My Hero Academia- Yaoyorozu and Nitinol
In season 2 episode 22 of My Hero Academia Todoroki and Yaoyorozu must battle against Aizawa during their final exams. Now the teacher in me can’t really see how having students fight against their teacher in any way counts as a final exam, but this is anime so I’ll leave that discussion for another time. The real reason that I am creating this post is that Yaoyorozu’s plan to capture Aizawa involves using Nitinol, which she describes as a shape memory alloy that returns to its original shape when heated. This might surprise some of you, but, yes, this is actually a real metal that does exist and is used in the real world, unlike the other substances I have written about in the past, Gundarium and Sakuradite.
Nitinol- Nickel titanium alloy
As I mentioned in my Gundarium post an alloy is a mixture of two or more metals, and the resulting material has its own unique set of properties separate from the elements used to create it.
Properties of Nitinol
What makes Nitinol so special is that it is a shape memory alloy. Shape memory alloys are metals that return to their original shapes when heated. While the idea of memory metals began to be studied in the 1930’s, the first memory metals weren’t made until the 1960’s, and as a fortunate accident to boot. I am not a chemist so I’m only going to be giving some broad strokes on the subject, and any chemists feel free to chime in.
On a basic level what shape memory means is that when the metal is heated above a certain temperature, it will return to its original shape. It is possible using various manufacturing processes to determine the shape the metal will return to and the temperature at which this happens. The process of determining the metal’s original shape is called training and occurs by heating the metal to 400-500 degrees Celsius for a period of time and then rapidly cooling the metal while holding the metal in the desired shape. The temperature is important because it removes deformities in the crystal structure of the metal without changing its overall structure.
I was unable to determine how the temperature needed to make the metal return to its original form was set, but I know it is something that can be changed. This is because anyone who is wearing those indestructible bendable glasses, sometimes marketed as Flexon, are wearing glasses made with a memory metal. In this case the metal of the frames has the temperature needed to return to its original shape set below room temperature. This why you can sit on them and they return to normal.
Chemical Composition of Nitinol
In the case of Nitinol, the nickel and titanium are present in equal amounts. However, it is not quite that simple, because 1kg of nickel, and 1kg of titanium would not be an equal amount.
“But Mr. Anime Science isn’t 1kg equal to 1kg?”
Yes, of course it is, but titanium is lighter than nickel, so 1kg of titanium contains 19% more atoms than nickel does. For the alloy to be created and function properly there needs to be an equal number of titanium and nickel atoms. Yes, it is possible determine the exact amounts needed to have a 1 to 1 ratio of nickel and titanium atoms. I don’t think that Yaoyorozu is doing this in her head, but she probably did the math ahead of time as she needs to know the exact make up of the material she is going to make using her creation quirk. Let’s assume that Yaoyorozu wants to make 2kg of Nitinol wire to restrain Aizawa, so what are the exact amounts of nickel and titanium needed to make the wire?
The mole is simply a unit of measurement that is used in chemistry, like a dozen eggs is used in baking. Unlike baking where a dozen means 12, a mole stands for the number of atoms in 12 grams of carbon 12, which comes out to be 6.022×10^23 (Avogadro’s number), which I realize is an insanely huge number, but it has its benefits. By using moles, you can determine the exact number of atoms you need to make a particular reaction occur, or to make a particular substance, and this greatly improves the accuracy and precision of measurements, experiments, and production processes.
While I know that there is math involved, it’s not quite as complicated as you might imagine. The first step is to determine how many moles of Nitinol are in 2kg of Nitinol. To do this you need the molecular formula of Nitinol, which is NiTi. FYI- this explains the name: Nickel, Titanium, or NiTi, and the nol part is short hand for Naval Ordinance Laboratory, where some of the original research on Nickel Titanium alloys was done. Knowing the formula for Nitinol allows us to determine the weight in grams of one mole of Nitinol by adding up the atomic mass of all of the atoms in the formula.
One mole, or 6.022 x10^23 atoms of an element, is equal to its atomic mass listed on the periodic table, so the weight in grams of 1 moles of Nitinol (NiTi) is equal to the atomic mass of Nickel plus the atomic mass of Titanium.
Atomic Mass of Nickel + Atomic Mass of Titanium = Molecular Mass of Nitinol
Ni- 58.7g + Ti- 47.9g = NiTi- 106.6g
Next you divide the total mass of Nitinol Yaoyorozu created, 2kg, by the mass of one mole of Nitinol, 106.6g, to determine how many moles are in 2kg.
2kg = 2000g
Mass of substance/ Mass of one mole of the substance = # of moles of the substance
2000g NiTi/ 1 mole of NiTi 106.6g = 18.8 moles of NiTi
Now for the easy part- since Nickel and Titanium exist in a one to one ratio in Nitinol, Yaoyorozu needs 18.8 moles of Nickel, and 18.8 moles of Titanium to make 18.8 moles of Nitinol. Working backwards we can determine how many grams of nickel and titanium she needs to make the 2kg of Nitinol.
Moles of Nickel x Molar mass of Nickel = grams of Nickel
18.8 x 58.7 = 1,103.6 grams of Nickel
Moles of Titanium x Molar mass of Titanium = grams of Titanium
18.8 x 47.9 = 900.5 grams of Titanium
Thus, Yaoyorozu has to combine 1,103.6 grams of Nickel, and 900.5 grams of Titanium to make her 2kg of Nitinol to capture Aizawa.
But how does she actually make it?
Yaoyorozu’s quirk is called creation and it is not one that I have covered in my previous posts on the various quirks of My Hero Academica, Part 1, and Part 2. The reason I didn’t is that there is no biological basis for her to be able to secrete metal ribbon from her chest. That being said however, the manga states that she can manipulate her fat cells to create any material that she needs, which means that she is essentially playing with Einstein’s famous equation E=mc^2.
Using our previous example let’s consider her creation of 2kg of Nitinol. Using Einstein’s formula, we can determine the amount of energy her fat cells need in order to create the Nitinol alloy.
E = MC^2
E = 2kg x (3×10^8 m/s) x (3×10^8 m/s)
E = 1.8 x 10^17 joules or 7.5 x10^17 calories
Now the question becomes how many pounds of fat does she need to burn to create 7.531 x10^17 calories.
7.5 x 10^17 calories / 3,500 calories in a pound = 2.2 x10^14 pounds
Ok, so clearly Yaoyorozu is using some kind of sparkly magical hero bullshit, because there is no way she weighs as much as some asteroids. Clearly she is converting 2kg of body fat into Nitinol through some quirky magical process that can not be explained by modern physics or chemistry.
FYI- Just in case anyone out there is thinking, but couldn’t she have used the titanium and nickel in her own body to make the metal? Let me explain a few things. The average human body has around 0.8milligrams of titanium, but that is only just passing through the body as the body has no use for titanium and it was absorbed by accident. Also, while it is not toxic even in large doses it might contribute to yellow nail syndrome. Nickel on the other hand, may have a role in the symbiotic bacteria that live in our digestive systems. Unlike titanium nickel is toxic to humans in large doses, and causes skin, eye, and lung problems. Additionally, some nickel compounds are carcinogenic, clearly not something anyone would want to expose themselves to.
Yes, Nitinol is a real metal that will change shape when heated, but My Hero Academia probably took some liberties as all of the Nitinol I could find was sold as a wire of some sort and not a fabric like rope. Also, while I like Yaoyozoru and her creation ability, I can’t see any way in which it is anywhere close to being realistic, not that this should really surprise anyone. The capture of Aizawa by Shoto and Momo is completely busted, even if it is a really cool scene.