Ductility is a mechanical property describing how readily a material is drawn into a wire. In other words, it is a material’s ability of sustaining plastic deformation under tensile stress before failure. A material with high ductility is ductile, while one with low ductility is brittle.
What Is the Most Ductile Metal?
Gold is the most ductile metal. According to Guinness World Records, one gram of gold can be drawn into a wire 2.4 kilometers long, while one ounce of gold can be drawn into a wire 43 miles long.
Platinum, silver, and copper are other highly ductile metals.
How Ductility Works
Metals are ductile because they form metallic bonds. This is a consequence of metallic character, which in turn depends on valence electrons. In ductile metals, electrons are delocalized and shared between atoms. This allows atoms to slide past one another.
There is a relationship between the number of electron shells of an atom and that element’s ductility. Usually, the greater the number of electron shells, the higher an element’s ductility. The valence electrons also matter. Gold, lead, and platinum have six electron shells. All three metals display extremely high ductility. Iron, zinc, manganese, and chromium each have four electron shells and lower ductility. Beryllium is brittle rather than ductile and has two electron shells.
Two Ways of Measuring Ductility
There are two common methods of measuring ductility: percent elongation and reduction of area. In other words, one measures how long a wire gets while the other measure how thin it gets.
Percent elongation is a measure of the length of a wire drawn of a material. It compares the final length to the initial length:
percent elongation = (final length – initial length)/initial length x 100% = (lf – l0)/lo x 100
Percent reduction of area compares how thin a wire cross section becomes before breaking by comparing the initial thickness (gauge) to the final thickness (gauge):
percent reduction of area = change in area/original area x 100% = (Ao – Af)/Ao x 100
Percent reduction of area measures the minimum thickness of the “neck” of the wire. A brittle material breaks without necking. Most ductile materials display necking. Some highly ductile materials draw to a fine point before breaking.
Factors Affecting Ductility
The main factors affecting ductility are temperature, crystal structure, and grain size.
Solids are brittle at low temperatures and more ductile at higher temperatures. The ductile-brittle transition temperature (DBTT) is the temperature where a material changes from brittle fracture to ductile fracture. Below the DBTT, even a normally ductile material is brittle.
Face-centered cubic (FCC) structure are more ductile over a wider temperature range than other crystal structure. Body-centered cubic (BCC) materials are only ductile at high temperatures. Hexagonal close packed (HCP) structures often are brittle over a wide temperature range.
Smaller grain size tends toward higher ductility and a lowering of the ductile-brittle transition temperature. The small grain size means it takes a larger stress to cause dislocation at the grain boundary.
Grain size also correlates with malleability. Metals which are ductile tend to be malleable.
Table of Ductility
Here are some representative ductility values for various elements. In general, metals are more ductile than metalloids, which are more ductile than nonmetals.
| Element | Symbol | Physical Ductility |
|---|---|---|
| Perfect Ductility | — | 1 |
| Gold | Au | 0.93 |
| Lead | Pb | 0.93 |
| Niobium | Nb | 0.82 |
| Palladium | Pd | 0.80 |
| Platinum | Pt | 0.76 |
| Silver | Ag | 0.73 |
| Vanadium | V | 0.73 |
| Tin | Sn | 0.69 |
| Aluminum | Al | 0.65 |
| Copper | Cu | 0.62 |
| Tantalum | Ta | 0.62 |
| Titanium | Ti | 0.54 |
| Nickel | Ni | 0.50 |
| Cobalt | Co | 0.50 |
| Magnesium | Mg | 0.43 |
| Iron | Fe | 0.43 |
| Tungsten | W | 0.40 |
| Zinc | Zn | 0.30 |
| Manganese | Mn | 0.23 |
| Uranium | U | 0.23 |
| Silicon | Si | 0.20 |
| Plutonium | Pu | 0.18 |
| Chromium | Cr | 0.18 |
| Carbon | C | 0.16 |
| Beryllium | Be | 0.00 |
| Total Brittleness | — | 0 |
Alloy ductility depends on their composition and other factors. Steel ductility decreases as carbon content increases.
Common Questions About Ductility
Is Ductility a Physical Property or a Chemical Property?
Ductility is a physical property of matter. It is observable without the need for a chemical change or reaction.
Why Do Metals Become Brittle at Low Temperatures?
Metals become brittle at low temperatures because their atoms have less kinetic energy. Essentially, this makes it harder for them to slip against each other.
What Metal Has the Highest Ductility?
The accepted answer is gold. Gold holds the record for highest ductility in the Guinness World Records book.
Which metal has the highest ductility depends on how it is measured. Are you comparing ductility per gram of metal or per cube of metal? For all practical purposes, gold is the most ductile metal. However, gold is very dense, so if you compare a wire from a cube of gold versus a wire from the same size cube of another metal, you can get a different answer. According to one article, platinum has the highest ductility, but drawing it requires special techniques. It is embedded in silver and drawn into a fine wire and then the silver is dissolved away.
References
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- Lah, Che; Akmal, Nurul; Trigueros, Sonia (2019). “Synthesis and modelling of the mechanical properties of Ag, Au and Cu nanowires”. Sci. Technol. Adv. Mater. 20 (1): 225–261. doi:10.1080/14686996.2019.1585145
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