Anatomy of an Alloy

Not a brass farthing. I hope this post is worth more than a brass farthing. Oh, Audrey Hepburn, always playing the most loveable roles. (Are you wondering if this is a science blog or not? Haha, me to.) I watched this video from MEL science and it made me think of the scene in My Fair Lady when Eliza tells her father “Not a brass farthing”. Just another day in the life of Me; making connections that aren’t really there.

So, why do you need this post? Well, because you are in desperate need of an understanding of alloys since you use them in your everyday life…and because they are really cool, especially shape memory alloys. Don’t you agree?

Well, let’s get started…..

Alloys are different forms of a metal. Not a compound with that metal in it (i.e. rust is iron (III) oxide and it is definitely not an alloy). Alloys are a mixture of one metal and at least one other element. This mixture is so well mixed that the atoms of “the other element(s)” fit themselves into the crystal structure of the metal (that takes a lot of stirring but mostly heat).  Did you know that metals are just crystals? Check out my growing crystals post here.

There are two basic categories of alloys, interstitial and substitutional. Alloys are considered interstitial if the element being mixed in occupies empty space in the crystal lattice. Substitutional alloys are mixtures in which the metal and “other element(s)” are similar in atomic size and “the other element(s)” replaces metal atoms in the crystal lattice.



Alloys have been used for centuries, nay millennia. Steel was developed in the iron age (hmm, imagine that). Since it seems to have been around the longest, we can start there.

 

Iron vs. Steel

So, is steel, iron? or is iron, steel? Iron and steel are like multiple personalities of the same person. Steel is made when Carbon is forced into the crystal structure of iron. (I think steel is iron)

The individual iron atoms arrange themselves into a body-centered cubic (BCC) structure. (Just think of a cube and individual iron atoms at each corner and one more in the very center of that cube (or body) all bonded together.)

You know you want to try this: Use some soft candy and toothpicks to create a visual for yourself.

When that spatial pattern repeats, then you get a nice sample of pure iron. If you heat up this pure iron to around 1000 degrees F those iron atoms start to move their structure and create a face-centered cubic crystal that is more space efficient (the atoms are packed in pretty much as close as they can get.) That means instead of an atom in the middle of the cube there are additional atoms on each of the faces of the cube (Thankfully, for once in science this naming system makes obvious sense).

Even though this is a more compact structure there are actually more spaces available to squeeze in some carbon atoms, which will change the nature of that sample of pure iron we started with.

Body centered cubic modelface centered cubic model

Can you guess in which category of alloy steel would fit?

Not a compound

I’m not talking about creating iron carbide, just steel. What is the difference you say? No electrons are involved in creating an alloy. Iron carbide is a compound that is created with an exchange of electrons. Steel can have iron carbide in it, but steel is just a solid mixture of carbon and Iron and some other elements and/or compounds.

Properties

Mixing things up in this way, allows for a change in the chemical and physical properties of metals without completely changing the identity of the substance. Thus, Iron and steel are two different “personalities” of the same substance. Pure iron is grey and soft and kinda ductile and malleable but not enough to really be useful. Whereas, steel is strong and quite malleable and can be used in all sorts of applications. (See? Multiple personalities.)

pssst….steel is an interstitial alloy.  Just in case you were wondering.

Nitinol

Another cubic crystal structure alloy is nitinol. Nitinol is composed of nearly equal parts nickel and titanium. The sizes of these atoms are similar and therefore create a substitutional alloy. It is a nickel-titanium alloy but it is a special one because it is known as a shape memory alloy.

When Nitinol is heated, its crystal structure moves about, similar to the process that iron undergoes when heated to create steel but now we are heating the alloy not the pure element. This heated crystal structure is its “happy state”. Once cooled, nitinol will look the same but you can bend and twist it however you want. When it is heated again to the right temperature it will “remember” its “happy crystal structure” which causes it to actually change shape on a large scale, not just an atomic one. AMAZING!

This is the part where people usually think, “That’s cool and all, but where am I ever going to see or use nitinol in real-life?” You know those eyeglasses that claim to be able to get smashed or twisted out of shape and then go right back to where they are supposed to be? Their frame is made up of nitinol! There are a bunch of other uses that are mostly medical, like stents, and I hope you don’t have to see nitinol in those applications. If you do find yourself in some kind of medical situation involving an alloy, perhaps it will be nitinol.

Brass

Now, back to the beginning. A brass farthing doesn’t exist. They were made of copper or bronze which explains a lot about this saying, but let’s get on to the chemistry of brass since that is why we are all here.

Brass is a substitutional alloy made of copper and zinc. Once again the atoms are similar in sizes. (if you don’t believe me, check the periodic table. Copper and Zinc are right next to each other which indicates that they are very similar in size)

Different amounts of zinc added to the copper will yield differing brasses.

This alloy is so common that you can actually try to make your own. YouTube has a bunch of videos on how to do it. Basically, all you need to do is coat a piece of copper with some zinc dust in a warm sodium hydroxide solution. Once it is coated, heat it up so that those zinc atoms can move into the copper crystal structure. Flinn Scientific suggests a safer version with zinc pellets and zinc chloride solution.  Here are a couple of videos that I like. It’s pretty cool and definitely worth trying if you are up for it.

https://www.youtube.com/watch?v=bhus7L3cNrE

https://www.youtube.com/watch?v=P_mj3xBAQxQ&t=149s

 

US pennies are actually zinc coated in copper. (You could cut one in half to find out) I wonder if we could use that zinc and copper already in the penny to create brass. Remember heat is the key. Also, if you change it to brass you can’t use it to buy anything (wait, could you use it to buy anything anyway? When was the last time you used a penny to buy something?)

 

So how do you feel?

Are there other alloys that you are familiar with? Do you need to go research what is in them? And whether they are substitutional or interstitial? Oh good. I thought that might be the result of this; a revelation of your subconscious need for more chemistry in your life.

 

What are you going to do now?

  • Leave a comment below about your preferred alloy.
  • Share this post on facebook, twitter, or pinterest.
  • Create your own crystal structures using candy or playdough and post the pics with a link to this post.

 

Disclaimer: I am not liable or responsible in any way for any of your actions or consequences thereof relating to this post. If you try any experiments, be sure to use safe lab practices and disposal techniques.

Info on the alloys nitinol, steel and brass.

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