The Atmel Atmega 328PU Double Inline Package Microcontroller, which the Arduino utilises, hasa maximum input voltage of 6 volts and a normal system voltage of 5. Other AVR microcofntrollers by Atmel/Microchip also have this limitation, and the Arduino Due can handle only 3.3 volts due to the ARM Cortex chip used. You overvolted it.