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Diffusion Solids Fundamentals Diffusion Controlled Solid State Episode 2 Part 5

Tham khảo tài liệu 'diffusion solids fundamentals diffusion controlled solid state episode 2 part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 348 20 Diffusion in Binary Intermetallics A o B i Vacancy 100 Straight Cycle Fig. 20.5. Schematic illustration of six-jump vacancy cycles in the B2 structure. The arrows show vacancy jumps the numbers indicate the jump sequence both components Da Db lies within the following fairly narrow limits 1 A q. 20.3 q Db q was first estimated to be 2 29 5 and by including correlation effects was later slightly corrected to q 2.034 30 . The upper lower limit is attained when vacancies are preferentially formed on the B A sublattice As the chemical composition deviates from the stoichiometric one and as disorder increases at high temperatures antisite atoms appear. As shown by Belova and Murch 32 interaction of the six-jump-cycles with antisite atoms remarkable widens the limits of Eq. 20.3 . Thus in a B2 alloy with some disorder values of Da Db beyond the limits of Eq. 20.3 cannot be considered as an indication that the 6JC mechanism does not operate. - Triple-defect mechanism In a B2 compound triple-defect disorder can occur according to the reaction Va Vb 2Va Ab . 20.4 Va Vb denotes a vacancy on the A B sublattice and Ab an A atom on the B sublattice see also Chap. 5 . Triple-defect disorder does not change the composition. Instead of forming equal numbers of vacancies 20.3 B2 Intermetallics 349 Fig. 20.6. Illustration of the triple-defect diffusion mechanism in the B2 structure. The arrows show vacancy jumps the numbers indicate the jump sequence on both sublattices two vacancies on one sublattice and an antisite atom on the other sublattice can appear. Triple-defect formation according to Eq. 20.4 is favoured in intermetallics with high formation enthalpies of VB vacancies. Vacancies and antisite defects can associate to form bound triple defects see also Chap. 5 . A triple-defect mechanism involving bound triple defects was proposed by Stolwijk ET AL. 33 for the B2 compound CoGa. The triple-defect mechanism in CoGa was attributed to two nearest-neighbour jumps of Co .