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Diffusion Solids Fundamentals Diffusion Controlled Solid State Episode 1 Part 5

Tham khảo tài liệu 'diffusion solids fundamentals diffusion controlled solid state episode 1 part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 5.4 Intermetallics 87 in Chap. 20. Some intermetallics are ordered up to their melting temperature others undergo order-disorder transitions in which an almost random arrangement of atoms is favoured at high temperatures. Such transitions occur for example between the Ị3 and Ị3 phases of the Cu-Zn system or in Fe-Co. There are intermetallic phases with wide phase fields and others which exist as stoichiometric compounds. Examples for both types can even be found in the same binary alloy system. For example the Laves phase in the Co-Nb system approximate composition Co2Nb exists over a composition range of about 5 at. whereas the phase Co7Nb2 is a line compound. Some intermetallics occur for certain stoichiometric compositions only. Others are observed for off-stoichiometric compositions. Some phases compensate off-stoichiometry by vacancies others by antisite atoms. Thermal defect populations in intermetallics can be rather complex and we shall confine ourselves to a few remarks. Intermetallic compounds are physically very different from the ionic compounds considered in the previous section. Combination of various types of disorder are conceivable vacancies and or antisite defects on both sublattices can form in some intermetallics. As self-interstitials play no role in thermal equilibrium for pure metals it is reasonable to assume that this holds true also for intermetallics. To be specific let us suppose a formula A By for the stoichiometric compound and that there is a single A sublattice and a single B sublattice. This is for example the case in intermetallics with the B2 and L12 structure see Fig. 20.1 . The basic structural elements of disorder are listed in Table 5.3. A first theoretical model for thermal disorder in a binary AB intermetallic with two sublattices was treated in the pioneering work of Wagner and ScHOTTKy 2 . Some of the more recent work on defect properties of intermetallic compounds has been reviewed by Chang and Neumann 42 and Bakker 43 .