Crystals can be used to purchase loot crates.
You need 110 crystals to buy the lowest cost crate, the hero crate. If you have less than that then you'll have to complete more achievements that reward crystals before you'll be able to use them.
In a way, yes crystals can be exchanged for credits. If you buy a loot crate with crystals you will be paid credits for any duplicates and there is a chance of getting credits as one of the items out of the crate. You are also guaranteed at least 35 crafting parts per crate as well.
