Homework # 4 Due Sunday 2 November 2014 in class. 1. Briefly explain the difference between self-diffusion and interdiffusion. 2. Self-diffusion it is not subject to observation by compositional changes, as with interdiffusion. Suggest one way in which self-diffusion may be monitored. 3. Compare interstitial and vacancy atomic mechanisms for diffusion. 4. Cite two reasons why interstitial diffusion is normally more rapid than vacancy diffusion. 5. Briefly explain the concept of steady state as it applies to diffusion. 6. Briefly explain the concept of a driving force. 7. What is the driving force for steady-state diffusion? 8. Write Fick’s first and second laws in equation form, and state the kind of diffusion for which each of these equations is normally applied. 9. How can the mathematics for non-steady state described? 10. What can be inferred from the magnitude of the diffusion coefficient? 11. A sheet of steel 2.5 mm thick has nitrogen atmospheres on both sides at 900 oC and is permitted to achieve a steady-state diffusion condition. The diffusion coefficient for nitrogen in steel at this temperature is 1.2 × 1010 m2/s, and the diffusion flux is found to be 1.0 × 10-7 kg/m2- s. Also, it is known that the concentration of nitrogen in the steel at the high-pressure surface is 2 kg/m3. How far into the sheet from this high pressure side will the concentration be 0.5 kg/m3? Assume a linear concentration profile. 12. What is the effect of the presence of impurities and deviations from stoichiometry on the diffusion rate of ions in ionic crystals?