Phase transformation in delta Pu alloys at low temperature: in situ dilatometric study

The purpose of this work is to precisely study the martensitic transformation in a plutonium-gallium alloy. Thus, the thermodynamics and kinetics of the δ → α' + δ transformation in a Pu-Ga alloy were studied under isochronal and isothermal conditions. The activation energy of the δ → α' + δ phase transformation at a constant cooling rate (0.5 K. min-1) was determined by using Kissinger and Ozawa models. The average value of the activation energy was found to be at -56 kJ.mol-1. Dilatometry measurement was also used to trace 'in situ' the entire transformation for several temperatures. The kinetics of the δ → α' + δ transformation were modelled under isothermal conditions in the theoretical frame of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory. It is proposed that the transformation consists of three stages. The α' transformation begins with a nucleation of pre-existing embryos. Then, both nucleation and rapid growth of α' occurs simultaneously and finally, the plates width expend. Apparent activation energies for nucleation and growth transformation were determined from the temperature dependence of the constant K at respectively -34 kJ.mol-1 and -60 kJ.mol-1. Dler et al. [1] investigated also the thermodynamics and the kinetics of the martensitic transformation in Pu alloys. These nucleation energies were found by modelling of heterogeneous martensitic nucleation via strain interaction with observed superdislocation-like nucleation sites in PuGa alloys. The values obtain by this model was very close to those we find. Investigations in steels alloys indicate that these energies are of the same order for nucleation near dislocation. Then, it could be indicating a strong relationship between these dislocations and martensitic nucleation sites. (authors)