Crystallographic Aspects of Dislocations Outline Slip Systems BCC

Crystallographic Aspects of Dislocations

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 Metallurgy Prof. Michael P. Short, P. 2

Slip Systems http: //ilan. schnell-web. net/physics/fcc/ • Systems of planes and directions that make dislocation movement easy Different views of FCC supercell 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 3

Slip Systems http: //ilan. schnell-web. net/physics/fcc/ • Systems of planes and directions that make dislocation movement easy Different views of FCC supercell 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 4

Slip Systems http: //ilan. schnell-web. net/physics/fcc/ • Systems of planes and directions that make dislocation movement easy Different views of FCC supercell 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 5

Counting Slip Systems • Multiply: • Number of non-parallel planes • Number of close packed directions per plane l h k Same slip planes! 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 6

In Class Draw primary slip systems for FCC, BCC, and HCP crystal systems 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 7

Evidence of Slip Systems http: //www. doitpoms. ac. uk/tlplib/slip/printall. php 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 8

Side Note: Twinning http: //www. doitpoms. ac. uk/tlplib/miller_indices/printall. php • Bands can “flip” to mirror image of surrounding crystal Annealing twins in brass 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 9

Side Note: Twinning • 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 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 10

http: //dcg. materials. drexel. edu/? page_id=14#nuclear Twinning Differently oriented dislocations inside/outside twin boundary! MIT Dept. of Nuclear Science & Engineering 22. 74: Radiation Damage & Effects in Nuclear Materials 22. 71: Physical Metallurgy Prof. Michael P. Short Page 11 Prof. Michael P. Short, P. 11

Evidence of Slip Systems http: //www. doitpoms. ac. uk/tlplib/miller_indices/printall. php A scanning electron micrograph of a single crystal of cadmium deforming by dislocation slip on 100 planes, forming steps on the surface 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 12

Evidence of Slip Systems N. Friedman et al. Phys. Rev. Lett. 109, 095507 (2012) • Nanopillar compression tests using a diamond flat punch • Clear 45 degree angles observed • Slip systems activated by shear 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 13

Evidence of Slip Systems S. Brinckmann et al. Phys. Rev. Lett. 100, 155502 (2008) • Nanopillar compression tests using a diamond flat punch • Clear 45 degree angles observed • Slip systems activated by shear 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 14

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 can’t activate? 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 15

Cross Slip Derek Hull and David J. Bacon, Introduction to dislocations, 4 th ed. (Butterworth-Heinemann, Oxford, 2001). • Dislocation switches slip systems if it get stuck • Example: pinned screw dislocation time 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 16

Cross Slip Allen & Thomas, p. 100 [101] l h 22. 71: Physical Metallurgy k Prof. Michael P. Short, P. 17

Slip Systems Allen & Thomas, “The Structure of Materials, ” p. 116 • Slip directions partially or fully enclose slip planes 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 18

HCP Slip Systems Ideal c/a = 1. 63299 c [0001] {101 1} {112 2} a 2 a 1 22. 71: Physical Metallurgy � {112 4} Prof. Michael P. Short, P. 19

Partial Dislocations • Look carefully at the (111) plane in FCC • How many ways can atom A move to location B? B B A 22. 71: Physical Metallurgy A Prof. Michael P. Short, P. 20

Partial Dislocations • Look carefully at the (111) plane in FCC • How many ways can atom A move to location B? B B A 22. 71: Physical Metallurgy A Prof. Michael P. Short, P. 21

Partial Dislocations Allen & Thomas, p. 119 • A “perfect” dislocation can split into two “partials” These move in unison 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 22

Partial Dislocations Allen & Thomas, p. 117 • A “perfect” dislocation can split into two “partials” 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 23

Partial Dislocation Separation • After formation, the two partials repel each other • Why? Opposite screw parts attract Parallel edge parts repel 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 24

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 Metallurgy Prof. Michael P. Short, P. 25

Stacking Fault Energy (SFE) • 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 26

The Thompson Tetrahedron http: //imechanica. org/files/Partial%20 Dislocation%20 Tutorial%20 for%20 FCC%20 Metals. pdf 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 27

Lomer-Cottrell Dislocation http: //imechanica. org/files/Partial%20 Dislocation%20 Tutorial%20 for%20 FCC%20 Metals. pdf • 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 Metallurgy Prof. Michael P. Short, P. 28

Lomer-Cottrell Dislocation http: //imechanica. org/files/Partial%20 Dislocation%20 Tutorial%20 for%20 FCC%20 Metals. pdf Lomer-Cottrell Dislocation Determination 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 29

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 Metallurgy Prof. Michael P. Short, P. 30

What Happens When Dislocations Get Stuck? • When bits get pinned, they can bow out… creating Frank-Read sources http: //youtu. be/Db 85 w. OCWJk. U 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 31

Dislocation Loops • Loops have mixed edge/screw character • May be circular planes of atoms between two planes 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 32

Frank-Read Loop Sources • Come from sessile sections of dislocations Old strain direction 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 33

Frank-Read Loop Sources http: //virtualexplorer. com. au/special/meansvolume/contribs/wilson/Generation. html http: //www. numodis. fr/tridis/TEM/recordings/FR_loin_53. mpg 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 34

Forces Between Dislocations • X & Y forces, no Z-force Peach-Kohler Equation Burgers vector of dislocation (2) transposed Line vector of dislocation (2) transposed Force vector on dislocation (2) Stress tensor induced by dislocation (1) 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 35

Forces Lead to Pileup Dislocations moving & piling up in Inconel 617 (Ni-based alloy) under insitu straining in the TEM http: //youtu. be/r-ge. Dw. E 8 Z 5 Y 22. 71: Physical Metallurgy Prof. Michael P. Short, P. 36

Forces Lead to Grain Boundaries http: //www. tf. uni-kiel. de/matwis/amat/def_en/kap_7/backbone/r 7_2_1. html Tilt grain boundary in Al 22. 71: Physical Metallurgy http: //moisespinedacaf. blogspot. com/ Prof. Michael P. Short, P. 37