Exhaust valves for turbo

[Ameer Antar]

I've heard some talk about other metals being better than sodium filled
valves. Does anyone know of any materials that are? (Hopefully something
cheaper than titanium!) And does it match sodium-types' heat dissipation or
are they just more able to take the heat? Is there any  down-side to
different material...ie. will it be bad if the valves could stand up to the
heat, but affect other parts in the head or something? Any experience or
tips much appreciated.

-ameer