4. The diffusivity of carbon in a-ferrite and self-diffusivity of iron in a-ferrite is given at two temperatures. Temperature [C] Self-Diffusivity [m/s] Fe in a-Fe 3.0 x 10-21 1.8 x 10-15 don 500 900 Diffusivity [m/s] C in a-Fe 2.4 x 10-12 1.7 x 10-10 a) Calculate the diffusivity, D, of Fe in a-Fe at 625C (10). Judg lqub leplago nA (d (P) 07 b) Adding small amounts of certain alloying elements (e.g., Ni, Cr, Mo, Mn) to plain carbon steel increases the hardenability of the resulting alloy without changing the crystal structure. Comment on how the values of diffusivity for C and Fe a-Fe listed above might change when such alloying elements are added to increase the hardenability (8).