MSE306Chap5DislocationsPlasticity

# MSE306Chap5DislocationsPlasticity - Forces on Dislocations...

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Forces on Dislocations We discussed forces due to line tension It seems that dislocations will experience a force due to applied stresses Consider a force, which performs work by causing a ┴ to sweep an area, A. T F? τ τ ( 29 b L f F bLλ W A Ab W A τ = = τ = = = τ = = ; b F λ

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Peach-Koehler Force Consider work required to form a loop with surface S ( 29 [ ] ( 29 ( 29 ( 29 [ ] ( 29 ξ σ b f r ξ σ b r ξ σ b r ξ σ b r ξ S d d d d d W d d C C C C × = δ × = δ × = δ × = δ δ × = d S S δ r The force is always normal to ┴ line Shears perpendicular to plane promote cross-slip screw ┴s to Normal stresses can promote climb Stresses may be external or internal, applied or due to defects d ξ ·
Dislocation Climb Glide is considered “conservative” motion, requiring no mass transport (diffusion) In contrast, climb is “nonconservative” motion whereby diffusion enables the dislocation to move out of its slip plane.

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Dislocation Climb ) Consider an edge dislocation under normal stress parallel to b The P-K equation predicts that there will be a “climb force”, i.e. a force normal to the glide plane. How do dislocations respond to such forces? “It is reasonable to expect that the more the vacancy concentration [departs from] the equilibrium value appropriate for the temperature of the crystal, the more difficult will it become for the dislocation line to [force further departure from that] value.” At static equilibrium (or constant velocity) the climb force will equal the Chemical Force exerted by vacancies.
Chap 5 – Overview of Plasticity Skip sections 5.3, 4, 16, 18 We have already covered: 5.5 – slip planes and directions

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MSE306Chap5DislocationsPlasticity - Forces on Dislocations...

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