Answer:
All US nickels from 1866 (when the coin was first made) to mid-1942, and from 1946 to today are made of an alloy of 25% nickel and 75% copper.
During WWII nickel was a strategic metal, so from mid-1942 to the end of 1945 US nickels didn't contain any nickel metal. They were made of an alloy containing 56% copper, 35% silver, and 9% manganese. These coins can be identified by the large mint mark letter above the dome of Monticello.