I’ve done a bit of research into alloys, how they form and what they are. I’ve come across two terms, insterstitial and substitutional, when classifying alloys. My understanding is that substitutional alloys are characterized by relatively higher ratios of alloying components, and typically form heterogeneouscompounds between elements with similar atomic radii, where the alloying component will replace atoms in the parent metal’s crystal structure. Examples of this would be bronzes, brasses, and pewter. The other kind of alloy is interstitial. My understanding is that they formwith the introduction of a small amount of a smaller atom, where the alloying component sits inside the crystal structure, steel being an interstitial alloy. An “average” carbon steel might have 1.5% carbon by mass whereas an “average” tin bronze might be 10% tin by mass, and zinc brasses can contain upwards of 30-40% zinc. My question is this: Is my understanding correct and what happens if you try to make a different type of alloy with metals like iron and copper? Is it possible to make an intetstitial alloy with copper as a base metal or a substitutional alloy with iron as a base? What are the properties of these alloys?
Just a few comments. I have never known the terms “substitutional” or “interstitial” applied to alloys. It usually applies to elements or atoms. You are correct in your usage, in that substitutional atom will replace an atom in the crystal structure, while an interstitial atom fits in the spaces between the lattice sites.
A carbon steel would not have 1.5% carbon. Maybe 0.15% C. Many low-alloy steels will have both interstitial and substitutional elements, say carbon as interstitial ann chromium and molybdenum as substitional.
In addition, to make it more complicated, in the case of carbon in steel, the amount of interstitial carbon you can have is dependent on temperature. At lower temperatures, you form iron carbide as a second-phase structure, but at higher temperatures, the crystal structure changes to one that will tolerate more carbon, a feature that allows steels to be heat treated to control mechanical properties.
Huh, I found a range of 0.8-2% as the range for carbon steel, with 0.15% being closer to a wrought iron
A carbon content of 0.80-2% is more what you would find in a cast iron. A wrought iron would probably be less than 0.10, but most ornamental wrought irons today are mostly just low-carbon steel. In general, a steel will be between 0.08 and 0.80 carbon. Over that, and a steel (not cast iron), and you are probably talking a tool steel.
FYI in the US, the last two digits of the steel designation give the carbon content. 1018 is a common mild steel and it is .18% carbon. 4130 is a low alloy with .30 carbon. Anything above .4 is considered to be hardenable but can be difficult to weld. The first two digits will indicate what type of alloying elements. Tool steels don’t follow this convention with usually a just letter and number.
An alloy is usually just a mixture. Lattice substitution or interstitial placement of either carbon or nitrogen is done. Re: carbon content. With powder metallurgy one can increase carbon way past 2% but mainly as metal carbides. My knife is made from zdp 189 (Hitachi) it has a lot of chromium carbide and some vanadium carbide mixed in. Rockwell hardness is 64 and total carbon is 3%.
really? one of the classes I took my first semester at university was materials science, and interstitial vs. substitutional alloys was covered like a month in. I still have the textbook. an example of an interstitial alloy was steel, where carbon atoms “sat” in the gaps between the iron atoms. A substitutional alloy would be something like brass, where zinc atoms would replace various copper atoms in the crystal lattice.
Iron and carbon don’t form an alloy. It is a chemical change. There is some lattice replacement for copper and zinc at the surface of the individual crystals but brass is mainly a jumble of zinc and copper crystals. A mixture, not a compound.
Well, I went into a little more depth than an intro to material science, earning a BS in Metallurgical Engineering, and add to that over 30 years in industry getting paid well for my understanding of metals, alloys, and their processing.
It could be that your textbook was a little loose with the terms, to help freshmen gain a basic understanding of how metals can be mixed (alloyed) to control properties. But, I can truthfully say that in the past 35 years dealing with these terms, I had never heard of an “interstitial alloy” before last Thursday. “Interstitial elements” used in producing alloys, yes, but not an “interstitial alloy”. If you look up the definition of the words, you’ll find that “interstitial alloy” doesn’t even make much sense, while “interstitial alloying element” does.
Technically, as I mentioned above, carbon does not even exist (much) as an interstitial element in the iron crystal structure at room temperature (and essentially never as a substitutional element). You can get about 0.008% C (or about 80 parts per million, based on mass) to exist interstitially in the iron structure at room temperature. Above that, you end up with iron carbide, a compound with the formula of Fe3C, where the carbon atom occupies a regular lattice position in the iron carbide crystal. The iron carbide will occur as a second-phase particle in the iron structure. While steel is often referred to as an alloy (mixture) of iron and carbon, it is more correctly an alloy (mixture) of iron and iron carbide.
Stainless steels (specifically, austenitic stainless steels) are a better illustration of the use of interstitial alloying elements. Because of high amounts of substitutional elements such as chromium and nickel, you can get much higher amounts of carbon and/or nitrogen to exist interstitially in the austenitic structure, and they can greatly influence the strength of the resulting alloy. Nitrogen is much more useful in this than carbon since carbon has a strong tendency to form carbides with some of the (substitutional) alloying elements, such as chromium and/or molybdenum. If the Cr and Mo are tied up as carbides, they cannot exist as substitutional elements in the iron crystal structure, so the carbon can essentially “de-alloy” the stainless steel, usually with detrimental results. While nitrogen will form nitrides, the elements that easily form nitrides (notably aluminum and vanadium) are not as common as Cr and Mo in stainless steels. For this reason, nitrogen in stainless steel is a better illustration of the use of an interstitial alloying element than carbon. But, because stainless steel makes much more use of substitutional alloying elements, trying to call it an “interstitial alloy” would be confusing. Like I said to begin with, the term “interstitial alloy” really does not make sense.
After looking up 41xx steel on wikipedia, I understand where the confusion arises. I was talking about carbon steel- being steel that consists of just carbon and iron. Many modern steels contain levels of other elements like manganese or silicon. Because of the presence of the other elements, the % of carbon is lower. I believe that in simple steels with just carbon and iron, the carbon content is higher.
Came along this by accident via Google, but I do not like to let the last answer stand: None of what excavating says below is incorrect, but I want to mention that - coming more from chemistry than metallurgy - the terms interstitial alloy and substitutional alloy are completely correct. Different fields, slightly different nomenclature.
I. e. chromium and iron provide a substitutional alloy, i. e. Cr occupies the place of iron in the lattice. Carbon and iron form an interstitial alloy, since Carbon occupies the interstitital spaces (= holes) in the lattice. Both can happen at the same time, i. e. when you mix Carbon, Chromium and Iron.
Iron carbide (or cementite), Fe3C, is an intermetallic compound with a structure different from that of iron. Neither iron nor carbon is here a substitutional or interstitial element… this is a new compound with (relatively) fixed stoichiometry.