251684(a) Discuss how diffusion in metals is assisted by imperfections in the crystalline\ structure - include diagrams to aid with your answer.\ 6 marks\ (b) State four factors which determine the rate of diffusion in solids.\ 4 marks\ (c) EN36 grade steel, a nickel, chromium, molybdenum alloy, is often case\ hardened making it suitable for a range of applications including aeroplane and\ motor crankshafts. Describe how the alloy is processed during case hardening\ and discuss the properties arising through the process.\ 5 marks\ (d) For the following systems, rank the diffusion coefficients in order from greatest\ to least:\ N in Fe at 750\\\\deg K; Cr in Fe at 750\\\\deg K; N in Fe at 923\\\\deg K; Cr in Fe at 923\\\\deg K\ Justify your ranking given both Fe and Cr have BCC crystal structures, and the\ atomic radii for Fe, Cr, N are 0.124, 0.125, 0.065 nm, respectively.\ 4 marks\ (e) It is proposed to harden a low carbon steel component to a depth (x) of 0.2 mm\ through a diffusion process at 1000oC.\ (i) If Do and Qd are 8 x 10-6 m2/s and 131 kJ/mol, respectively, calculate the\ diffusion coefficient at 1000oC.\ 4 marks\ (ii) Calculate the processing time, given time (t) is approximately equal to the\ diffusion depth squared (x2) divided by the diffusion coefficient.\ 2