Main Difference – Ductility vs Malleability

Ductility and malleability are properties related to deformation of metals. Ductility is the ability of a metal to undergo tensile stress. Malleability denotes the ability to undergo compressive stress. This is the main difference between ductility and malleability. These two remarkable properties are due to the unique metallic bonding found only in metals.

This article studies,

1. What is Ductility
      – Definition, Features, Examples
2. What is Malleability
      – Definition, Features, Examples
3. What is the difference between Ductility and Malleability

What is Ductility

When a force is applied at the two ends of a material to pull each other away, a stress is applied on the material. This is called the tensile stress. Plastic deformation occurs due to tensile stress.  Tensile stress is applied along a single axis and the material can be rolled into a wire. Most metals show great ability to withstand this tensile stress. Copper, as an example, shows high ductile properties whereas Bismuth shows comparatively low ductility and tend to rupture easily due to tensile stress.

Ductility depends on the grain size of the material. Lower the grain size, harder the motion of dislocations due to greater resistance; therefore, ductility decreases. With larger grain sizes, vice versa occurs.

Ductility owes to the ability of metal atoms to slip over each other and deform under stress. This is also proportional to temperature. When metals are heated their ductility increases. However, lead shows an exception by becoming more brittle when it is heated.

The process of stretching metal is called twining. Chains and necklaces are produced by twining valuable metals such as gold and silver.

Ductility is understood by the tensile stress of a material. Higher the tensile stress, higher the ductility and easier the material to be stretched.

Ductility is measured by bend test. This is done by bending the specimen to a predetermined angle or until it fractures. Ductile materials are used to produce tubes, wires, and various other vehicle parts.

Alloys are highly ductile as the compositions are not pure. Materials such as carbon are less ductile. By increasing the composition of carbon, steel can be made more ductile.

Figure 1: Ductile material can be rolled into wires.

What is Malleability

Malleability correlates to the ability of plastic deformation of a material under a compressive test. Compressive stress results in shortening the dimensions of a material, making its volume smaller. Metals are highly malleable as the sea of electrons surrounding the positive metal ions can adjust themselves to withstand their small volume.

A malleable material can be rolled into thin sheets, pressed or hammered without breaking it. Different materials show different malleability owing to their arrangement of the crystal structure. NaCl has an ionic lattice structure which requires positive and negative ions to be located in specific places. Therefore when pressure is applied, the ions are unable to dislocate and the structure is broken. Therefore, NaCl is not a malleable material. Cu, in contrast, can adjust its crystal structure when pressure is applied. Hence, it is highly malleable.

Some examples of highly malleable materials include gold, silver, iron, copper, aluminum, tin and lithium. Antimony and bismuth are much harder as their atoms don’t line up when pressure is applied. Therefore, the material is harder and brittle.

The increase of temperature increases the malleability as well. Even impurities affect malleability.  They make the dislocations hard to move. Malleability is useful for making various objects by changing the shape of metals.

Figure 2: Ductile materials can be rolled into sheets.

Difference Between Ductility and Malleability

Definition

Ductility: Ductility refers to the ability of a material to stretch under tensile stress.

Malleability: Malleability refers to the ability to deform and change shape under compressive stress.

Shape

Ductility: Ductile materials can be rolled into wires.

Malleability: Malleable materials can be rolled into sheets.

Measurement 

Ductility: Ductility is measured by bend test.

Malleability: Malleability is measured by the ability to withstand pressure.

Factors that Affect Malleability and Ductility

Ductility: Ductility is affected by the grain size.

Malleability: Malleability is affected by the crystal structure. 

Conclusion

Ductility refers to the ability of a material to stretch under tensile stress and malleability is the ability to deform and change shape under compressive stress. This is the main difference between ductility and malleability.

Both these properties increase with increasing temperature, however, lead and tin shows decreasing ductility and malleability when heat is provided. Most ductile materials are malleable. Gold is both highly ductile and malleable. Therefore, very popular in making jewelry.

Alloys show resistance to pressure as the grain size becomes more workable due to the metal mixture. Ductility depends on the grain size of the material whereas malleability depends on the crystal structure.

Reference:
1. “Malleability .” Infoplease. N.p., n.d. Web. 15 Feb. 2017.
2. “Malleability in metals.” Physics Stack Exchange. N.p., n.d. Web. 15 Feb. 2017.
3. Truitt, Benjamin.”Compressive Stress: Definition, Formula & Maximum.” Study.com. N.p., n.d. Web. 15 Feb. 2017.
4. Bell, Terence. “Malleability Explained | Compressive Stress and Metals.” The Balance. N.p., n.d. Web. 15 Feb. 2017.
5. “How the ductility of metal change when grains is decreased ?” Physics Forums – The Fusion of Science and Community. N.p., n.d. Web. 15 Feb. 2017.

Image Courtesy:
1. “Enamelled litz copper wire” By Alisdojo – Own work  (CC0) via Commons Wikimedia
2. “Mg sheets and ingots”By CSIRO (CC BY 3.0) via Commons Wikimedia

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