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Crystallographic Aspects of Dislocations

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Outline Slip Systems BCC, FCC, HCP Cross Slip Partial Dislocations Stacking Faults The Thompson Tetrahedron Fancy Stuff Frank rule, Frank loop, Lomer lock, Lomer-Cotrell dislocations, prismatic dislocations 22.71: Physical MetallurgyProf. Michael P. Short, P. 2

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Slip Systems Systems of planes and directions that make dislocation movement easy 22.71: Physical MetallurgyProf. Michael P. Short, P. 3 Different views of FCC supercell http://ilan.schnell-web.net/physics/fcc/

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Slip Systems Systems of planes and directions that make dislocation movement easy 22.71: Physical MetallurgyProf. Michael P. Short, P. 4 Different views of FCC supercell http://ilan.schnell-web.net/physics/fcc/

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Slip Systems Systems of planes and directions that make dislocation movement easy 22.71: Physical MetallurgyProf. Michael P. Short, P. 5 Different views of FCC supercell http://ilan.schnell-web.net/physics/fcc/

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Counting Slip Systems Multiply: Number of non-parallel planes Number of close packed directions per plane 22.71: Physical MetallurgyProf. Michael P. Short, P. 6 k h l Same slip planes!

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In Class Draw primary slip systems for FCC, BCC, and HCP crystal systems 22.71: Physical MetallurgyProf. Michael P. Short, P. 7

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Evidence of Slip Systems 22.71: Physical MetallurgyProf. Michael P. Short, P. 8 http://www.doitpoms.ac.uk/tlplib/slip/printall.php

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Side Note: Twinning Bands can flip to mirror image of surrounding crystal 22.71: Physical MetallurgyProf. Michael P. Short, P. 9 Annealing twins in brass http://www.doitpoms.ac.uk/tlplib/miller_indices/printall.php

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Side Note: Twinning 22.71: Physical MetallurgyProf. Michael P. Short, P. 10 Alternate plastic deformation mechanism http://moisespinedacaf.blogspot.com/ http://dcg.materials.drexel.edu/?page_id=14#nuclear Twinning observed in irradiated reactor pressure vessel steel

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22.71: Physical MetallurgyProf. Michael P. Short, P. 11 Twinning MIT Dept. of Nuclear Science & Engineering 22.74: Radiation Damage & Effects in Nuclear Materials Prof. Michael P. Short Page 11 http://dcg.materials.drexel.edu/?page_id=14#nuclear Differently oriented dislocations inside/outside twin boundary!

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Evidence of Slip Systems 22.71: Physical MetallurgyProf. Michael P. Short, P. 12 A scanning electron micrograph of a single crystal of cadmium deforming by dislocation slip on 100 planes, forming steps on the surface http://www.doitpoms.ac.uk/tlplib/miller_indices/printall.php

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Evidence of Slip Systems 22.71: Physical MetallurgyProf. Michael P. Short, P. 13 Nanopillar compression tests using a diamond flat punch Clear 45 degree angles observed Slip systems activated by shear N. Friedman et al. Phys. Rev. Lett. 109, 095507 (2012)

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Evidence of Slip Systems 22.71: Physical MetallurgyProf. Michael P. Short, P. 14 Nanopillar compression tests using a diamond flat punch Clear 45 degree angles observed Slip systems activated by shear S. Brinckmann et al. Phys. Rev. Lett. 100, 155502 (2008)

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Secondary Slip Systems When something blocks a primary slip system, a secondary slip system may activate Only if it is energetically favorable to continue deforming What happens if a secondary system cant activate? 22.71: Physical MetallurgyProf. Michael P. Short, P. 15

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Cross Slip Dislocation switches slip systems if it get stuck Example: pinned screw dislocation 22.71: Physical MetallurgyProf. Michael P. Short, P. 16 time Derek Hull and David J. Bacon, Introduction to dislocations, 4th ed. (Butterworth-Heinemann, Oxford, 2001).

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Cross Slip 22.71: Physical MetallurgyProf. Michael P. Short, P. 17 Allen & Thomas, p. 100 k h l [101]

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Slip Systems Slip directions partially or fully enclose slip planes 22.71: Physical MetallurgyProf. Michael P. Short, P. 18 Allen & Thomas, The Structure of Materials, p. 116

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HCP Slip Systems 22.71: Physical MetallurgyProf. Michael P. Short, P. 19 Ideal c/a = 1.63299 a 1 c a2a2 1122 24 [0001] 1011

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Partial Dislocations Look carefully at the (111) plane in FCC How many ways can atom A move to location B? 22.71: Physical MetallurgyProf. Michael P. Short, P. 20 A B A B

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Partial Dislocations Look carefully at the (111) plane in FCC How many ways can atom A move to location B? 22.71: Physical MetallurgyProf. Michael P. Short, P. 21 A B A B

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Partial Dislocations A perfect dislocation can split into two partials 22.71: Physical MetallurgyProf. Michael P. Short, P. 22 Allen & Thomas, p. 119 These move in unison

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Partial Dislocations A perfect dislocation can split into two partials 22.71: Physical MetallurgyProf. Michael P. Short, P. 23 Allen & Thomas, p. 117

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Partial Dislocation Separation After formation, the two partials repel each other Why? 22.71: Physical MetallurgyProf. Michael P. Short, P. 24 Opposite screw parts attract Parallel edge parts repel

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Stacking Faults The shifted portion of the partial dislocation is a stacking fault Atomic stacking order into the screen has changed Was ABCA / BCABCABC Now it is ABCA / CABCABC 22.71: Physical MetallurgyProf. Michael P. Short, P. 25

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Stacking Fault Energy (SFE) 22.71: Physical MetallurgyProf. Michael P. Short, P. 26

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The Thompson Tetrahedron 22.71: Physical MetallurgyProf. Michael P. Short, P. 27 http://imechanica.org/files/Partial%20Dislocation%20Tutorial%20for%20FCC%20Metals.pdf

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Lomer-Cottrell Dislocation Two partials hit at 60 degree angles Each consists of a leading and trailing partial Leading partial intersections will form a new full edge dislocation 22.71: Physical MetallurgyProf. Michael P. Short, P. 28 http://imechanica.org/files/Partial%20Dislocation%20Tutorial%20for%20FCC%20Metals.pdf

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Lomer-Cottrell Dislocation 22.71: Physical MetallurgyProf. Michael P. Short, P. 29 http://imechanica.org/files/Partial%20Dislocation%20Tutorial%20for%20FCC%20Metals.pdf Lomer-Cottrell Dislocation Determination

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Lomer Lock Both original dislocations (BC and DB) were in slip planes Is the new dislocation in any slip planes? What happens next? 22.71: Physical MetallurgyProf. Michael P. Short, P. 30

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What Happens When Dislocations Get Stuck? When bits get pinned, they can bow out creating Frank-Read sources 22.71: Physical MetallurgyProf. Michael P. Short, P. 31 http://youtu.be/Db85wOCWJkU

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Dislocation Loops 22.71: Physical MetallurgyProf. Michael P. Short, P. 32 Loops have mixed edge/screw character May be circular planes of atoms between two planes

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Frank-Read Loop Sources 22.71: Physical MetallurgyProf. Michael P. Short, P. 33 Come from sessile sections of dislocations Old strain direction

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Frank-Read Loop Sources 22.71: Physical MetallurgyProf. Michael P. Short, P. 34 http://virtualexplorer.com.au/special/meansvolume/contribs/wilson/Generation.html http://www.numodis.fr/tridis/TEM/recordings/FR_loin_53.mpg

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Forces Between Dislocations 22.71: Physical MetallurgyProf. Michael P. Short, P. 35 X & Y forces, no Z-force Burgers vector of dislocation (2) transposed Line vector of dislocation (2) transposed Stress tensor induced by dislocation (1) Force vector on dislocation (2) Peach-Kohler Equation

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Forces Lead to Pileup 22.71: Physical MetallurgyProf. Michael P. Short, P. 36 http://youtu.be/r-geDwE8Z5Y Dislocations moving & piling up in Inconel 617 (Ni-based alloy) under in- situ straining in the TEM

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Forces Lead to Grain Boundaries 22.71: Physical MetallurgyProf. Michael P. Short, P. 37 http://www.tf.uni-kiel.de/matwis/amat/def_en/kap_7/backbone/r7_2_1.html Tilt grain boundary in Al http://moisespinedacaf.blogspot.com/