Aluminium or Titanium frames make the best travel bikes.

- Unlike steel, they don't rust.

- Unlike carbon, they can cope with rough treatment (for example when loaded into the hold on a plane). They are less likely to be damaged, and the damage is more likely to lead to a gradual failure.

Titanium is reputed to be better but modern Aluminium frames are pretty light, pretty comfortable, and vastly cheaper.

I have an aluminium bike that I use whenever my trip involves a train ride or a flight.