I saw Bill got the first response in. He is the site expert, too. I saw your video. Very impressive. I study the theory a lot, but I envy people who can build such fine little machines.
As for graphite? I have seen a number of thermoacoustic Stirlings that only use graphite for some reason I have to assume it good. The displacer and power piston are always one-in-the-same on these. Power output is so low, I guess the graphite is strong enough, light enough, and self lubricates the cylinder.
Look up the thermoacoutics on the web, and you will find much about the preference on graphite.
Good luck, and keep those way cool videos coming!