Ductility

"Malleability" redirects here. For the property in cryptography, see Malleability (cryptography).

Tensile test of an AlMgSi alloy. The local necking and the cup and cone fracture surfaces are typical for ductile metals.

Tensile test of a nodular cast iron with very low ductility.

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

In materials 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.

Scientific fields

Geology

In Earth science the brittle-ductile transition zone is a zone, at an approximate depth of 15 km (9 mi) 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 m (100 ft) depth. It is still possible for material above a brittle-ductile transition zone to deform ductilely, and possible 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

Gold leaf is possible due to its malleability

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 $\varepsilon _{f}$, 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 $q$.

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.

Ductile-brittle transition temperature

Schematic appearance of round metal bars after tensile testing.
(a) Brittle fracture
(b) Ductile fracture
(c) Completely ductile fracture

The ductile-brittle transition temperature (DBTT), nil ductility temperature (NDT), or nil ductility transition temperature of a metal 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. A similar phenomenon, the glass transition temperature, occurs with glasses and polymers, although the mechanism is different in these amorphous materials.

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 of 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 (K_iC). 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

One important 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 samples located within the reactor pressure vessel. The vessel's nil ductility temperature is likely to be the limiting factor in plant life, at least for pressurized water reactors (PWR).

Periodically, all thermal power plants, including nuclear power plants, are shut down for refueling and maintenance. Nuclear power plants use schedules of approximately 18 months between outages, as these are called, for PWRs, and 24 months for boiling water reactors (BWRs). At this time, the reactor pressure vessel is cooled down from above 600 °F (316 °C) (for PWRs) or above 285 °C (545 °F) (for BWRs) to ambient temperatures, the same as in any thermal power plant experiencing a maintenance outage, such as coal, natural gas, oil, geothermal, or solar thermal power plants, though other thermal power plants often have much sharper temperature gradients. This cooling down and warming up afterward creates temperature gradients and thus induced stresses between the different components and areas of the reactor. As the reactor gets older, neutron radiation causes embrittlement and the stresses must be below a certain value.

So as to ensure that neutron embrittlement does not cause the RPV to go out of specification, numerous material samples of the same material that the RPV was made out of are located within the RPV for retrieval at every outage. These samples are then tested to analyze their ductility; lack of ductility within the bounds of the vessel specification, the limits of the nameplate of the vessel, and/or the bounds specified within the ASME Boiler and Pressure Vessel Code would require that a mechanical engineer and/or a nuclear engineer be sought to advise as to the situation, and what corrective actions, if any, should be taken, such as modification of plant operations protocols (longer periods of heat-up or cool-down) or eventually temporary shutdown of the plant for replacement of the RPV, an expensive task, but one that pales in comparison to the loss of reliable electricity that a modern nuclear power plant provides.

See also

• Deformation
• Work hardening, which reduces ductility

External links

• Ductility definition at engineersedge.com