What years were nickels struck in silver?

The US made special "war nickels" from late 1942 to the end of 1945. They were made of an alloy of 56% copper, 35% silver, and 9% manganese due to wartime metal shortages, and can be identified by a large mint mark letter over the dome of Monticello on the back.

Regardless of date, all other US nickels are made of an alloy of 25% nickel and 75% copper.