This book is probably worth having on your shelf if you are at all serious about professional optics and their manufacture. There are chapters devoted to alloy optical substrates pertaining to thermal properties, reflective properties, methods of manufacture, etc)
http://www.amazon.com/Reflecting-Tel...ref=pd_vtp_b_6
It has been a while since I read it, but from memory, discs of any real size when cut out of sheet alloy inevitably suffer from astigmatism. (The substrate has a directional grain) One solution to this that does work is to use something like a MIG welder to spray the metal on to a rotating mandril and slowly grow the size of the disk 'snowball' style. Of course you will also need to acquire the correct alloy in wire format, which may be a deal breaker in itself.
One other point that Wilson notes is that irrespective of the metal the substrate is made from, a machined and polished surface will
NEVER achieve the same reflectivity as a vacuum deposition coating using exactly the same alloy. Implicitly therefore, you will still need to (or at least be better off) aluminising then quartz over-coating the finished optic.
Considering the cost (and relative quality) of optics coming out of GSO these days, the case for alloy substrates is not a very strong one. The exception to this is if you simply wanted to do it for the sake of the exercise, in which case the effort is entirely justified.
fwiw) There is one unconventional material that Wilson intimated had great promise for large diameter optical substrates, and that is Tungsten Carbide... I would hope I don't need to elaborate as to why that material is likely to remain outside of the amateur domain for a while at least.
best,
~c
