Silicon part is greater than 50%
Has plasticity comparable to iron.
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The only alloys I can think of where silicon would be present in that high a quantity would be ferrosilicon, silumin (where you very rarely get 50% Si), and things like silicon-germanium which are usually used as semiconductors for very small scale applications. Otherwise, you're looking at ceramics like silicon carbide.
All of those materials tend to be very brittle on a macro scale: on a nano scale, some ceramics show ductility. I'd be very surprised if you can find anything that matches your criteria though.