If reports from Japanese site Macotakara are to be beleived, Apple and Foxconn have been trying to source unknown parts from a company who deals in carbon fiber materials:
According to my source, some engineer of Apple and Foxconn Technology came on Japanese company, which has carbon production in mid-March, and they requested to develop some sample.
I don't have information which is ordered by Apple, source told the number of parts is too large to be called "sample".
Apple have been toying with the idea of carbon fiber-based products for a while, with rumors a few years ago we could've seen, or could still see, a carbon fiber-based MacBook Air. With carbon fiber being stronger material, while weighing less, it would make sense for Apple to build a MacBook Air with carbon fiber.