Rheanimum

Rheanimum is a substance is in the Generation 1 continuity family.

Would you stay if she promised you heaven? Will you ever win?

Rheanimum is a rare gas that is mined on Tykos, a moon of E'Brutoc. It improves the density and durability of metals by filling the gaps between molecules, strengthening the material to near-indestructibility. Handy stuff if you're made of metal. Behind the scenes, the fictitious Rheanimum is possibly based on the rare high-density transition metal Rhenium.

Fiction

Marvel Comics continuity

Megatron led his newest warriors (the Laser Rods and Rotor Force) to capture the rheanimum stores on Tykos, intending to use the plundered substance to render his army unstoppable. New Dawn

When the Swarm attacked the combined Autobot-Decepticon forces on Earth, the rheanimum was instead distributed among the allied Transformers, in hopes that its properties would render them invulnerable to the Swarm. It worked. However, in Jhiaxus' flagship in Earth orbit, Optimus Prime refused the rheanimum offered to him by Starscream; in order for his desperate gamble to work, he needed to be consumed by the Swarm! A Rage in Heaven!

TransTech

The processing center that provides ingress to Axiom Nexus has (among other defenses) rheanimum-reinforced walls. Withered Hope

Transformers Animated

Omega Supreme's armor is enhanced with rheanimum. AllSpark Almanac