What makes some metals stronger than others?

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Why do we use steel instead of just normal iron or aluminum or something?

In: Engineering
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“Stronger” is a nebulus term. Do you want strength in relation to mass, or absolute strength? Do you want tensile strength, compressive strength, shear strength? All these properties differ from metal to metal.

However, why is steel stronger than wrought iron, for example? It’s all to do with the [crystal structure](https://www.lff-group.com/posts/carbon-steel-fundamentals-part-1#:~:text=Carbon%20steel%20is%20fundamentally%20an%20alloy%20of%20iron%20and%20carbon.&text=Below%200.008%20wt%25%2C%20the%20structure,the%20centre%20(Figure%201).) of the metals. In basic terms, every so often in your steel, you hit a “wall” of carbon, which keeps the iron from shifting and moving about, making the steel harder.

The same is true of steel-vs-aluminum, the crystal structures of steel is better at staying static with tensile stresses (about 5x better), so in cases where your part is under tension, and weight is not an issue, we will use steel over aluminum.

Metal atoms are organized by arranging themselves into many repeated pieces called [unit cells](https://www.merriam-webster.com/dictionary/unit%20cell). Pure metals such as iron, gold, tin, and aluminum only have one type of atom in their unit cell structure. For example, a piece of aluminum foil is made up of tons of organized aluminum atoms. Metallic alloys, such as steel, brass, and bronze all have more than one type of element in their structures. For example, steel has a mostly iron structure with carbon atoms dispersed throughout.

The different crystal structures of alloys and pure metals directly affect their physical properties, such as strength or hardness. For example, pure iron (a.k.a Ferrite) forms a [body-centered cubic](https://www.lff-group.com/posts/carbon-steel-fundamentals-part-1) (BCC) unit cell, a cube with an iron atom in the center and atoms at each corner. The unit cell of steel is called [face-centered cubic](https://www.lff-group.com/posts/carbon-steel-fundamentals-part-1) (FCC) with atoms on all corners as well as faces.

Generally speaking, pure metals have BCC structures and alloys have FCC structures. The Bitesize webpage of the British Broadcasting Corporation states that alloys are [harder and stronger](https://www.bbc.co.uk/bitesize/guides/z8db7p3/revision/2) than pure metals because there are more atoms occupying the FCC structure than the BCC structure. As seen on Professor Marzari’s illustrations of unit cells in her MIT [lecture slides](https://ocw.mit.edu/courses/materials-science-and-engineering/3-012-fundamentals-of-materials-science-fall-2005/lecture-notes/lec16b_note.pdf), there are two total atoms in the BCC unit cell and four total atoms in the FCC unit cell. This difference in microstructures results in a “[greater force required [to distort] the layers](https://www.bbc.co.uk/bitesize/guides/z8db7p3/revision/2)” of atoms in alloys than in elemental metals. In other words a higher concentration of atoms leads to a stronger material. This also explains why an elemental metal like titanium, [which has a hexagonal close-packed crystal structure](https://www.phase-trans.msm.cam.ac.uk/2004/titanium/titanium.html) (HCP) is so strong. HCP has a very tightly packed structure making it difficult to pull the atoms and move them around. There are many other factors that cause metals to be stronger than others, such as oxidation, temperature, and heat treatment, but the atomic structure is the most basic and important reason.