Flattening a piece of wire by hammering can work-harden it, which can make it brittle. If, after flattening (but before file shaping) you heat the sword blank to red and let it cool, it will re-soften the metal and make it much less brittle.
I have a little butane flambe torch that I use for that sort of thing, but with such small pieces of metal a candle will do the job just fine.