Past exam of the materials science course of the University of Cambridge / 2017 / Paper 1 / 4 / d / Solution

The disc volume is $\pi d^{2}h/4$, the combined area of its two flat faces is $\pi d^2/2$, and its cylindrical edge area is $\pi dh$. With the signed bulk driving term $\Delta G$ per volume,For favourable transformation $\Delta G$ is negative and the interfacial terms are positive. This classical nucleation theory expression neglects a separate elastic term; if coherency strain is important, its appropriate energy must be added or included explicitly in the effective volume term. $W_n$ has energy units, whereas $\Delta G$ here has energy-per-volume units.