Nucleation and growth of the α′ martensitic phase in Pu-Ga alloys

In a Pu-2.0 at.% Ga alloy, it is observed experimentally that the amount of the martensitic alpha' product formed upon cooling the metastable 5 phase below the martensite burst temperature (M-b) is a function of the holding temperature and holding time of a prior conditioning (annealing) treatment. Before subjecting a sample to a cooling and heating cycle to form and revert the alpha' phase, it was first homogenized for 8 h at 375 degrees C to remove any microstructural memory of prior transformations. Subsequently, conditioning was carried out in a differential scanning calorimeter apparatus at temperatures in the range between -50 and 370 degrees C for periods of up to 70 h to determine the holding time and temperature that produced the largest volume fraction of alpha' upon subsequent cooling. Using transformation peak areas (i.e., the heats of transformation) as a measure of the amount of alpha' formed, the largest amount of alpha' was obtained following holding at 25 degrees C for at least 6 h. Additional time at 25 degrees C up to 70 h, did not increase the amount of subsequent a' formation. At 25 degrees C, the Pu-2.0 at.% Ga alloy is below the eutectoid transformation temperature in the phase diagram and the expected equilibrium phases are alpha and PU3Ga, although a complete cutectoid decomposition of 8 to these phases is expected to be extremely slow. It is proposed here that the influence of the conditioning treatment can be attributed to the activation of alpha-phase embryos in the matrix as a beginning step toward the eutectoid decomposition, and we discuss the effects of spontaneous self-irradiation accompanying the Pu radioactive decay on the activation process. Subsequently, upon cooling, certain embryos appear to be active as sites for the burst growth of martensitic alpha' particles, and their amount, distribution, and potency appear to contribute to the total amount of martensitic product formed. A modeling approach based on classic nucleation theory is presented to describe the formation of alpha-phase embryos during conditioning. The reasons why the holding times during conditioning become eventually ineffective in promoting more alpha' formation on cooling are discussed in terms of the differences in the potency of the embryos created in the 6 matrix during conditioning and in terms of growth-impeding volume strains in the matrix resulting from an increasing number of martensite particles, thus opposing further growth. It is suggested that the disparate amounts of the alpha' formation reported in the literature following various studies may be in part a consequence of the fact that conditioning times at ambient temperatures are inevitably involved in any handling of radioactive samples prior to testing. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.