Structural materials that are intended for service at very high temperatures are usually strengthened by precipitation hardening, using precipitates that involve substitutional solute species. Examples include the Fe-based and Ni-based 'superalloys' used in jet engines, which are strengthened by precipitates of intermetallic compounds such as Ni3Ti. Why choose precipitation hardening? Why should the precipitates employ substitutional solutes?

i think   the answer is c hope i'm right

that's what i got you can count the number of atoms

answer: semi-infinite solid initially at a uniform temperature of 400 ◦c is cooled at its surface by convection. the cooling fluid at a constant temperature of 200 ◦c has a convective coefficient (ha) of 250 w/m2.k. the physical properties of the solid: k = 20 w/m.k and α = 4 * 10-5  m2/s. using δx = 0.04 m and δt = 10 s. write the finite difference equations describing the transient heat transfer of the first five δx and the temperature profile after 30 s.