My story involves sword of very high quality; they use lasers to cut, but have a metal reinforcement. The monsters the characters encounter and fight have lava as their 'life liquid'. If someone were to cut such a monster, the lava would seep out. This leads me to wonder: is there a metal with a high enough melting point to be resistant to lava?
Upon a very quick check it looks like titanium has a melting point of around 1600ºC and magma has an average temperature of around 700ºC to 1300ºC. Given that you could pretty quickly assume that as long as your characters aren't just running every enemy creature straight through and leaving their weapons inside their adversaries for long periods the swords would hold their tensile strength pretty well.
OK, I think melting point is the wrong thing to think about here. The first thing is specific heat capacity, which tells us how much heat energy is needed to raise the temperature of 1kg of a substance by 1 Kelvin.
So, a metal with high specific heat capacity can absorb more energy without ill effects. Here's a table to peruse.
Of course, this does have to be balanced against the other physical properties of the sword - to quote Donal Noye of the Night's Watch: "Robert was the true steel. Stannis is pure iron, black and hard and strong, yes, but brittle, the way iron gets. He'll break before he bends. And Renly, that one, he's copper, bright and shiny, pretty to look at but not worth all that much at the end of the day". So that will also have an influence on the metal you choose.
Secondly, we have to think about the construction of the sword. Many traditional designs balance just forward of the crossguard, which means a large proportion of their mass is in the hilt end. This can be used as a heat sink for the blade portion, allowing a quantity of energy to be absorbed with less effect to the whole.
There's a better alloy.
The exotic alloy, which is a combination of the rare metal hafnium, carbon and nitrogen, would only begin to melt into a liquid at temperatures of more than 4,126°C (7,460°F) - two thirds the temperature of the surface of the sun.
I think that would be strong enough to suit most purposes.
My perfect fantasy blade is made of -- real substance, not a joke -- transparent aluminum. Aluminium Oxynitride: Melting point of about 4000 deg F. Light weight, slightly flexible, and at 1" thick it is capable of parrying a 50 caliber bullet fired point blank.
Flowing lava typically burns around 1200 (red) to 1600 (orange) degrees Fahrenheit. Using this page at Engineering Toolbox.com to view the various melting points of metals and alloys, you can see that there are a lot of various options.
Some of these options include:
- Aluminum Bronze
- Red Brass
If you're looking for heat resistance, the first thing that comes to mind is a refractory material. Unfortunately, these are usually ceramics and somewhat brittle - but that's where composite materials come in!
You could have a ceramic sword with steel reinforcement inside - think of reinforced concrete. Or, a steel core with ceramic coating/insulation.
You're cutting through monsters, which implies fairly brief contact with the high-temperature lava. You're not submerging this sword in a lava pit. The ceramic outer layer should sufficiently protect the metal inside from the higher temperatures. It helps that ceramics have a significantly lower thermal conductivity than most metals, so it won't even absorb as much heat from its surroundings.
This also adds a cooldown element, if you want.
You're still going to want to avoid large shocks that might crack the ceramic. But you have a laser cutting edge anyway, and lava has a rather high viscosity - so it's unlikely to flow into small cracks.
This is also kind of a solved problem. Where else do we encounter high temperatures in the order of 1300ºC? Jet engines!
The top gas temperature in a modern jet turbine is more like 1500°C, and the turbine blades tolerate temperatures of around 1200°C.
Why to use bulk materials only?
You can use conventional material for the body, cover it with thermall barrier coating and protect it with hard coating.
So when you start with titanium blade, cober it with 200 um TBC and then cover it with 200 um alpha-alumina you will get weapon that sustain 2000 °C for reasonable time. The blade will be also very hard through wide range of temperatures. One can prepare coatings with hardness ranging from ~5 to ~15 GPa (HV 5000 ~ HV 15 000). Nanohardness in martensite vary from 0.2-1 GPa (HV 200 ~ HV 1000).
If you chose more complex material for the surface finish, you can top the hardness up to 30 GPa (HV 30 000)...
Does it have to be metal? Carbon fiber has a very high melting point. Bonus point, it would be light and a high quality carbon fiber blade could keep its shape for a long time.
Why not create a sword with internal channels that circulate a COOLING FLUID? Could be like the executioner sword from Gene Wolfe's "New Sun" that has a slug of mercury that can flow to the tip. But in this case it is liquid nitrogen or something, the sword hilt reservoir is filled right before battle.
The sword also doesn't have to be homogenous, it could be more like a Koa sword (Polynesian weapon with a wood "blade" embedded with shark tooth cutting edges). The "blade" is material with low heat conductivity and high strength and only the edges are high temp edged metal. This way as the metal edges lose temper and dull, they can easily be replaced after the battle but the weapon as a whole retains integrity and doesn't heat up because it has low conductivity. For bashing apart lava monsters this should suffice. Enough cutting ability to pierce the skin, then enough heat resistance to widen the opening and puncture any (?) internal organs.
Most metals we'd consider making a sword of can take a splash of lava. We're not talking about immersing the blade in a lava flow, here.
After any fight, swords require maintenance. Your friendly armorer will retemper the blade as necessary.
Take a look at the metal cups that are used to scoop up lava for analysis. Guessing they may just be stainless steel or wrought or cast nickel.
You could also actively cool by making passages and injecting compressed air.