eh? whats with this 50lbs number being thrown around? That would be so thick that it would just be a solid pole, more than a few inches wide to be 50 pounds at 5 feet.
To the OP, are you sure you really want the swords to be extremely heavy? Weight didn't do a lot for the sword, but add inertia that made it take longer to swing. Too big and it might even be destroyed by its own momentum, as it loses some of its ability to flex as it gets bigger.
Historically accurate blades weighed on average between 3.5 and 5.5 lbs.
To cut through sufficient steel armour (an average of 3 to 4 millimeteres) you would need stronger steel, not heavier. Your super soldiers are incredibly strong, so the extra weight would probably slow them down. The tungsten carbides would work, as would mithril or bologinum. Magically enhanced steel would work too.
If you want them to be able to just kill anyone inside of any armour, a really big flail works better. Armor was designed to turn any blow that didn't land at a right angle to its face off of the body, and the armor is shaped weird to stop right angle blows. So a good suit of steel is going to turn away a great deal of sword strikes, no matter who is swinging them.
Now stepping off of the soap box.
I'm in the planning-stages for a fantasy trilogy, which features among other things a group of magically-enhanced soldiers with super-strength and enormous swords that no ordinary human can wield. Now I'm wondering just how big and heavy to make them.
People used five foot swords easily, normally when they super size a solider they make him bigger so give him a bigger sword, so that he would have more reach against his enemies.
The idea is that the swords are created with a combination of metallurgy and magic to be super-dense and nearly unbreakable, so as not to snap when wielded with inhuman strength, or lose their edge when used to cleave armor. Can someone suggest to me, theoretically, how heavy a 5-foot steel blade would have to be, to be nearly indestructible?
Sword often did not break because of how they were made. Normally they would flex and twist to avoid breaking, this was due to a spring action given to them by the fullers and the ridges. To not break while being used with superhuman strength the blade would probably be much wider, to encompass more fullers and ridges. Nothing you do to normal steel will stop it from loosing its edge as it tries to cut through steel, or even bronze, armor. That will have to come from the magic.
So, in total. I'm giving the sword I'm thinking of a weight of 6 to 7 pounds.