Role of martensitic interface on the discontinuous precipitation of U-Ti alloys at 400 °C

The microstructure evolution of U-0.8 wt%Ti (U-0.8Ti) during aging at 400 ºC was investigated through SEM and TEM techniques. Three kinds of decomposition routes were identified. One route is through continuous precipitation (CP) process while the other two are through discontinuous precipitation (DP) process. In the early stage of aging, CP reactions of U 2 Ti occurred at the boundaries of martensitic plates. DP reactions were discovered both around grain boundaries and inside large-sized martensitic plates. The colonies of the former one were found at prior grain boundaries but were soon suppressed in the later stage of aging. DP reactions inside large-sized martensitic plates did not take place until the final stage of aging. By applying the Hatt martensitic transformation theory to the electron backscatter diffraction results, it was identified that most of the boundaries of martensitic plates were incoherent high-angle boundaries. Such boundaries compensate the opposing energy of the nucleation of the U 2 Ti during CP process and offer extra surface energy to promote the DP process around grain boundaries. This paper provides an overview of the decomposition process and an undocumented understanding of the role of martensitic plate interfaces on the DP and CP processes during aging at 400 ºC in U-0.8Ti alloy, which is also worth considering in other systems with acicular martensitic structure, such as U-0.8 wt%Mo and U-1 wt%Nb alloys. EBSD analysis indicates most of the martensitic interfaces in U-0.8Ti alloys belong to incoherent high-angle boundaries, and STEM investigations show that is no preferred orientation relationship between the parent and decomposed phases in the discontinuous precipitation colony. • Three decomposition routes (one CP & two DP reactions) were identified during aging. • The CP reaction preferred to occur in the high-angle martensitic boundaries. • The DP reactions around grain boundaries were driven by interface energy reduction. • The DP reactions inside martensitic plates were driven by chemical potential reduction. • No preferred orientation relationship was found between the parent and decomposed phases.