high ductility. Ductility is a focus of rheology, the study of how materials deform and flow in response to force.

Ductility is a mechanical property used to describe the extent to which materials can be deformed plastically or "stretched" into "wires", without fracture. Ductility is the most important parameter to consider in metal forming operations such as rolling, extrusion, and drawing. Examples of highly ductile metals are silver, gold, copper, and aluminium. The ductility of steel varies depending on the alloying constituents. Increasing levels of carbon decreases ductility.

Ductility can be quantified by the fracture strain 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.

Scientific fields

Geology

In Earth science the brittle-ductile transition zone is a zone, at an approximate depth of 15 km 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 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.

Materials science

In 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 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 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.

The most accurate method of measuring the BDT or DBT temperature or a material is by fracture testing. Typically four point bend testing at a range of temperatures is performed on pre-cracked bars of polished material. For experiments conducted at higher temperatures dislocation activity increases. At a certain temperature dislocations shield the crack tip to such an extent the applied deformation rate is not sufficient for the stress intensity at the crack-tip to reach the critical value for fracture (KiC). The temperature at which this occurs is the ductile-brittle transition temperature. If experiments are performed at a higher strain rate more dislocation shielding is required to prevent brittle fracture and the transition temperature is raised.

Nuclear power plant reactor pressure vessel embrittlement

Perhaps the most critical ductility concern is the embrittlement of nuclear power plant reactor vessels. Neutron radiation causes embrittlement of some materials, neutron-induced swelling, and buildup of Wigner energy, thus affecting the nil ductility temperature of the vessel's metal. This effect is now rigorously scrutinized by the operators, including by periodic testing of metal "coupons" emplaced inside the reactor vessel. The vessel's nil ductility temperature is likely to be the limiting factor in plant life, at least for pressurized water reactors.

Every year a PWR is opened and needs to be cooled down for maintenance works. This cooling down and warming up afterwards creates temperature differences and great stresses in between the different components of the reactor. As the reactor gets older, neutron radiation causes embrittlement and the stresses must be below a certain value. Thus cooling down and warming up must be done slower, but is still possible.

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