l9 - CIVE 2700 Civil Engineering Materials, Winter 2007...

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CIVE 2700 – Civil Engineering Materials, Winter 2007 1 Diffusion Professor George Hadjisophocleous, Ph.D., P.Eng. Carleton University Department of Civil and Environmental Engineering Ottawa, Ontario CIVE 2700 – Civil Engineering materials, Winter 2007, 8.2 Outline • Diffusion couple • Steady-state diffusion • Transient diffusion
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CIVE 2700 – Civil Engineering Materials, Winter 2007 2 CIVE 2700 – Civil Engineering materials, Winter 2007, 8.3 Introduction • Diffusion is the phenomenon of material transport by atomic motion. • This lecture discusses the mechanisms and mathematics of diffusion, and the influence of temperature and diffusing species on the rate of diffusion. • Example of diffusion is the diffusion couple, which is formed by joining bars of two different metals together. CIVE 2700 – Civil Engineering materials, Winter 2007, 8.4 Diffusion Couple • This couple is heated for an extended period of time at an elevated temperature (but below the melting temperature of both metals), and cooled to room temperature. • The process, whereby atoms of one metal diffuse into another, is termed inter-diffusion or impurity diffusion. • There is a net drift or transport of atoms from high to low concentration regions.
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CIVE 2700 – Civil Engineering Materials, Winter 2007 3 CIVE 2700 – Civil Engineering materials, Winter 2007, 8.5 Diffusion Couple • Diffusion also occurs for pure metals, but all atoms exchanging positions are of the same type; this is termed self-diffusion. CIVE 2700 – Civil Engineering materials, Winter 2007, 8.6 Diffusion Mechanisms • Diffusion is the stepwise migration of atoms from lattice site to lattice site • For an atom to make such a move, two conditions must be met: – 1. there must be an empty adjacent site, and – 2. the atom must have sufficient energy to break bonds with its neighbor atoms and then cause some lattice distortion during the displacement. • The fraction of the total number of atoms capable of diffusive motion increases with rising temperature. • Models of metallic diffusion are: vacancy diffusion and interstitial diffusion
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CIVE 2700 – Civil Engineering Materials, Winter 2007 4 CIVE 2700 – Civil Engineering materials, Winter 2007, 8.7 Vacancy Diffusion • Interchange of an atom from a normal lattice position to an adjacent vacant lattice site or vacancy. • The extend to which vacancy diffusion can occur is a function of the number of vacancies present. • Significant concentrations of vacancies may exist in metals at elevated temperatures • Since diffusing atoms and vacancies exchange positions, the diffusion of atoms in one direction corresponds to the motion of vacancies in the opposite direction . CIVE 2700 – Civil Engineering materials, Winter 2007, 8.8
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This note was uploaded on 11/13/2011 for the course CIVE 2*** taught by Professor - during the Spring '11 term at Carleton CA.

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l9 - CIVE 2700 Civil Engineering Materials, Winter 2007...

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