MECH 210 081006 - Diffusion

MECH 210 081006 - Diffusion - MECH 210 081006 Diffusion...

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MECH 210 081006 Diffusion Diffusion is material transport by atomic motion. The primary driving force for atom diffusion is concentration gradient – atoms migrate from a region of higher concentration of a type of atoms to a region of lower concentration of the same type of atoms. Other factors such as temperature and species of atoms involved also affect the diffusion of atoms. Mechanisms of diffusion: vacancy diffusion and interstitial diffusion (1) Vacancy diffusion: Interchange of an atom from a normal lattice position to an adjacent vacant lattice site (vacancy). Since diffusing atoms and vacancies exchange positions, the diffusion of atoms in one direction corresponds to the motion of vacancies in the opposite direction. Self diffusion in pure metals and interdiffusion of different species of atoms in alloys normally occur by vacancy diffusion. (i) Self diffusion: same type of atoms exchange position (ii) Interdiffusion: atoms of one type metal diffuse into another (2) Interstitial diffusion: Atom migrates from an interstitial site to a neighboring vacant interstitial site. Small impurity atoms such as hydrogen, carbon, nitrogen, and oxygen can fit into interstitial positions more easily, so they migrate by interstitial diffusion. Host or substitutional atoms are too large for interstitial positions, so they rarely migrate
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MECH 210 081006 - Diffusion - MECH 210 081006 Diffusion...

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