Nucleation and growth of the Alpha-Prime Phase 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-prime product formed upon cooling the metastable delta phase below the martensite burst temperature (M{sub 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-prime phase, it was first homogenized for 8 hours at 375 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 C and 370 C for periods of up to 70 hours to determine the holding time and temperature that produced the largest volume fraction of alpha-prime upon subsequent cooling. Using transformation peak areas (i.e., the heats of transformation) as a measure of the amount of alpha-prime formed, the largest amount of alpha-prime was obtained following holding at 25 C for at prime least 6 hours. Additional time at 25 C, up to 70 hours, did not increase the amount of subsequent alpha-prime formation. At 25 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 Pu{sub 3}Ga, although a complete eutectoid decomposition of delta 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-prime particles, and their amount, distribution, and potency appear to contribute to the total amount of martensitic product formed. A modeling approach based on classical 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-prime formation on cooling are discussed in terms of the differences in the potency of the embryos created in the delta 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-prime 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.
- Research Organization:
- Lawrence Livermore National Laboratory (LLNL), Livermore, CA
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 897989
- Report Number(s):
- UCRL-JRNL-209612
- Journal Information:
- Acta Materialia, vol. 54, no. 15, September 1, 2006, pp. 4001-4011, Journal Name: Acta Materialia, vol. 54, no. 15, September 1, 2006, pp. 4001-4011
- Country of Publication:
- United States
- Language:
- English
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