I know first hand how hard it is to design carbon fibre preasure bulk heads, for aircraft with a much lower preasure delta (I wrote my first thesis about how to produce something like that). So, on the sirface, titanium makes sense. Using both, carbon fiber and titatium is just, well, not a good idea. Especially since I have the feeling this whole things wasn't properly calculated in the first place.
...With titanium hemispheres at each end, so some sort of poorly understood titanium carbon-fibre interface.