Investigation of the discontinuous precipitation of U-Nb alloys via thermodynamic analysis and phase-field modeling

U-Nb's discontinuous precipitation, gamma(bcc)(matrix) -> 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. In this work, we examine different possible mechanisms by which the discontinuous precipitation product in the U-Nb system can be stabilized. We put forward a mechanism by which the bulk free energy of the gamma can develop a non-monotonic dependency with composition. Additionally we posit that local contributions due to lattice mismatch between the alpha and gamma phases may be responsible for the generation of metastable states that may stabilized by thermodyna mics as well as by kinetics. Our work suggests that local misfit strain tends to play a crucial role in the growth of the discontinuous precipitation product. Depending on the magnitude of strain developed at the alpha /gamma' interfaces, either an increasing gamma' composition or a random distribution of gamma' compositions around the equiatomic value with respect to increasing temperature could be expected. Moreover, we show how it is possible to stabilize the discontinuous precipitation front through highly anisotropic and fast interface diffusion.