> All,
> Can I drive a 6v DC motor with a 12v battery by using PWM and a 50% (or
> less) duty cycle?  How will this affect the life of the motor?


Voltage really only determines the top end speed. FAR more important is
current control. If the motor is stalled, it will draw a lot of current, and
tend to overheat. Of course a 12V supply makes this a more serious problem.
If you sense motor current, and chop the voltage to limit the current to a
safe level, then you won't have problems.
This can be done in hardware or in software but actually hardware is
probably better.