What were us nickels made of in 1940?

In 1940 US nickels were composed of 75% copper and 25% nickel.

In fact, all US nickels except special "war nickels" made from late 1942 to 1945 are struck in that same alloy. War nickels were made of an alloy of 56% copper, 35% silver, and 9% manganese because nickel metal was needed for the war effort. These coins are distinguished by a large mint mark letter over the dome of Monticello on the back.