Complementary observations concerning cast grain refining in uranium alloys. Influence of small additions

In the introduction, the advantages of refining the primary γ crystallization in uranium and its alloys are rapidly presented. The refining or the control of this γ grain can be obtained either by modifying the solidification kinetics, or by the addition of very small amounts of alloying elements. This second method will be considered here and more particular attention will be paid to the refining role played by elements of low atomic number. 1. Influence of carbon - The presence of carbon in concentrations of less than 1000 x 10-6 in U-Mo alloys leads a definite refining effect on the primary γ crystallization. For a given rate of solidification, this effect is a maximum for a carbon content of between 500 and 800 x 10-6. The importance of this maximum decreases with the cooling rate; it is very marked for fast rates and disappears in the case of low cooling rates. The addition of carbon gives rise to the formation of uranium carbides whose nature and dispersion state depend on the solidification kinetics. The grain refinement mentioned above depends both on the temperature of formation of these precipitates, i.e. the equilibrium diagram, and on their size. The authors propose a mechanism for interpreting the various observations. 2. Influence of boron - Boron has a still faster action than that of the carbon, even in the case of much lower concentrations. This addition of boron leads to the formation of boride whose morphology has been related to the equilibrium diagram and to the solidification process. Finally, the simultaneous addition of boron and of carbon leads to a larger relative refining than that corresponding to only one of these elements. 3. Other addition elements - A systematic search has been carried out amongst the elements having a low atomic diameter. Oxygen and sulphur, amongst others, proved to have no effect; beryllium and phosphorus on the other hand displayed a definite ability to refine the grain. This effect is studied in more detail in the case of beryllium. (authors)