THE FORMATION OF VACANCIES IN ORDERED Fe-Al ALLOYS WITH B2 STRUCTURE

The vacancies retained by quenching or at thermal equilibrium were studied experimentally in ordered Fe-Al alloys with B2 structure over a range of compositions from 46 to 51 at % Al. The study, which was initially mainly dilatometric, shows that : - The formation of vacancies is a property of the B2 structure in iron-aluminium. - Quenching is very efficient for retaining vacancies at least up to 1 000°C. - At constant temperatures, the concentration of vacancies increases monotically with the aluminium content and can reach very high values (> 2 %). - At constant aluminium content, the vacancy concentration as a function of temperature exhibits an anomaly characterized by a negative deviation from the classical exponential law. The temperature at which this anomalous behaviour is observed is independent of aluminium content and is equal to 1 000°C. A systematic study of the possible causes allows one to conclude that this anomaly must be related to a decrease in the degree of order. This study was completed by observations using field ion microscopy which shows that : - The vacancy interpretation of the effects previously observed by different methods is correct. - It is possible to evaluate the distribution of vacancies among both sub-lattices, and that the quenched vacancies preferentially occupy iron sites.