4020-F-2011-Lec 9 Precip.Hardening-Cambridge

4020-F-2011-Lec 9 Precip.Hardening-Cambridge - Obstacles to...

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13 AM Donald Crystalline Solids Obstacles to Dislocation Motion Many objects can impede dislocation motion: ? Other dislocations ? Precipitates ? Grain boundaries Dislocation Interactions When dislocations intersect, jogs and kinks are formed. A kink is a step in the dislocation line in the slip plane: A kink in an edge dislocation (left) and a screw dislocation (right).
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14 AM Donald Crystalline Solids A jog is a step in the dislocation line onto another slip plane : A jog in an edge dislocation (left) and a screw dislocation (right). A kink, lying in the slip plane provides no impediment to motion. This is the case when edge dislocations meet. But if a jog with edge character is formed in a screw dislocation it cannot glide since the glide plane for the jog is different from that for the main dislocation line.
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15 AM Donald Crystalline Solids In the case illustrated of a jog introduced by a screw intersecting with either another screw or an edge dislocation, the jog has edge character and a different glide plane. The jog is pinned and the dislocation is said to be 'sessile' . In this case motion can only occur by the dislocation line moving out of its existing glide plane – this is known as non-conservative motion ; the length of the dislocation line is not conserved. (Motion on the glide plane is known as conservative). b b glide plane for jog glide plane for screw
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16 AM Donald Crystalline Solids Climb – Diffusion-Controlled Creep Dislocation climb allows dislocations to climb round obstacles which are impeding their glide, thus allowing slip to continue. B A vacancy diffuses to the position of atom B, causing the dislocation to climb one lattice vector. Diffusion can occur either through the bulk of the crystal - as shown (' lattice diffusion ') - or along the dislocation core (' pipe diffusion '). This non-conservative dislocation mechanism gives rise to high temperature creep deformation. It only occurs at
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4020-F-2011-Lec 9 Precip.Hardening-Cambridge - Obstacles to...

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