  U-Nb's discontinuous precipitation, $\gamma^{bcc}_{matrix} \rightarrow
\alpha^{orth}_{cellular} + \gamma'^{bcc}_{cellular}$, is intriguing in the
sense that it allows formation and growth of the metastable $\gamma'$ phase
during the course of its occurrence. Previous attempts to explain the
thermodynamic origin of U-Nb's discontinuous precipitation hypothesized that
the energy of $\alpha$ forms an intermediate common tangent with the first
potential of the double-well energy of $\gamma$ at the $\gamma'$ composition.
While this hypothesis is eligible and consistent with the experimental
observation of gradual increase in $\gamma'$ composition at increasing
temperature, it is in conflict with recent experiments whose results indicated
a distribution of $\gamma'$ compositions in the vicinity of 50 at\%Nb. To shed
some light onto this issue, the current work investigates the origin of U-Nb's
discontinuous precipitation in view of fundamental thermodynamics and kinetics,
taken from the perspective of phase-field theory. It has been showed that local
misfit strain tends to play a crucial role in the formation and growth the
discontinuous precipitation. Depending on the magnitude of strain developed at
grain boundaries, either an increasing $\gamma'$ composition or a random
distribution of $\gamma'$ composition around the equiatomic value with respect
to increasing temperature could be expected.
