Thick single crystalline Tin $($Sn$)$ and Indium $($In$)$ metal plates are attached against each other $(=$semi$-$infinite diffusion couple$)$ at the temperature of $100C.$

a$)$ Based on the phase diagram $($see$.$ Fig. $1$a$),$ draw the structure of the diffusion couple with superimposed composition profile for Indium, when sufficient amount of time has elapsed so that local equilibria is achieved in all interfaces. $($NB$.$ Mark clearly the compositions of the phases at all interfaces!$)(3$p$)$

b$)$ Please discuss on the possible diffusion mechanisms and types in that diffusion couple? $(1$p$)$

c$)$ Based on the Gibbs energy $-$ composition diagram $($see Figure $2)$ define the driving force for $Sn$ to diffuse through $$ phase $($i$.$e from $Sn|$ interface to $|$ interface$)$ at $100C.(2p)$

Figure $2.$ Gibbs energy $-$ composition diagram at $100C$ from $Sn-$In system