GRAIN-BOUNDARY DIFFUSION IN B2 INTERMETALLIC COMPOUNDS: EFFECT OF ORDERING ON DIFFUSION IN THE Fe_3Al AND FeCo COMPOUNDS

摘要

Grain-boundary (GB) self diffusion in the Fe_3Al and FeCo compounds was investigated in the B-kinetic regime by the radiotracer method over temperature ranges including the bulk order-disorder tran- sition temperatures. For iron diffusion in Fe_3Al the A2-B2 transition did not lead to any modification of the Arrhenius behavior, while a slight decrease of the triple product values below the B2-DO_3 transition is observed. In FeCo both for iron and cobalt the A2-B2 transition evokes a decrease in the pre-exponential factor of the triple product, while the diffusion activation energy is not altered. These results are tentatively interpreted in view of recent segregation experiments carried out on the free surfaces of the compounds and it is proposed that owing to segregation the order-disorder transition temperature can be considerably different in GBs from that in the bulk.