Evidence of Embryo Formation as a Precursor to the Delta to Alpha-Prime Transformation in a Pu-Ga Alloy

摘要

It was experimentally observed that a single Pu-2.0 at% Ga sample can be thermally cycled many times, with nearly the same amount of transformation and reversion during each cycle, provided that the sample is annealed at 375℃ for 8 hours and then conditioned at 25℃ for at least 6 hours. The ambient temperature conditioning period is crucial for obtaining the same amount of transformation to alpha-prime in each thermal cycle. Here, we present results from a series of experiments that investigate the effects of conditioning time and temperature on alpha-prime formation. When the sample is conditioned at 25℃ for times between 0 and 6 hours, the amount of alpha-prime formed is a function of conditioning time. For conditioning treatments between 6 and 70 hours, however, the amount of alpha-prime formed is nearly constant. Conditioning treatments at -50℃, 150℃, and 370℃ all resulted in less alpha-prime formation than the 25℃ treatments. We hypothesize that embryos of the thermodynamically stable alpha phase form during the conditioning treatments, and these embryos form alpha-prime upon subsequent cooling. This result is an indirect confirmation of the Russian Pu-Ga equilibrium phase diagram.