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Dislocation Dynamics in Multilayers: Interaction and Strengthening Mechanisms

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25 diapositives · 387 Views · Oct 31, 2014

This presentation by Rajlakshmi Purkayastha, under the guidance of Prof. Prita Pant at IIT Bombay, explores the dynamics of dislocation interactions in multilayer structures. Multilayers, composed of thin films of varying materials, exhibit extraordinary mechanical properties such as enhanced strength, hardness, and corrosion resistance. Key topics include image forces, interface strength, and various strengthening mechanisms resulting from dislocation interactions in multilayers like Cu-Ni. The findings are substantiated through experimental and simulation results, contributing to the understanding of materials at the nanoscale.

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  1. Dislocation Dynamics Studies of dislocation interaction in multilayersUROP 03 – Stage II PresentationbyRajlakshmiPurkayastha(04011013)under the guidance ofProf. Prita Pant Metallurgical Engineering and Materials Science Department IIT Bombay

  2. Outline • Introductionand Background • Image Forces • Interface Strength • Strengthening Mechanisms in Multilayers • Conclusions

  3. Introduction : Multilayers • A series of thin films of different materials deposited on top of each other • Extraordinary strength, hardness, friction and wear, higher temperature operation, corrosion resistance, fracture toughness, low electrical resistivity, heat-resistance • E.g. NiCrAlY bondcoat used as a thermal coating for turbine blades • Higher strength-to-weight ratios Co-Ni Multilayers (http://cms.tnw.utwente.nl/magnetic_multilayers/magn_mult.htm)

  4. Introduction : Experimental vs Simulation • Experimental Results [A. Misra, M. Verdier, Y.C. Lu, H. Kung, T.E. Mitchell, M. Nastasi, J.D. E Embury. Scripta Materialia, 39(4/5):555-560, 1998.] • Simulation Results [F. Akasheh ,H. M. Zbib, J. P. Hirth, R. G. Hoagland, and A. Misra. Journal of Applied Physics, 101:08414, 2007.]

  5. Introduction : Problem Formulation • Two layers each 400 nm thick • Each having different moduli (Cu =41.35 GPa , Ni = 80.15 GPa) • Sharp interface with misfit dislocations Nickel • Factors to be considered while determining strengthening mechanism • Image forces • Interface Strength • Dislocation Interactions Copper

  6. Outline • Introductionand Background • Image Forces • Interface Strength • Strengthening Mechanisms in Multilayers • Conclusions

  7. Image Forces : Theory • Origins :Motion of a dislocation towards a free surface • Violate laws of conservation • Concept of an image dislocation, from electrostatics • On left side : • On right side : (a) A screw dislocation placed at distance -x from the interface, along the z axis. (b) Dislocation and its image which produce the stress for x<0.. (c) Image dislocation which gives the stress for x>0..

  8. Image Forces : Koehler Barrier • Screw Dislocation • Edge Dislocation • Net stress where, 1

  9. Image Forces : Simulation Results Screw Dislocation • Mixed Dislocation

  10. Image Forces : Simulation Results • Mixed Dislocation Screw Dislocation

  11. Outline • Introductionand Background • Image Forces • Interface Strength • Strengthening Mechanisms in Multilayers • Conclusions

  12. Interface Strength : Misfits • Misfit dislocations at the interface of two crystal lattices with a 0.05 mismatch • [ www.tf.uni-kiel.de/matwis/amat/semien/kap5/illustr/mistdislocations.gif ] • Spacing of dislocation array is

  13. Interface Strength : Results • Loop percolating through

  14. Interface Strength : Results • Cu-Ni interface • Interface Strength vs Misfit Dislocation Spacing • Cu-Cr interface

  15. Interface Strength : Comparison • Array of Misfit Dislocations • Strength arrived at = 975 MPa • Literature : 1100 MPa from atomistics simulations • Comparison with literature • [Hoagland RG, Mitchell TE, Hirth JP, Kung H. Philos Mag A 2002;82(4):643.]

  16. Outline • Introductionand Background • Image Forces • Interface Strength • Strengthening Mechanisms in Multilayers • Conclusions

  17. Strengthening Mechanisms • Self stress due to curvature of the loop • Image forces • Repulsion due to the array of misfit dislocations at the interface • Interactions between different dislocations in the same layer

  18. Strengthening Mechanisms • All factors combined

  19. Strengthening Mechanisms: Dislocation Interactions • Ordered System • Semi-Random System

  20. Strengthening Mechanisms: Dislocation Interactions • 20 loops completely random • (a) Initial configuration • (b) Some loops start shrinking • (c) Some loops have already begun to merge with each other • (d) activity starts to take place at the interface • (e) Loops merge at the interface • (f) More loops collide

  21. Strengthening Mechanisms: Dislocation Interactions • A close up the interface interactions • (a) The blue loop in the Copper layer is very close to the red loop in the Nickel layer. • (b) The loop is so close that the motion of the red loop is hindered. However the blue loop in the Nickel layer advances. • (c) The two blue loops merge • (d) As the merged loop starts moving away , the red loop can now advance.

  22. Strengthening Mechanisms: Image Forces • Comparison to see the effect of Image forces

  23. Outline • Introductionand Background • Image Forces • Interface Strength • Strengthening Mechanisms in Multilayers • Conclusions

  24. Conclusions

  25. Thank You ! Metallurgical Engineering and Materials Science Department IIT Bombay