DUCTILITY: PROPERTIES, EXAMPLES, EXPERIMENTS - CHEMISTRY - 2024

The ductility is a proprietary technology of materials allowing them to deform a stretch stress; that is, the separation of its two ends without there being a prompt fracture at some point in the middle of the elongated section. As the material elongates, its cross section decreases, becoming thinner.

Therefore, ductile materials are mechanically worked into threadlike shapes (threads, cables, needles, etc.). In sewing machines, bobbins with wound threads represent a homemade example of ductile materials; otherwise the textile fibers could never have acquired their characteristic shapes.

Source: Emilian Robert Vicol via Flickr.

What is the purpose of ductility in materials? That of being able to cover long distances or attractive designs, whether for making tools, jewelry, toys; or for the transport of some fluid, such as electric current.

The last application represents a key example of the ductility of materials, especially that of metals. Fine copper wires (top image) are good conductors of electricity, and along with gold and platinum, they are used in many electronic devices to ensure their operation.

Some fibers are so fine (just a few micrometers thick) that the poetic phrase "golden hair" takes on all real meaning. The same goes for copper and silver.

Ductility would not be a possible property if there were not a molecular or atomic rearrangement to counteract the incident tensile force. And if it did not exist, man would never have known the cables, the antennas, the bridges, would disappear, and the world would remain in darkness without electric light (in addition to innumerable other consequences).

What is ductility?

Unlike malleability, ductility warrants more efficient structural rearrangement.

Why? Because when the surface where the tension lies is greater, the solid has more means of sliding its molecules or atoms, forming sheets or plates; whereas when the stress is concentrated in a smaller and smaller cross section, the molecular sliding must be more efficient to counteract this force.

Not all solids or materials can do it, and for that reason they break when subjected to tensile tests. The breaks obtained are on average horizontal, while those of ductile materials are conical or pointed, a sign of stretching.

Ductile materials can also break past a stress point. This can be increased if the temperature is increased, since heat promotes and facilitates molecular slippage (although there are several exceptions). It is then thanks to these slides that a material can exhibit ductility and be ductile.

However, the ductility of a material encompasses other variables, such as humidity, heat, impurities, and how force is applied. For example, newly molten glass is ductile, adopting threadlike shapes; But as it cools, it becomes brittle and can break with any mechanical impact.

Properties

Ductile materials have their own properties directly related to their molecular arrangements. In this sense, a rigid metal rod and a wet clay rod can be ductile, even though their properties differ greatly.

However, they all have something in common: a plastic behavior before breaking. What is the difference between a plastic and an elastic object?

The elastic object is reversibly deformed, which initially occurs with ductile materials; but increasing the tensile force, the deformation becomes irreversible and the object becomes plastic.

From this point on, the wire or thread takes on a defined shape. After continuous stretching, its cross section becomes so small, and the tensile stress too high, that its molecular slides can no longer counteract the stress and it ends up breaking.

If the ductility of the material is extremely high, as in the case of gold, with one gram it is possible to obtain wires with lengths of up to 66 km, with 1 µm thickness.

The more elongated the wire obtained from a mass, the smaller its cross section (unless tons of gold are available to build a wire of considerable thickness).

Examples of ductile metals

Metals are among the ductile materials with countless applications. The triad is made up of metals: gold, copper and platinum. One is gold, the other pinkish orange, and the last silver. In addition to these metals, there are others of less ductility:

-Iron

-Zinc

-Brass (and other metal alloys)

-Gold

-Aluminum

-Samarium

-Magnesium

-Vanadium

-Steel (although its ductility may be affected depending on its carbon composition and other additives)

-Silver

-Tin

-Lead (but within certain small temperature ranges)

It is difficult to ascertain, without prior experimental knowledge, which metals are really ductile. Its ductility depends on the degree of purity and how the additives interact with the metallic glass.

Also, other variables such as the size of the crystal grains and the arrangement of the crystal come into consideration. In addition, the number of electrons and molecular orbitals involved in the metallic bond, that is, in the "electron sea" also plays an important role.

The interactions between all these microscopic and electronic variables make ductility a concept that must be thoroughly addressed with a multivariate analysis; and the absence of a standard rule for all metals will be found.

It is for this reason that two metals, although with very similar characteristics, may or may not be ductile.

Grain size and crystal structures of metals

The grains are portions of glass that lack noticeable irregularities (voids) in their three-dimensional arrangements. Ideally, they should be completely symmetrical, with their very well defined structure.

Each grain for the same metal has the same crystalline structure; that is, a metal with a compact hexagonal structure, hcp, has grains with crystals with the hcp system. These are arranged in such a way that before the force of traction or stretching they slide over each other, as if they were planes made up of marbles.

Generally, when planes made of small grains slide, they must overcome a greater friction force; while if they are large, they can move more freely. In fact, some researchers seek to modify the ductility of certain alloys through the controlled growth of their crystalline grains.

On the other hand, regarding the crystalline structure, usually metals with a crystalline system fcc (faced centered cubic, or cubic centered on the faces) are the most ductile. Meanwhile, metals with crystalline structures bcc (body centered cubic, cubic centered on the faces) or hcp, tend to be less ductile.

For example, both copper and iron crystallize with an fcc arrangement, and are more ductile than zinc and cobalt, both with hcp arrangements.

Effect of temperature on the ductility of metals

Heat can decrease or increase the ductility of materials, and the exceptions apply to metals as well. However, as a general rule, the softer metals are, the easier it is to turn them into threads without breaking.

This is due to the fact that the increase in temperature makes the metallic atoms vibrate, which consequently causes the grains to unify; that is, several small grains come together to form one large grain.

With larger grains, ductility increases, and molecular slippage faces fewer physical impediments.

Experiment to explain ductility for children and adolescents

Source: Doug Waldron via Flickr.

Ductility becomes an extremely complex concept if you start to analyze it microscopically. So how do you explain it to children and adolescents? In such a way that it appears as simple as possible to their prying eyes.

Chewing gum and play dough

So far there has been talk of molten glass and metals, but there are other incredibly ductile materials: chewing gum and modeling clay.

To demonstrate the ductility of chewing gum, just grab two masses and start stretching them; one located to the left, and the other will be carried to the right. The result will be a hanging gum bridge, which will not be able to return to its original shape unless it is kneaded with the hands.

However, there will come a point where the bridge will eventually break (and the floor will be stained with gum).

The image above shows how a child by pressing a container with holes makes the plasticine emerge as if it were hair. Dry putty is less ductile than oily putty; Therefore, an experiment could simply consist of creating two worms: one with the dry clay, and the other moistened in oil.

The child will notice that the oily worm is easier to mold and to gain length at the expense of its thickness; While the worm dries up, it is likely to end up breaking down several times.

Plasticine also represents an ideal material to explain the difference between malleability (a boat, a gate) and ductility (hair, worms, snakes, salamanders, etc.).

Demonstration with metals

Although adolescents will not manipulate anything at all, being able to witness the formation of copper wires in the first row can be an attractive and interesting experience for them. The demonstration of ductility would be even more complete if one proceeds with other metals, and thus be able to compare their ductility.

Next, all the wires must be subjected to a constant stretching to their breaking point. With this, the adolescent will visually certify how ductility influences the resistance of the wire to break.

References

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