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

Abstract

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 U2Ti 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 U2Ti 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.