Thermomechanical process optimization of U-10 wt% Mo – Part 1: high-temperature compressive properties and microstructure

Abstract

Nuclear power research facilities require alternatives to existing highly enriched uranium alloy fuel. One option for a high density metal fuel is uranium alloyed with 10 wt% molybdenum (U-10Mo). Fuel fabrication process development requires specific mechanical property data that, to date has been unavailable. In this work, as-cast samples were compression tested at three strain rates over a temperature range of 400 to 800°C to provide data for hot rolling and extrusion modeling. The results indicate that with increasing test temperature the U-10Mo flow stress decreases and becomes more sensitive to strain rate. In addition, above the eutectoid transformation temperature, the drop in material flow stress is prominent and shows a strain-softening behavior, especially at lower strain rates. Room temperature x-ray diffraction and scanning electron microscopy combined with energy dispersive spectroscopy analysis of the as-cast and compression tested samples were conducted. The analysis revealed that the as-cast samples and the samples tested below the eutectoid transformation temperature were predominantly phase with varying concentration of molybdenum, whereas the ones tested above the eutectoid transformation temperature underwent significant homogenization. *Manuscript Click here to view linked References

α-Phase transformation kinetics of U – 8 wt% Mo established by in situ neutron diffraction

“…The eutectoid transformation of the γ -phase in U-Mo proceeds in this temperature range by a cellular reaction, producing lamellar colonies of α + γ [6] [15]. For the U -8 wt% Mo hypo-eutectoid composition, these colonies will eventually stop growing and come into a kinetically arrested metastable thermodynamic equilibrium with the remaining γ-phase grains [6] [16] [17], after which the γ-phase in both the γ-phase grains and lamellar colonies will proceed to slowly form the γ'-phase. As the α-phase has a low (1 at%) solubility for Mo atoms, the surrounding γ-phase will enrich in Mo during transformation, especially the γ-phase portion of the lamellar colonies.…”

The Microstructure of Rolled Plates from Cast Billets of U-10Mo Alloys

“…A good example is the optical metallography of Figure 3.1a where Mo banding cannot be seen by optical microscopy and the U-10Mo is a very consistent equiaxed grain sized material. Given that the process looks viable, the following recommendations can be made:  The billet must be homogenized to eliminate Mo segregation; given the thickness of the billet and the reduced solidification and cooling rates it would be expected that the homogenization conditions would be different than those recommended by PNNL for the 5 mm castings (Joshi et al 2013). A homogenization study is needed.…”

Modeling the homogenization kinetics of as-cast U-10wt% Mo alloys

“…In order to fabricate U-10Mo monolithic plates consistently, at a lower cost and with high yield, a thorough understanding of the microstructure evolution kinetics from as-cast to the final formed structure after various material processes is required. The U-10Mo as-cast microstructure is an inhomogeneous dendritic structure with molybdenum-rich and -lean regions [4][5][6][7]. It has been observed in U-10Mo that, depending on the casting process adopted [5],microstructures with different grain size, molybdenum inhomogeneity,secondary dendrite arm spacing and carbide distribution wereobserved.…”

“…Based on the earlier work [1][2][3][4][5][6][7] on the thermomechanical processing of U-10Mo it was determinedthat different combinations and sequences of material processes may result in a variety of microstructures with different Mo segregation, grain morphology, phase volume fractions, carbide inclusions, and so on. In order to fabricate U-10Mo monolithic plates consistently, at a lower cost and with high yield, a thorough understanding of the microstructure evolution kinetics from as-cast to the final formed structure after various material processes is required.…”

“…The kinetics of the homogenization is dependent on the grain size and the other aforementioned parameters, and experimentally determining the time and temperature to homogenize is time-consuming and labor-intensive. Recent work on homogenization of U-10Mo was performed by Leenaers [7] and Joshi et.al [4][5][6]. The homogeneity or inhomogeneity was observed using the x-ray techniques in scanning electron microscope (SEM).…”