"alloy" technically means any two metals combined. Steel is an alloy as is aluminum, nickel, almost any metal commercially used is actually an alloy. 14 karat gold is an alloy of pure gold, and some other metal to give it a bit more durability. For some reason when talking about bicycle materials, a lot of people mis-use the term alloy to mean aluminum.

Almost all cranks these days are made out of aluminum. You would be hard pressed to find a decent quality steel one. Carbon fiber ones are readily available for a price. So you don't really have that much choice about the material, it will almost certainly be aluminum, but there are a lot of different aluminum alloys used, and a lot of different processes used to manufacture them. Generally speaking a cold forged hollow arm crank will give the best combination of low weight, and stiffness, but you don't have to spend a fortune for a light crank. 1980s vintage Shimano 600 cranks for example are very light, durable, and inexpensive to buy used. Chucks bikes has a Shimano 105 crank (square taper) new for $30 right now.