TAILIEUCHUNG - Diffusion Solids Fundamentals Diffusion Controlled Solid State Episode 1 Part 7

Tham khảo tài liệu 'diffusion solids fundamentals diffusion controlled solid state episode 1 part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Pressure Dependence 139 vation volumes are considered as evidence among others for self-interstitial mediated diffusion in silicon see 6 15 and Chap. 23 . Activation Volumes of Solute Diffusion For diffusion of interstitial solutes no defect formation term is required. Then from Eqs. and the activation volume is AV VM where VM is the migration volume of the interstitial solute. As already mentioned small atoms such as C N and O in metals diffuse by this mechanism. The effect of pressure was studied for C and N in O-Fe for C in Co and for C in Ni and for N and O diffusion in V for references see Chap. 10 in 1 . Interstitial diffusion is characterised by small values of the activation volume. For example for C and N diffusion in o-iron small values between and Q were reported see Table . This implies that interstitial diffusion is only very weakly pressure dependent. Diffusion of substitutional impurities is mediated by vacancies. According to Sect. the diffusivity can be written as D ga2n exp - exp H f kB 1 J k kB 1 J where GB is the Gibbs ebergy of binding between solute and vacancy. HG denotes the activation enthalpy for defect-impurity exchange and f2 the correlation factor of impurity diffusion. Using Eq. we get for the activation volume of solute diffusion AV V-v - VB VM Bil - dp Ch The term Vjv VB represents the formation volume of the impurity-vacancy pair. It is different from the formation volume of the vacancy in the pure solvent due to the volume change VB associated with pair formation. V2M is the migration volume of the vacancy-solute exchange which in general is different from the migration volume V-v of the vacancy in the pure matrix. Finally the term C2 arises from the pressure dependence of the solute correlation factor. V2m C2 can be interpreted as the migration volume of the solute-vacancy complex. The activation volumes for various solutes in aluminium listed in Table show a .

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