High-temperature atomic defect properties and diffusion processes in intermetallic compounds

Data on thermal vacancy formation in intermetallic compounds obtained from positron lifetime spectroscopy yield high effective formation enthalpies H(V)(F) in close-packed structures and low values in bcc-type structures which can be well understood theoretically. The vacancy migration enthalpy H(V)(M) could be determined at high temperatures for B2-FeAl by studying the equilibration process after temperature changes. As demonstrated here in a comparative study on B2-FeAl the thermal formation and migration of defects can also be sensitively investigated by time-differential length-change studies after temperature changes in the vicinity of the equilibration temperatures. The present vacancy data can explain the wide variation of the transition metal self-diffusivities in intermetallic compounds. For B2-FeAl it is shown that the high-temperature mechanical properties are closely linked to the formation of thermal defects as evidenced by the temperature variation of the yield stress anomaly and its time dependence after fast heating. (C) 1999 Elsevier Science Ltd. All rights reserved.