Microstructural and phase changes in alpha uranium investigated via in- situ studies and molecular dynamics

FG Di Lemma and TK Yao and D Salvato and L Capriotti and F Teng and AM Jokisaari and BW Beeler and YH Wang and CJ Jensen, JOURNAL OF NUCLEAR MATERIALS, 577, 154341 (2023).

DOI: 10.1016/j.jnucmat.2023.154341

A deeper knowledge of thermally induced microstructural and phase evolution in nuclear metallic fuel can be obtained using novel in-situ microscopic analyses. Such studies can provide information on the dynamics of phase transitions which is not possible with conventional postmortem characterization (post-irradiation examination). In this work, the behavior of alpha uranium (alpha-U) was investigated via in- situ heating tests in a transmission electron microscope. The main objective is to understand the microstruc-tural and phase changes, such as defect annihilation and beta phase formation and retention, observed in reactor in-pile transient studies at the Transient Reactor Test facility. Indeed, defect migration and re-arrangement were observed within the alpha phase starting at 673 K; alpha ->beta phase transition was observed between 773 K and 1,073 K during the heating ramp (which is in the temperature window reported for alpha ->beta transition temperatures). Recrystallization and formation of nano grains was observed at high temperatures (over 1,073 K). Such recrystallization was possibly related to the formation of the gamma phase. Finally, it was indeed observed that the beta phase (but not gamma phase) was retained at room temperature upon rapid cooling. Molecular dynamics studies support these experimental results and shows that the gamma phase of pure uranium cannot be retained at room temperature if not stabilized with the addition of an alloying element.Published by Elsevier B.V.

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