Are composites lighter than aluminum alloys, and aren't they able to with stand more heat then aluminum and titanium alloys?
Composite refers to a class of material that you combine together at macroscopic level. Example reinforced concrete. There is metal bar inside cement matrix. Similarly, carbon composite, commonly are carbon fiber embedded inside epoxy matrix. alone, the single material has a lot of weaknesses, but combined in some arrangement they compliment each other resulting in much superior quality.
it is possible to combine all sort of material, including metal in metal matrix. eg. thin silicon carbide, tungsten carbide or carbon fiber filament inside metal (aluminum, titanium, etc). Very similar in concept like reinforced concrete. (except costing gajillion bucks) I think newer landing gear are made out of titanium with carbon filament inside. We only know how to do MMC in small part, inches long. Compared to feet and yard scale in carbon composite.
A-380 airbus uses aluminum, glass, epoxy laminate for wing edges. 787 entire fuselage are composite.
Heat resistance of composite naturally depends on its constituent. Organic based composite are nothing more than glorified high temperature plastic. (think your mom's pot and pans cooking handle) It can withstand 5-600 degres in some material, but it won't withstand thousand.
The advantage of organic based composite? You can mold it into very complicated shape and we know a good deal how to machine and manufacture it. (so you can make entire small wing section without complicated rivet and welding, massive weight saving there. Plus reducing part counts and doesn't rust)
to go higher, one can do ceramic composite. But ceramic is brittle, so it's not suitable for a lot application. And finally, metal matrix composite. Yes, they can perform better in heat, but these are insanely expensive and very hard to manufacture.
I think ultimately things will be things like diamond matrix. (imagine the strength of that type of material. ... and imagine the manufacturing cost.. heh.)