73 lines
3 KiB
Markdown
73 lines
3 KiB
Markdown
#SMMS
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cooking recipe -> microstructure -> mechanical behavior
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Elastic domain should be big (to resist constraints)
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But Plastic domain should be easy to attain (to shape the material)
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In a isotropic material with a mix of constituants :
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$$E_{tot}=\sum\%_iE_i$$
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The stress-strain curve has a big hypothesis : $\dot{\varepsilon}=cste$
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### Everything creep with enough time
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(see [[Rheology]])
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# Movement of dislocations
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Dislocation by slipping
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![[dislocations]]
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With $\vec{b}$, Burger's vector
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The energy needed to dislocate is mainly to start the dislocation (breaking the first bond $+\Delta E$) because when a bond is formed, the energy of breaking is regained ($-\Delta E$). A bit of energy is lost to move the atoms ($\delta E$), but is is negligible in front of the breaking energy ($\Delta E$).
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Energy for one step :
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$$\Delta E + \delta E - \Delta E = \delta E$$
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Covalent bonds : strong bond, so the material does not dislocate very much -> cracks instead -> brittle material (like rocks)
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Metallic bonds : weak bond, so the material can dislocate easily -> plastify -> ductile material.
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# Dependency to temperature
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![[Temperature effect on stiffness]]
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Heat brings energy to the material, so the dislocations become easier to start -> the material softens
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Also : $T\downarrow\ \Rightarrow$ hardness $\uparrow\ \Rightarrow$ ductility $\downarrow\ \Rightarrow$ brittleness $\uparrow$
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There exist a temperature of brittle/ductile transition.
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# Describing dislocations
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![[Burgers_Vector_and_dislocations_(screw_and_edge_type).svg.png]]
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$\Psi =$ angle between $\vec{b}$ and $\vec L$ (*in french "caractère"*)
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$\Psi = 90° \Rightarrow$ edge dislocation (*coin*)
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$\Psi = 0° \Rightarrow$ screw dislocation (*vis*)
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## Work of a dislocation
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![[dislocation energy]]
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$$W=FL\ \ \ W=F_a b=\tau(Ll)b\ \Rightarrow\ \frac{F}{l}=\tau b$$
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## Interactions between dislocations
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Dislocations can attract or repel each other depending on their burger vector and their dislocation line.
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Dislocations can even disappear if two opposite ones come into contact.
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Dislocations can be generated from one (Frank-Read sources).
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Due to the repelling forces between dislocations, their is a saturation level of the material, when the material is filled with dislocations, the only way to dissipate energy is fracture.
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Density of dislocations : $\rho = \frac{l}{V}$ where $l$ is the total length of the dislocations and $V$ the volume.
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($10^6$ is the lowest density we can produce and $10^{16}$ the highest before failure)
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## Crystal Twinning (*maclage*)
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$\vec{b}$ can be smaller than the movement needed to realign perfectly the crystal.
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In that case, multiple dislocations can make a perfect alignment, and that makes a twin crystal, causing an orientation change.
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![[Transform_twin.png]]
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#### Shape Memory alloy
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Using martensite, one can make a stable configuration of twinned crystal. When under a load it then undergoes "de-twinning" which stores the mechanical load.
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It can be recovered instantly, but sometimes need a bit of energy (as heat).
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