DISLOCATION DYNAMICS STUDIES OF DISLOCATION INTERACTION IN MULTILAYERS UROP 03 – STAGE II PRESENTATION by Rajlakshmi Purkayastha ( ) under the guidance of Prof. Prita Pant Metallurgical Engineering and Materials Science Department IIT Bombay

Outline  Introduction and Background  Image Forces  Interface Strength  Strengthening Mechanisms in Multilayers  Conclusions

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 ( magnetic_multilayers/mag n_mult.htm)

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): , 1998.]  Simulation Results [F. Akasheh,H. M. Zbib, J. P. Hirth, R. G. Hoagland, and A. Misra. Journal of Applied Physics, 101:08414, 2007.]

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

Outline  Introduction and Background  Image Forces  Interface Strength  Strengthening Mechanisms in Multilayers  Conclusions

Image Forces : Theory (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..  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 :

Image Forces : Koehler Barrier  Screw Dislocation  Edge Dislocation  Net stress where, 1

Image Forces : Simulation Results  Screw Dislocation  Mixed Dislocation

Image Forces : Simulation Results  Screw Dislocation  Mixed Dislocation

Outline  Introduction and Background  Image Forces  Interface Strength  Strengthening Mechanisms in Multilayers  Conclusions

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

Interface Strength : Results  Loop percolating through

Interface Strength : Results  Interface Strength vs Misfit Dislocation Spacing  Cu-Ni interface  Cu-Cr interface

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.]

Outline  Introduction and Background  Image Forces  Interface Strength  Strengthening Mechanisms in Multilayers  Conclusions

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

Strengthening Mechanisms  All factors combined

Strengthening Mechanisms: Dislocation Interactions  Ordered System  Semi-Random System

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

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.

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

Outline  Introduction and Background  Image Forces  Interface Strength  Strengthening Mechanisms in Multilayers  Conclusions

Conclusions

Thank You ! Metallurgical Engineering and Materials Science Department IIT Bombay