Phase transformation studies in U–Nb–Zr alloy.

Phase transformation diagrams provide fundamental informations for designing thermomechanical processes being a must regarding uranium alloys nuclear fuels. The work shows the evaluation of a kinetic transformation diagram for U–7.5Nb–2.5Zr (wt.%) based on both calorimetry experiments and dilatometry allied to X-ray diffraction analysis. Calorimetry measurements in scanning and drop modes can detect enthalpies of heating and transformation onset points from ambient up to select isotherms while the dilatometer is used to scan for sample volume changes related to phase transformations. The resulted kinetic diagram shows the gamma phase is stable for this alloy, guiding the rolling deformation process to temperature ranges where this phase remains for longer periods. Comparing to the literature results, the low temperature transformation (300–400 °C) is shifted to longer times accordingly to the disclosed TTT kinetic diagram. Therefore, two forming process windows can be proposed at 200 °C and 400 °C neighborhood where gamma-phase remains for enough time to accomplish total reductio

Competitive precipitation and recrystallization in U-7.5Nb-2.5Zr alloy

Metallic nuclear fuel plates are nowadays an alternative to the ceramic ones in the sense that the uranium density can be increased at lower enrichment. Higher thermal conductivity is also a key factor favouring such fuels for power reactors. Uranium reacts promptly with oxygen and nitrogen at high temperatures to catastrophic corrosion due to non-protective oxide layers, which imparts hot forming processes. The gamma phase body centred cubic structure can be retained at room temperature by annealing the U-7.5Nb-2.5Zr (wt.%) alloy followed by quenching, where the deformation can be extensive. The resulted highly deformed gamma supersaturated structure is subjected further to competitive recovery/recrystallization and phase precipitation phenomena whose are studied in the work. The U-7.5Nb-2.5Zr alloy was melted into plasma and induction furnaces and afterwards annealed to gamma phase. The normalized alloy was cold rolled and underwent isochronal and isothermal treatments. The microstructure evolution was monitored by optical microscopy, X-ray diffraction analysis and hardness measurements. The results show the precipitation events of α” and α+γ3 phases are dominant over recovery in the range 200ºC < T < 500ºC. Above 500ºC the recrystallization is the main process leading to softening and initial Vickers hardness recovery. One refined gamma phase grain structure was obtained (~8.0 μm) after annealing at 700ºC for 2.5 hours.LAT

Gamma-phase homogenization and texture in U–7.5Nb–2.5Zr (Mulberry) alloy.

This work investigates the phenomena of homogenization and texture of the γ phase in U–7.5Nb–2.5Zr (Mulberry) alloy prepared by induction melting and cold-rolling. The microstructural characterization of the as-cast and homogenized alloy (heat treated in γ phase region and then quenched in water), as well as the deformed state, was performed using optical and electron microscopy techniques, hardness testing and X-ray diffraction, employing the Rietveld method. The as-cast microsegregation was qualitatively observed by optical microscopy whereas the quantitative evaluation was obtained by electronprobe micro-analysis (EPMA). The homogenization state of the structure was evaluated after heat treatment at 1000 °C in a tube furnace for 5 h. It was found that this treatment is effective in eliminate dendritic segregation in this alloy. The texture of the Mulberry alloy was studied by X-ray diffraction (XRD) in the γ-phase stabilized condition and deformed state (rolled at room temperature). The stabilized γ alloy showed moderate texture mainly on the components (0 2 3)〈1 0 0〉 and (0 3 2)〈1 0 0〉. After 80% deformation, the sample showed a fiber texture (0 0 1)〈u v w〉, not commonly found in BCC metals, besides the γ fiber (1 1 1)〈u v w〉 with intermediate intensity

TG/DTA-MS evaluation of methane cracking and coking on doped nickel–zirconia based cermets

Cermet materials based on metallic nickel and cubic zirconia are the key material for applications on solid oxide fuel cells and high temperature water electrolysis. The main advantage is the possibility of direct feeding a hydrocarbon fuel, like methane, or even an alcohol as a source of hydrogen. The reforming reaction on the Ni catalyst surface can produce hydrogen continuously. However, the resulting catalyst poisoning by carbon deposition (coking) imparts their broad application. The work shows the evaluation of coking tolerance of some cermets prepared by mechanical alloying techniques and compares new additives specially chosen in order to avoid coking and increase the catalytic activity. Refractory metal additives besides copper were added to the basic cermet. While copper is a known doping agent that avoids coking, the refractory metals (Mo and W) have a twofold effect: promote sintering at lower temperatures and increase Cu activity due to their mutual immiscibility. Results of TG/DTA-MS analysis demonstrate both refractory metals have increased the coking tolerance as well as the catalytic activity during diluted methane cracking. Molybdenum and tungsten additives are promised regarding the improvement of these cermet materials for high temperature electrochemical devicesCNPqFAPESPIPE