Kinetics of the β→α transformation in unalloyed plutonium using differential scanning calorimetry

Differential scanning calorimetry measurements, both isothermal and on cooling, were made during the β→α transformation of unalloyed plutonium. The calorimetric measurements, because they directly measure the rate of heat flow, provide a direct quantitative measure of the rate of the reaction. Isothermal measurements at temperatures from 56 C to 68 C showed that the reaction required incubation times, presumably for nucleation of the alpha phase, of from 300 to 550 seconds. The reaction rate decreased with increasing temperature in agreement with results of prior studies. The measured value for the heat of the reaction also corroborated earlier findings. The fraction-transformed curves were analyzed using the Johnson, Mehl, Avrami model. Analysis showed clearly that the reaction kinetics do not fit this model. The Avrami exponent, instead of being constant as predicted, decreased continuously (by more than a factor of four) during the first 80% of the reaction. Possible reasons for this apparent deviation from classical transformation kinetics are discussed. Measurements in the temperature-scanning mode were made on a sample consisting of ten separate pieces of plutonium. With successive cooling cycles through the β→α transformation, the heat release peak changed in shape, width, and position. This effect is believed to be due to changes in the residual stresses in different pieces of the sample