Grain-boundary diffusion in B2 intermetallic compounds:: Effect of ordering on diffusion in the Fe3Al and FeCo compounds

Grain-boundary (GB) self-diffusion in the Fe3Al and FeCo compounds was investigated in the B-kinetic regime by the radiotracer method over temperature ranges including the bulk order-disorder transition temperatures. For iron diffusion in Fe3Al 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-DO3 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. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.