various aspects of deformation twins

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Department of Metallurgical and Materials Engineering

Various aspects of deformation twining in

nanocrystalline materials

Supriya NandyRoll. No.: 14MT60R25

Indian Institute of Technology, Kharagpur 721302, India September 3 rd , 2014



Outline

Introduction

Twinning in coarse f.c.c. materials

Twinning in nanocrystalline (nc) materials• Techniques to observe twinning mechanism• Various twinning mechanisms• Twinning in non-f.c.c. nc materials

Effect of various factors on twinning

Effect of twinning on various properties

Concluding remarks

References



Introduction

Twining is the mirror symmetry arrangementof atoms across a coherent boundary , calledtwin boundary.

MechanicalTwinning

Twinning

AnnealingTwinning

MechanicalTwinning

Lowtemperature of

deformation

High strainrate

Limited slipsystem

Strain rate sensitivityDeformation twinning in nc metals results :

Strength and HardnessductilityStrain hardening rateConductivity

CrystalStructure

Twin plane Twindirection

Typicalexamples

bcc (112) [111] α -Fe

fcc (111) [112] Au, Cu

hcp (10 1 2) [ 1 011] Zn, Cd



Twinning in coarse fcc materials

Grain size

Stress required to twin



Twinning in nanocrystalline (nc) materials

Atomistic simulation approach to study the deformationmechanisms of nanocrystalline (nc) materials.Contributions• Observation of emission of partial dislocations from

grain boundary• Mechanisms of twin nucleation(a) Overlapping of two extended dislocations(b) Successive emission of Shockley partials(c) Grain boundary splitting and migration

Molecular dynamic simulation

It involves HREM, in situ HREM and diffraction analysis.HREM provides atomic resolution, diffraction analysis

provide the twin boundary density etc.Contributions • Fivefold twins• Twins with reduced macroscopic strain•

Inverse grain size effect on twins

Experimental Observations

Observation Techniques


Advantages• Illustration of the atomic level deformation

mechanisms in nc materials• Capable to depicts the real time deformation scenario• Ideal approach to study the deformation mechanisms• Simulation with high dislocation densities (severe

plastic deformation) is readily availableLimitations• Generally applied to very small grain size na materials

Short time duration leads to high strain rates ( 10 -7s-1),that are physically plausible

Molecular dynamic simulation

Experimental Observations

Advantage• Experimental techniques provide to observe relatively

wider grain distributionLimitations• Lower spatial and temporal resolution than MDS• Difficult to observe real time deformation

Observation Techniques




Various mechanisms for twinning:

Overlapping of SF ribbons

Partial emission form grain

boundary

Twinning with low macroscopic

strain

Grain boundary splitting and

migration

Sequential twinning mechanism

Partial multiplication at twin

boundaries

Dislocation rebound mechanism

Overlapping of SF ribbonsPartial emissions with same burgers vector

Partial emissions (diff. burgers vector)




Twinning in nc b.c.c. material:

Twinning in nc h.c.p. material:



Effect of twinning on various properties

Strain rate sensitivity

Twining in nc material increases the stain ratesensitivity (SRS).

Twin density Coherent Twin Boundary SRS

Strain hardening rate

Twining in nc material increases the stain

hardening rate.

Twin density Dislocation accumulation at TB s

Strain hardening rate

Strength and ductility

Twining in nc material increases the strength and

then ductility.

Transition of mechanism:H-P strengthening and cutting through twin plane

dislocation nucleation controlled

wt.%Zn SFE

Fatigue

Electrical Conductivity



References

1. Y.T. Zhu, X.Z. Liao, X.L. Wu, Prog Mater Sci, 57(2012) pp. 1-62.

2. J.W. Christian, S. Mahajan, Progress in MaterialScience, 39 (1995) pp. 1-157.

3. X.H. Chen, L. Lu, Scripta Materialia, 57 (2007) pp.133-136.

4. B. Li, E. Ma, Acta Materaialia, 57 (2009) pp. 1734-

1743.5. D.J. Siegel, Applied Physics Letters, 87 (2005) pp.121901-03.

various aspects of deformation twins

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