Reading the PHB chapter 5 equipment pg. 151. It describes an arcane focus as such:
Arcane Focus. An arcane focus is a special item — an orb, a crystal, a rod, a specially constructed staff, a wand like length of wood, or some similar item — designed to channel the power of arcane spells. A sorcerer, warlock, or wizard can use such an item as a spellcasting focus, as described in chapter 10.
Now I have been coming up with a dwarven war mage character concept, and was curious to know would it be allowed to have an arcane focus as warhammer?