Ick, these are steel disk platters. So smashing them is out of the question. I've smashed the plastic platters before, but it ain't gonna happen on this drive!
They shouldn't be made from steel. Aluminum is common but glass/ceramic discs exist too (especially smaller drives). If the disks are metal chances are they are aluminum disks and that means that they have a low melting point
I don't think a microwave oven would be an appropriate heat source for this purpose (chances are you'd kill the microwave in the process). I was thinking of something along the lines of a blowtorch (with the appropriate precautions).