Which matreial is used to make the gears in the Poulan xt 17.5hp lawn tractor transmission?

I own a Poulan Model PO17542LT Lawn Tractor. The gears are becoming very hard to move. I neighbor greased the gear shift as the manual said and it loosened up but today I could not get it in reverse. Any suggestions?

They can be cast iron, steel, sintered metal or other materials.