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Representing alloys using particulate models

An alloy is a mixture of two or more elements, of which at least one is a metal. There are two main types of alloys: interstitial alloys, which form between atoms of different radii, and substitutional alloys, which form between atoms of similar radii. In this video, we'll learn how to represent the different types of alloys using particulate models. Created by Sal Khan.

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  • duskpin tree style avatar for user Natrium Chloride
    It was mentioned that adding other elements into metals change their properties. But how exactly does that work? For example, how does adding carbon make steel stronger?
    (4 votes)
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    • leaf red style avatar for user Richard
      As a pure material, many metals like iron are easily malleable and ductile meaning they can bent and worked easily into shapes like wires which is of course useful. This ability of metals being able to bend is due to them being able to essentially slide past each other even as a solid. However this makes them lacking when it comes to strength and we need a metal object to not bend under pressure. So when interstitial elements like carbon, nitrogen, hydrogen, or boron are added they are smaller than the metal atoms and fit into the small spaces, or interstices, formed by the metal lattice. These interstitial elements act as hardening agents which makes it more difficult for the metal atoms to slide past each other and in effect makes the alloy more brittle than the original pure metal solid.

      As far as steel goes, as stated this includes the addition of carbon as the interstitial element to iron to act as the hardening agent. The amount of carbon added to iron determines how malleable or brittle the alloy is. With little to no carbon in your iron, your alloy is very malleable like pure iron. With around 0.002% -2.14% by weight of carbon in your alloy you make steel. And with a carbon content higher than 2.14% by weight you create what is known as pig iron which becomes too brittle and begins to break instead of bend at all because there's too much carbon. So steel possesses that nice balance between malleable and brittle which makes it suitable for various human uses.

      Hope that helps.
      (16 votes)
  • starky ultimate style avatar for user Michele Franzoni
    In one of the exercises there was a question about an interstitial alloy made of iron and hydrogen. I googled it but i couldn't find nothing about it, is such an alloy a real thing? If so, does it have a name? What is it used for?
    (1 vote)
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    • leaf red style avatar for user Richard
      So an interstitial alloy means you have metal atoms forming an order lattice, but holes throughout the lattice in between the atoms where smaller atoms can fit into. These smaller atoms could be something like hydrogen, boron, or carbon. Steel is actually one of these alloys where iron atoms form the main lattice and carbon atoms fit into the holes to make steel stronger than pure iron.

      So at normal atmospheric pressure iron can form different ordered crystalline lattices at certain temperatures. At room temperature (~25 degrees centigrade) iron forms a body-centered cubic lattice. Imagine a cube of repeating units filled with iron atoms where each corner of the cube is an iron atom and at the center is another iron atom. Each of these iron atoms forming metallic bonds with each other. At this temperature only a small amount of hydrogen can be inserted into the holes to make an alloy, but with the body-centered cubic lattice it forms ferritic iron hydride. At higher temperatures (~910 degrees centigrade) iron turns into a different crystalline lattice called face-centered cubic. Still a cube like before with iron atoms at each of the cube's corners but now we have irons atoms in the center of the six faces of the cube instead. This form of iron can have more hydrogen atoms inserted into the holes when it forms an alloy which is called austenitic iron hydride. The inclusion of hydrogen into the lattice acts as a softening agent which makes the iron more ductile sand malleable.

      However these alloys only form when iron is in an atmosphere more saturated with hydrogen gas. Our normal atmosphere lacks this so when these iron-hydrogen alloys are exposed to it they release the hydrogen as hydrogen gas and revert back to regular iron. So because of this they have no use as structural materials. However it has been hypothesized that iron-hydrogen alloys compose the earth's core where pressure and temperatures are more extreme than normal conditions and may explain its low density. Hope that helps.
      (9 votes)
  • blobby green style avatar for user hallrandall614
    Don't metalloids work the same way as alloys do? Or are alloys just another word for metalloids?
    (2 votes)
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    • mr pants purple style avatar for user Ryan W
      Alloys are a mixture of metals, or a metal and other elements. Bronze for example is an alloy of copper and tin, brass is copper and zinc.
      They’re quite distinct from metalloids which is a grouping of elements with properties somewhere in between metals and nonmetals.
      (7 votes)
  • aqualine ultimate style avatar for user Max Qiu
    Does substitutional alloy counts as a metallic bond? Since we know that Chromium have joined the sea of electrons, so that should count as a metallic bond.
    (1 vote)
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Video transcript

- [Instructor] In many videos we have already talked about metals and metallic bonds. And in this video we're going to dig a little bit deeper and in particular, we're going to talk about alloys, which are mixtures of elements, but still have metallic properties. So first of all, what are metallic properties? Well those tend to be things like they're shiny, they reflect light. This is actually a pure iron sample right over here. You can see that it reflects light. It tends to be malleable, which means you can bend it without breaking it. And it tends to conduct electricity. And alloys are when you can mix multiple elements together and still have most of these properties. And just as a review of where these properties come from, we can imagine metallic bonds. And there's a whole video on this, but in metallic bonds, let's say we were to take a bunch of iron and you can see right over here, iron, Fe, it is a transition metal. And what happens with metals is, is when they form bonds with each other, they're valence electrons, because each of the atoms aren't that electronegative, they don't want to hog the electrons. They don't want them, just for themselves. They're willing to share their valence electrons into a bit of a communal pool of electrons. And so even though you have a bunch of neutral, let's say iron atoms, you could actually view them as positively charged ions in a sea of electrons. And so you have a bunch of electrons here. And where did these electrons come from? Well these are the valence electrons from the neutral atoms that get contributed to this sea. And this is why most metals are good at conducting electricity. This is why they are malleable. And depending on the metal, if you're talking about a Group one metal, you could imagine that the charge of these ions right over here would be a plus one. But if we're talking about a Group two metal or a transition metal, they have more valence electrons that they might be able to contribute to this pool. And so if you're thinking about these ions, they can even have a positive two charge or a positive three charge. But as promised in this video we're gonna talk about the notion of alloys. And we're going to do these particulate diagrams that we have seen in other videos. And in the particulate diagrams, we're not going to show this sea of electrons, but they're going to help us visualize the structure of the alloys. So let's imagine what iron could look like. And we're just going to look at a two-dimensional slice of a solid of iron, where all the iron atoms have formed metallic bonds. And as I said, we're not going to draw this sea of electrons, but they might form a pretty regular structure, something like this. And so each of these circles represent an iron atom. But as promised, this video is about alloys. So let's imagine what steel might look like. This is a steel blade and steel is a bunch of iron, so once again, we can visualize each of these as an iron atom, but mixed in with that iron is a little bit of carbon. And when you look at the periodic table of elements you can see that carbon is a good bit higher on the periodic table of elements and to the right of iron. Neutral iron has 26 protons and 26 electrons, and neutral carbon only has six protons and six electrons. The valence electrons in carbon are in their second shell. The valence electrons of iron are in the fourth shell. So carbon is a good bit smaller. And so, when you mix that carbon in, because it is smaller, it's able to fit in the gaps between the irons. So you might have, I'll draw this right here. You might have a little bit of carbon there. You might have a little bit of carbon there. You might have a little bit of carbon there. And so when you form an alloy, where one atom has a larger radius or a significantly larger radius than the other, you tend to form things like this, which are known as interstitial alloys, and basic carbon steel is a good example of it. Now you have other situations where you have alloys between atoms of similar size. And this right over here, this is a brass, I don't know if this is a clock or an astrolabe, or something like this, but brass is made up of a mix of copper and zinc. And so when you have an alloy like this, that's between atoms of similar radius, this is called a substitutional alloy. You can imagine that some of the copper has been substituted with zinc. So this is substitutional alloy. Now the last thing you might be wondering about is can you have a combination of both? And you indeed can. This is over here are panels on the International Space Station, and it's made out of stainless steel. You're likely to have stainless steel in your kitchen. And stainless steel, you could view it as it's basic steel but instead of just iron and carbon, It also has a little bit of chromium mixed in. And so we can visualize this. If this is stainless steel, maybe the blue ones, we say are iron, but it has a little bit of chromium. I'll do that with red. Chromium has a similar radius to iron. It's not exactly the same, but it is close. So maybe a little chromium there, a little bit of chromium right over there, a little bit of chromium right over there. And if it was just iron chromium we would call it substitutional, but it also has carbon and carbon has a smaller radius. So maybe a little bit of carbon fitting in the gaps between the larger atoms there. A little bit of carbon there. A little bit of carbon right over here. And so this is an example of an alloy, that is both interstitial and substitutional. Now one final question, you're like okay this is all interesting, but why have we decided to put things like carbon in iron? Well it turns out that even by putting a little bit of carbon in or mixing in with other metals, you're able to change the properties and for example, steel as an alloy, is much stronger than iron, by itself. And stainless steel once you mix that chromium in, it's much more resistant to corrosion, than basic steel. So I'll leave you there. You just learned a little bit more about metals and alloys.