[en] In 20 K, 3 MeV electron irradiated Cu₃Au, single vacancies migrate at 260 K and form three dimensional vacancy clusters which evaporate above 400 K. Only slight differences are observed in previously ordered or disordered alloys (in ordered alloy, the clusters are smaller but in higher concentration). A model is given which explains the two steps of the long range ordering induced by the electron irradiation in terms of single vacancy long range migration occuring before (250-300 K) or after (400-450 K) the vacancy clustering