Differential scanning calorimetry study on annihilation behavior of supersaturated thermal vacancies in B2-type FeAl

In this article, our recent advances on supersaturated vacancy behavior in rapidly quenched FeAl investigated by DSC measurements coupled with TEM observations are reviewed. There are two exothermic peaks only at the first heating step of DSC for the rapidly quenched FeAl. The 1st peak is small and lies in a lower temperature range, and the 2nd peak is large lying in a higher temperature range. The apparent activation energy of the exothermic reactions changes not only with Al concentration but also with quenching temperature. This indicates that the apparent activation energy is not the material constant of FeAl but a variable that changes with the concentration of supersaturated thermal vacancies. The 2nd peak corresponds to the nucleation and growth of vacancy clusters and complexes inside the bulks due to the condensation and annihilation of supersaturated vacancies, i.e. the formation and growth of cuboidal voids for rapidly solidified ribbons and of 〈100〉-type prismatic loops for waterquenched single crystals. The morphology of the vacancy complexes grown under the 2nd peak is correctly expressed by the Johnson-Mehl-Avrami (JMA) equation extended by Matusita et al. for the DSC curves: the morphology factor, m, nearly equals to 2 for the formation and growth of 〈100〉-type prismatic loops and to 3 for those of cuboidal voids. Therefore, the extended JMA method would be useful to judge the morphology of vacancy complexes.