Phase decomposition of γ-U (bcc) in U-10 wt% Mo fuel alloy during hot isostatic pressing of monolithic fuel plate

Abstract

Eutectoid decomposition of γ-phase (cI2) into α-phase (oC4) and γ'-phase (tI6) during the hot isostatic pressing (HIP) of the U-10 wt.% Mo (U10Mo) alloy was investigated using monolithic fuel plate samples consisting of U10Mo fuel alloy, Zr diffusion barrier and AA6061 cladding. The decomposition of the γ-phase was observed because the HIP process is carried out near the eutectoid temperature, 555°C. Initially, a cellular structure, consisting of γ'-phase surrounded by α-phase, developed from the destabilization of the γ-phase. The cellular structure further developed into an alternating lamellar structure of α-and γ'-phases. Using scanning electron microscopy and transmission electron microscopy, qualitative and quantitative microstructural analyses were carried out to identify the phase constituents, and elucidate the microstructural development based on time-temperature-transformation diagram of the U10Mo alloy. The destabilization of γ-phase into α-and γ'-phases would be minimized when HIP process was carried out with rapid ramping/cooling rate and dwell temperature higher than 560°C.

Observed Changes in As-Fabricated U-10Mo Monolithic Fuel Microstructures After Irradiation in the Advanced Test Reactor

“…Depending on how a fuel plate is fabricated, it is possible for the U-10Mo alloy fuel microstructure to consist of elongated fuel grains in the rolling direction, areas of transformed c-(U,Mo) phase, areas of chemical banding, and impurity phases in different areas of the microstructure. 3,[10][11][12] Figure 1 shows examples of these microstructural features. The elongated grains results from the rolling process.…”

Atomistic simulation of deformation twinning in nanocrystalline body-centered cubic U–Mo alloys

“…2 Due to its cubic symmetry, the bcc γ-phase exhibits the best technical characteristics among the aforementioned phases. 3,4 Nevertheless, the γ phase exhibits significant instability when subjected to low temperatures. It is therefore common practice to alloy uranium with other bcc metals to keep the γ-phase stable at ambient temperature.…”

Solid-state phase transitions of two quaternary metallic fuel alloys (U-2.5Mo-2.5Ti-5.0Zr and U-1.5Mo-1.5Ti-7.0Zr in wt. %)

“…Mariani proposed an alloy containing Mo, Zr, Ti with U (or quaternary UMTZ alloy) [3,4], with the expectation of decreasing the  phase onset temperature (by Mo) and increasing the solidus/liquidus temperatures (by Zr and Ti) over that of pure uranium [3,4,5]. A low γ phase onset temperature will be beneficial due to the isotropic swelling in this cubic phase [4,6]. For pure uranium, the  phase onset temperature is 1049K.…”