Quenched-in Vacancies and Hardening of Fe-Al Intermetallics

The role of vacancies in hardening of Fe-Al intermetallic alloys were studied in the present work for a wide range of Al concentrations from 20 to 50 at%. The alloys quenched from 1000 degrees C as well as those annealed subsequently at 520 degrees C for 1 h were subject to study. Slow-positron beam experiments combined with Vicker's microhardness tests were utilised. Hardness of Fe-Al alloys exhibited a somewhat complex dependence on Al content which could not be fully explained purely by consideration of intermetallic phases formed. This happens due to additional hardening effect caused by quenched-in vacancies. The concentrations of vacancies were estimated from positron back-diffusion data and found to rise for Al content above 25 at%. Correlation of vacancy concentrations with hardness data for the quenched and annealed alloys has revealed that hardening of alloys with a low Al content (< 30 at%) is originated predominantly by anti-phase boundaries while hardening induced by quenched-in vacancies dominates for alloys with a higher Al content (30-50 at%).