Are there any material alternatives for the aluminium used on the aircraft leading edge? By alternatives, I mean composite alternatives.
If we're talking about transport category airplanes, the options depend to some extent on the wing. Wings with hard leading edges (no slats) and supercritical airfoils with poor stall characteristics are not very tolerant of what is called "runback ice"; ice that melts or is prevented from freezing by a warm but not hot leading edge and can run back and re-freeze just aft of the leading edge, forming a ridge, creating a dangerous risk of premature stall in the approach configuration.
These wings may require "evaporative" anti icing, which heats the leading edge to above the boiling point, so that supercooled drops that strike immediately turn to steam and can't flow back onto the wing plank as water.
Such a leading edge will run at about 110 degC when anti-ice is on and you will burn yourself pretty nicely if you touch one. The leading edges can be aluminum since the aluminum's heat treat state isn't affected until over 200C. So a composite material would have to have a transition temperature above 200C to have some margin to be tolerant of overheat cases.
If your wing has slats, runback icing isn't a concern in the approach configuration with slats extended, so the slats themselves may only need to be heated to about 50-60 degC, and the composite options become wider.
If you're not finding any high temperature composites on the web, try searching just for high temperature Epoxy laminating resins. There are all sorts of resins that can handle temperatures almost as high as aluminum and some that are higher.
In any case, the biggest reason composites aren't used for heat-anti-iced leading edges is poor conductivity. If you heat the leading edge with bleed air blown inside (typically) or with electrical elements bonded on the back of the skin (I think that's how Boeing does in in the 787), you need the heat to transfer to the outer surface immediately with almost no temperature loss (so that a sensor mounted on the inside surface is reading very close to the actual temperature on the outer surface) and you generally want a smooth surface, so aluminum is the ideal material.
Of course you could go with rubber boots and make the leading edge out of whatever you want, if you're willing to live with the inferior performance at ice shedding. Airplanes typically use boots because their engines don't make enough surplus bleed to heat a leading edge. Just about all turboprops and some small corporate jets like the earlier Citations are like that - and some are able to heat the leading edges but have to use boots on the tail, or, make the tail big enough to handle the ice so you don't have to anti-ice it at all (the CRJs do this).