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MS115a Lect 17 11-02-2011

MS115a Lect 17 11-02-2011 - Defects in Solids 0-D or point...

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Defects in Solids 0-D or point defects vacancies, interstitials, etc. control mass diffusion 1-D or linear defects dislocations control deformation processes 2-D or planar defects grain boundaries, surfaces, interfaces, heterophase boundaries 3-D or volume defects voids, secondary components (phases) (mechanical properties – yield, metals) (mechanical props – fracture, ceramics)
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Volume Defects / Heterophase Boundaries Composites Two or more distinct types of materials, “phases” Boundary between them is a heterophase interface A B At grain boundaries Second phase concentrated at triple contacts of host grain boundaries Typical when liquid phase forms at high temperature liquid / amorphous grain 1 grain 3 grain 2 Balance of forces interphase boundary γ SS γ LS γ LS φ γ SS = 2 γ SL cos( φ /2) Pores 2 nd phase is a void increases scattering thermal insulation white, not transparent
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Volume Defects and Mechanics A secondary (different) material: “phase” in metals: secondary phases tend to pin dislocations Pores in ceramics: tend to be source of failure σ = F/A ε = L/L ceramic metal Mechanical Behavior σ y σ frac Y Y Y – from chemical bonds σ y – due to dislocation glide σ y (obs) << σ y (theo) σ frac – due to volume defects σ frac (obs) <<
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