Fawkes' Super Sledge

Fawkes' Super Sledge is a Super Sledge equipped by Fawkes while escaping Vault 87 with her. She also will use it later on if you decide to have her join you after escaping Raven Rock. To obtain it, kill her or accept her as an ally and trade equipment with her. If you choose to take it, Fawkes is equipped with a normal sledgehammer. You may also pickpocket one from her. (She appears to have two in her inventory when pickpocketing.)

Also, if your karma is not high enough to get her in your party and you don't want to kill her or pick her pocket, wait for one of the super mutants in Vault 87 to knock it out of her hands. After the fight she will usually pick up the weapon that the super mutant used or the aforementioned sledgehammer. Then you are free to pick it up off of the ground