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Dislocation Lecture Summary for 22.71 on 10_4_2012

Dislocation Lecture Summary for 22.71 on 10_4_2012. Elliott Fray. Outline. What is a dislocation? Localized and Non-localized deformation Characterizing dislocations Dislocation types S tress field of a dislocation Edge type , Screw Type Slip Systems Applications.

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Dislocation Lecture Summary for 22.71 on 10_4_2012

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  1. Dislocation Lecture Summary for 22.71 on 10_4_2012 Elliott Fray

  2. Outline • What is a dislocation? • Localized and Non-localized deformation • Characterizing dislocations • Dislocation types • Stress field of a dislocation • Edge type , Screw Type • Slip Systems • Applications

  3. What is a dislocation? • A dislocation is a discontinuity at which a material’s crystal lattice shifts from an un-sheared to a sheared state • Dislocations enable a material to deform under shear stress levels ~ 1/10,000 of its theoretical yield stress

  4. Localized and Non-Localized Deformation • Crystals deform by dislocations 1 atomic layer thin () • Energy stored in the material after deformation is localized around the dislocations Dislocation Line

  5. Characterizing Dislocations • Burgers Vector • The burgers vector is a property of a dislocation which characterizes its orientation • The burgers vector closes the circuit in loop in an imperfect crystal

  6. Characterizing Dislocations • Burgers Vector • Dislocations with the same signs tend to repel, dislocations with the opposite signs tend to attract Dislocation Line ) ↔) Same, Stress fields repel one another ) ↔) Opposite, Stress fields repel attract one another

  7. A few Dislocation Types Screw Dislocation Edge Dislocation • Edge • b is perpendicular to the dislocation line • Screw • b is parallel to the dislocation line b b Mixed

  8. Stress Field of a Dislocation • Screw Dislocation • Edge Dislocation • Packing of dislocations is limited by the range of the stress field surrounding a dislocation (R1) where Ro~b

  9. Slip Systems • Materials tend to slip first along the closest packed plane • Smallest burgers vector: b • Largest spacing between planes: do • Alternatively one can find the directions of slip from the line tension Close packed plane of FCC Crystal ~ or where α= 0.5-1 Minimize this to find slip systems →

  10. Applications • FCC materials tend to yield more before failing than BCC systems due to the large number of possible slip directions • FCC Aluminum Alloys are commonly used in nuts for traditional rock climbing Nut Placing gear during trad climbing

  11. Thanks! • Dr. Meredith Aronson (2001). Materials Science and Art of Archaeological Objects. Retrieved from: http://www.ic.arizona.edu/ic/mse257/class_notes/disclocation.html • Dr. Ulrich T. Schwarz (2004). Research Topics – Semiconductor Optoelectronics. Retrieved from: http://www.physik.uni-regensburg.de/forschung/schwarz/research.html • UC Santa Barbara Materials Research Lab. Finding the Burgers Vector of a Dislocation. Retrieved from: http://www.mrl.ucsb.edu/~edkramer/LectureVGsMat100B/99Lecture11VGs/ • University of Cambridge. (2012). Slip in Close Packed Cubic Crystals. Retrieved from: http://www.doitpoms.ac.uk/tlplib/slip/slip_in_ccp.php • Iwona Erskine-Kelliw (2009). Trad Rock Climbing Gear. Retrieved from: http://www.flickr.com/photos/iwona_kellie/4584634424/ References

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