Could cobalt make a good sword? and what knownk alloy would be the closest to it?

Question

The setting is semi modern urban fantasy where there are mercenaries with the task of slaying monsters on the unpopulated areas, steel is the first metal to go when it comes to making weapons on large scale or for those who don't have the money for better materials. But because almost everyone and most of monsters can potentially bend fire, spit corrosive fluids etc. Making so the best weapon material that one those mercenaries could use must be better than classic spring steel in those cases; My best bet is to introduce cobalt weapons, I've read about the properties of cobalt and somewhat they match, but I'm not that sure of how good it would be for weapons like swords. Did a bit of research and found Elgiloy which is an alloy that's used for springs and from what I know spring metal works well especially for swords. But I'm not sure of how good would that work.

Answer 1

The main advantage of using iron alloys for making swords is that the properties of the material can be locally tuned to improve the global performance of the blade.

When you look at how a blade works, you want to have:

• a hard surface, to be able to resist the impact with the target
• a tenacious core, to be able to withstand the solicitations imparted by the blow

Now, a single alloy can be hard and brittle or tenacious and softer, and you see that you will have to compromise too much to make a good sword.

Using iron based alloys instead one can use I.e. quenching or carbon hardening to have a hard surface while maintaining a tenacious core, dramatically improving the performances of the blade.

From what I see, cobalt and its alloys are generally hard and resistant to high temperature, but that alone would make the cobalt blade brittle. And you don't want a sword that shatters on impact, don't you?

Answer 2

In additional to its brittleness, the oxidation products of cobalt are toxic. Not in a "poisoned sword that keeps on poisoning" sort of way, more in a "use this thing long enough, and you'll get deathly ill and may die" manner.

Now, there are cobalt alloys (with iron, which makes them steels) that work well, but in general, you want a carbon steel for weapons. It can be hardened by heat treatment, and tempered to make the hard edge and tough core that makes a sword durable. Adding a little molybdenum helps a lot for toughness, while cobalt can help with corrosion (not as much as chromium and nickel, though -- those go into stainless).

And contrary to the movies, there are no acids that will eat away your blade in the blink of an eye. Get a splash of concentrate nitric, sulfuric, or hydrochloric acid on the blade, and if you don't just let it sit there and eat away at the surface you'll get local darkening and etching (the kind that brings out the grain structure in a polished surface) -- not wholesale destruction.

Answer 3

It would definitely be hard to do.

just finding the material is going to be way harder than finding iron.

I found this through Wikipedia, definitely worth checking out this graph

You can see that Iron (Fe) has about 10^5 atoms for every 10^6 atoms of Si, whereas Cobalt (Co) has about 10^1 atoms per 10^6 atoms of Si.

1,000 Atoms of Fe for every atom of Co.

Let's talk melting point now.

Iron has a melting point of 1,538C. Cobalt has a melting point of 1,495C. that is only about a 40-degree difference. Therefore we will say it has no impact on anything essential.

Cobalt has a density of 8.86g/cm^3 whereas Iron has a density of 7.874g/cm^3. Let's remember that casting cobalt means you're probably going to have to anneal it as well.

after all of this work, you'd think it would be worth it right?

Absolutely not.

Cobalt is extremely brittle so if you decide to try to sharpen it we can assume it would just shatter into a million pieces.

Shortened Answer:

You'd be putting in too much time and effort to A) Mine cobalt, B) Melt it, C) anneal it, and D) Sharpen it, just to be presented with a heavy blade that would slowly poison you over time. Not worth all that effort if it's gonna kill you.