DISLOCATION DYNAMICS STUDIES OF DISLOCATION INTERACTION IN MULTILAYERS UROP 03 – STAGE II...

DISLOCATION DYNAMICS STUDIES OF

DISLOCATION INTERACTION IN MULTILAYERS

UROP 03 – STAGE II PRESENTATIONby

Rajlakshmi Purkayastha (04011013)

under the guidance of

Prof. Prita Pant

Metallurgical Engineering and Materials Science DepartmentIIT 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

(http://cms.tnw.utwente.nl/

magnetic_multilayers/magn_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):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.]

Microsoft

should know - hardness of Cr, Nb, Ni films also (Cu given in the plot). Is the multilayer hardness higher than what simple "rule of mixtures" would predict.

Introduction : Problem Formulation

Copper

Nickel

Two layers each 400 nm thick Each having different moduli

(Cu =41.35 GPa ,

Ni = 80.15 GPa) Sharp interface with misfit

dislocations

Factors to be considered while determining strengthening mechanism Image forces Interface Strength Dislocation Interactions

Outline


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.. (c) Image dislocation which gives 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 :

Microsoft

I would explain this just with the example of screw dislocation and air - why do you need an image dislocation - because some component of stress is not zero at free surface and it becomes zero when you put an image with -b. Now that you have an image there is attractive force on the existing dislocation Fimage = ..... For material with different modulus image dislocation burgers vector is ....... which reduces to -b for air.

Image Forces : Koehler Barrier

Screw Dislocation

Edge Dislocation

Net stress

where,

1

Microsoft

I would explain this just with the example of screw dislocation and air - why do you need an image dislocation - because some component of stress is not zero at free surface and it becomes zero when you put an image with -b. Now that you have an image there is attractive force on the existing dislocation Fimage = ..... For material with different modulus image dislocation burgers vector is ....... which reduces to -b for air.

Image Forces : Simulation Results

Screw Dislocation

Mixed Dislocation

Image Forces : Simulation Results

Screw Dislocation Mixed Dislocation

Outline


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

Microsoft

What are theta and phi in the equation? can you put in a tiny schematic?

Interface Strength : Results

Loop percolating through

Interface Strength : Results

Interface Strength vs Misfit Dislocation Spacing

Cu-Ni interface

Cu-Cr interface

Microsoft

This plot can be made smaller and the next two data points shown on the same plot - with the zoomed in version coming up when you click on the data points.

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.]

Microsoft

which number is being compared with what here?

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


Conclusions

Microsoft

Pure bulk Cu is abt. 50 MPa

Thank You !

Metallurgical Engineering and Materials Science DepartmentIIT Bombay

DISLOCATION DYNAMICS STUDIES OF DISLOCATION INTERACTION IN MULTILAYERS UROP 03 – STAGE II...

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