Vacancy Formation and Diffusion in FeAI-Alloys

摘要

Absolute concentrations of vacancies in FeAl-alloys have been measured by means of differential dilatometry and positron annihilation techniques, which have been calibrated by determination of positron trapping rates. At lower Al concentrations the defects appear to be single vacancies, whereas above 35 at.% Al aggregates as triple defects in B2(l) or additional divacancies have to be considered in B2(h). In this context the B2-phase field has to be split into several regions B2', B2(l) and B2(h). In B2(h) Arrhenius plots show sudden breaks followed by small slopes. These can be shown to be the result of temperature dependent thermodynamic changes in local order with higher order transitions. This has been deduced from positron lifetime measurements and their pressure dependence and the thus obtained formation volumes and the change in formation enthalpies and -entropies in the respective ranges. This view is further corroborated by coincidence spectra of the 511 keV positron annihilation line. Studying the annihilation of quenched-in defects by positrons, their mobility may be obtained, strongly decreasing with Al content. Several annealing steps can be found in some concentration ranges with different mobilities. Chemical diffusion D is characterized by an increase in A2 due to a strong Al-vacancy binding and the thermodynamic factor. At the A2-B2-transition D decreases sharply and stays relatively constant up to stoichiometry except in B2(h) above 35 at.% Al in agreement with. The magnitude and concentration dependence of the thermodynamic factor is still less clear. Kirkendall shift corrected for volume contraction indicates that the ratio of the intrinsic diffusivities D(Al)/D(Fe) is always larger than one in the order of 1.2-1.5 in ordered FeAl and 1.75-1.78 in disordered A2 at 20 at.% Al.