Positron-lifetime measurements are reported on electron-irradiated high-purity $\alpha$-iron and on iron doped with controlled amounts of carbon impurities. We show that in pure iron monovacancies are mobile at stage III around 220 K causing vacancy clustering into small three-dimensional agglomerates. These clusters anneal out between 500 and 600 K. In carbon-doped iron a competing mechanism is the formation of a highly asymmetric carbon-vacancy pair at 200 K. These pairs dissociate around 490 K, resulting in another release of free vacancies in the lattice. Controversial aspects on vacancy properties in earlier investigations are discussed.

• Received 1 June 1981

DOI:https://doi.org/10.1103/PhysRevB.25.762