(i) At a particular temperature, the coefficient DP describing diffusion through the perfect lattice differs by three orders of magnitude relative to that describing diffu- sion along grain boundaries (Dgb = 2 1011 m2 s1) in a polycrystalline material. Stating any assumptions, calculate the net diffusion coefficient that represents both fluxes, given that the grain size of the material is 100 m.

(ii) On the basis of your answer, giving reasons, comment on whether the diffusion data represent measurements made at a very high temperature, or not.

(iii) Why is it important to eliminate grain boundaries in a turbine blade that operates in the hottest part of the engine? Explain how the blade could be made into a single-crystal without any grain boundaries.