8.10 An Al-4% Cu alloy is heated to 550°C during heat treatment and quenched to room temperature. Immediately after quench, the diffusion rate of Problems 197 copper (which proceeds by a vacancy mechanism) was found to be 107 times faster than what would be expected from the listed diffusion data. What fraction of vacancies in equilibrium at 550°C is retained at room temperature by the rapid quenching? The enthalpy of motion of vacancy in this alloy is 50 kJ mol–1.