tailieunhanh - Diffusion Solids Fundamentals Diffusion Controlled Solid State Episode 2 Part 3

Tham khảo tài liệu 'diffusion solids fundamentals diffusion controlled solid state episode 2 part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 17 Self-diffusion in Metals General Remarks Self-diffusion is the most fundamental diffusion process in a solid. This is the major reason in addition to application-oriented motives why self-diffusion studies have consumed energies of many researchers. Self-diffusion in a metallic element A is the diffusion of A atoms. In practice in most cases tagged atoms A - either radioactive or stable isotopes - are used as tracers see Chap. 13 which are chemically identical to the atoms of the base metal. As already mentioned in Chap. 8 the temperature dependence of the tracer self-diffusion coefficient D is often but by no means always described by an Arrhenius relation1 D D0 exp AH kBT with a pre-exponential factor D and an activation enthalpy ÁH. The preexponential factor can usually be written as D0 gfv a exp AS kB where AS is called the diffusion entropy g is a geometrical factor of the order of unity . g 1 for the vacancy mechanism in cubic metals f the tracer correlation factor V0 an attempt frequency of the order of the Debye frequency and a the lattice parameter. For a diffusion process with a temperature-independent activation enthalpy the Arrhenius diagram is a straight line with slope AH kB. From its intercept - for T 1 0 -the pre-exponential factor D0 is obtained. The physical meaning of the activation parameters of diffusion depends on the diffusion mechanism and on the lattice geometry see also Chap. 8 . Self-diffusion in metals is mediated by vacancy-type defects 1-6 . Strong evidence for this interpretation comes from the following observations 1. The Kirkendall effect has shown that the diffusivities of different kinds of atoms in a substitutional metallic alloy diffuse at different rates see also 1 We use in this chapter again the upper index to indicate tracer diffusivities. 298 17 Self-diffusion in Metals Chaps. 1 and 10 . Neither the direct exchange nor the ring mechanism can explain this observation. It became evident that vacancies .