Effect of NB concentration on the y-̊--> a" martensitic transformation in U+NB alloys

The martensitic reaction in five Uranium-Nb alloys (14.5,15.0, 15.5, 15.5, and 16.0 at% Nb) was studied by means of x-ray diffraction experiments in order to establish the temperature range of this reaction. The experiments were carried out in a cryostat on a diffractometer in which minimum temperatures close to liquid N₂ temperature were obtained. All the samples were solution heat treated at 800°C and water quenched. They were then electropolished prior to diffraction experiments. The diffraction patterns were taken from about 25°C down to about -190°C. The intensity-Bragg angle data were digitized and the relative amounts of the parent phase γ° and the martensite phase α" were determined by means of a peak separation and analysis program.

The 14.5 at% Nb alloy was completely martensitic at 25°C, and the 16.0 at% Nb alloy remained completely γ° phase down to -192°C. For the other alloys, as the temperature decreased, the amount of the martensitic α" phase increased, and that of γ° decreased. For all three alloys, the temperature was below the lowest temperature of measurement (-192°C), and the M_s was above 25°C. At a given temperature, the amount of α" decreased with increasing Nb content.