If it's only made of aluminium (100%) then it is not an example of an alloy as alloys are always made up of 2 or more different elements. However, some pie plates may not be 100% aluminium, for example they might have small percentages of other elements in too in order to make it stronger and able to support a heavier pie. So a purely aluminium pie plate is not an alloy, but if you think it has other elements in to make it stronger then it would be an alloy.