What were US nickels made of in 1943?

All 1943 US nickels were struck in an alloy of 56% copper, 35% silver and 9% manganese.

Nickel metal was needed for the war effort, so special "war nickels" were minted from October 1942 to December 1945. They can be identified by a somewhat darker color and a large mintmark letter over the dome of Monticello. internet rumors to the contrary, these are the only US nickels that ever contained any silver.