Why aren't ceramics used in turbines?
Ceramic materials are starting to be used in jet engines.  E.g. the LEAP-1A is using a few ceramic matrix composite in the high pressure turbine.   (http://aviationweek.com/mro/ceramic-composites-enter-service). Future designs will likely expand the use of CMCs (e.g. http://www.geaviation.com/press/ge90/ge90_20150303.html)