Too much torque used when installing? Those are alloy, and don't hold up as well as steel. Less forgiving to exceeding maximum torque than steel would be. On a positive note, the actual front-derailleur (FD) seems to be fine. I don't know if a replacement for the band is available. So you may be stuck with getting a new FD - but call, or drop by, your LBS and see if they have something that will work in a parts-box somewhere.
A torque-wrench and installation instructions would help you in the future.