mse 6020: defects and microstructure in materials...

University of Virginia, MSE 6020: Defects and Microstructure in Materials, Leonid Zhigilei

Instructor: Leonid Zhigilei

Office: Wilsdorf Hall 303D

Office Hours: Tuesday 10:00 – 11:00 am & open

Telephone: (434) 243 3582E-mail: [email protected]

Class web page: http://www.people.virginia.edu/~lz2n/mse6020/

Class e-mail list: [email protected]

Contact Information:

Spring 2015, Tuesday and Thursday, 3:30 - 4:45 pm, Rice Hall 032

MSE 6020: Defects and Microstructure in Materials


laser-materials interactions

Research in Computational Materials Group:

Current projects include:

Development of computational methods for materials modeling at multiple length & time-scales Investigation of dynamic non-equilibrium materials processing, structure and properties of nanostructured and non-crystalline materials, mechanisms of phase transformations

structural self-organization & heat transfer in carbon nanotube based materials

Oxygen Al vapor

CW laser ablation of Al in a shear gas flow with oxidation

ablation

melting dislocations

structure of interfaces


Homework 25%2 mid-term exams 30%Final exam 35%Presentation/discussion of a controversial research problem 10%

Grading:

- discussions through e-mail list are encouraged

A pair of students debate a controversial/unresolved research issue related to crystal defects. Possible formats include

Discussion of a controversial research problem related to crystal defects:

• discussion of a comment to a research paper and reply by the authors (one student presents the point of view of the authors, another student represents the critics)

• one student proposes an approach addressing an open research question, another student reviews the approach and identifies strengths & weaknesses

• other ideas?

each student gives a brief (≤ 10 min) presentation followed by open discussion


Textbooks:Major references:D. Hull and D. J. Bacon, Introduction to Dislocations, 5th edition (Butterworth-Heinemann, 2011) or earlier editions, dislocations.D. A. Porter and K. E. Easterling, Phase Transformations in Metals and Alloys, 2nd ed. (Chapman & Hall, 1992); reprinted by CRC Press in 2003 (2nd ed.) and 2009 (3rd ed.), intro to thermodynamics, interfaces, microstructure development.S. Allen, E. L. Thomas, The Structure of Materials (John Wiley and Sons, 1999), Ch. 5.A. Kelly, G. W. Groves, P. Kidd, Crystallography and Crystal Defects (John Wiley and Sons, 2000) R. Swalin, Thermodynamics of Solids, 2nd edition (John Wiley and Sons, 1972), point defects in Chapters 11-15.J. M. Howe, Interfaces in Materials: Atomic Structure, Kinetics and Thermodynamics of Solid-Vapor, Solid-Liquid and Solid-Solid Interfaces (John Wiley & Sons, 1997), interfaces.

Reading from original journal articles will also be required in some cases. Journal articles will be provided.

Lecture notes: The lecture notes will appear at the class web page as course progresses. You can print out lecture notes before coming to class, or make your own notes and combine them with the printed lecture notes.

Required textbook: none


Crystal Defects and Microstructure in Materials Science

Defects have a profound impact on the various properties of materials: mechanical (plasticity, failure), optical (e.g., color centers), thermal and electrical transport (e.g., scattering of phonons and electrons), electronic (e.g., doping of semiconductors), etc.

Composition

Bonding Crystal Structure

ThermomechanicalProcessing

Microstructure

defects are introduced and manipulated

microstructure

processing

properties

performance


Classification of Crystal Defects

Defects may be classified into four categories based on their dimension:

0D, Point defects: atoms missing or in irregular places in the lattice

(1) vacancy(2) self-interstitial(3) interstitial impurity(4,5) substitutional impurities

Atomic configurations in the vicinity of point defects are distorted (the arrows show the displacements of atoms) → local stresses are introduced by point defects.Due to the local stresses, point defects can “feel”each other and other defects (interact) and “feel”external stresses.The interactions can give a directionality to otherwise random jumps of atoms.

1

2

3

4

5



0D, Point defects:

vacancies and self-interstitials are the only point defects that can exist in pure elemental crystals and are called intrinsic defectsinterstitial and substitutional impurities are called extrinsic point defectsIn ordered alloys or compounds atoms occupying sites on a wrong sublatticeare called antisite defectsThe requirement of charge balance in ionic crystals → Schottky and Frenkeldefects

from point to linear defects:

…

vacancy vacancy pair(still a point defect)

disc shaped agglomerate of vacancies (dislocation loop - linear defect)



1D, Linear defects: distorted atomic configurations are extended along a line and have microscopic dimensions in the directions perpendicular to the line

edge dislocation

TEM image of dislocations in NiManchester Materials Science Center

atomistic simulation of work-hardening IBM-LLNL collaboration

HRTEM image of a disclination dipole in Fe[Murayama, Howe, Hidaka, Takaki, Science 295, 2433, 2002]

The Volterra process of introduction of a negative wedge disclination

screw dislocation



2D, Planar defects: the interfaces between homogeneous regions of the material (e.g. grain boundaries, stacking faults, external surfaces)

disc shaped agglomerate of vacancies (stacking fault outlined by dislocation loop)

3D, Volume defects: Pores, cracks, foreign inclusions, …

grain boundary

CBA

stacking fault: ABCACABC

HRTEM image of small angle tilt boundary in Si

twin boundaries


Crystal Defects → Material Properties

Material properties that are strongly affected by defects can be called structure sensitive properties (e.g., yield strength and failure, diffusion, electrical & thermal conductivity of non-metals, corrosion, optical properties of “transparent materials,” …)

Mathiessien rule for phonon mean-free path:

LiFundeformed

Example: Dislocations - not just mechanical properties

plastically deformed

[Singh, Menon, Sood, PRB 74, 184302 2006]

1, 2, 3, 4 are theoretical curves accounting for scattering on dislocation core, static strain field, dynamic effects, and stacking faults individually.

vlck v31

=thermal conductivity:

surfacesdefectsphph llll1111

++=−

effect of dislocations on other properties:Ch. 17 in J. Friedel, Dislocations (Pergamon Press, 1964)


Crystal Defects → Material PropertiesSome other properties are less sensitive to defects, e.g., melting temperature, elastic moduli, thermal & electrical conductivity of metals, thermal expansion, etc. These properties are largely defined by the electronic structure and interatomic bonding.

Distance between atoms, rij, Å

Ene

rgy,

eV,

Forc

e,eV

/Å

2 4 6 8

-0.01

-0.005

0

0.005 12

1221 dr

)dU(r- F F =−=rr

2112 rrr rr−=

12 1Fr

2FrForce F2

Energy U

repulsion attraction

what characteristics of interatomic potential curve define

bulk modulusthermal expansionmelting temperature ?



Even properties that are normally not affected by defects can become sensitive to microstructure at very high defect densities.Example: melting of nanocrystalline materials

atomistic simulation of laser melting of nanocrystalline Au20 nm Au film irradiated by a 200 fs laser pulse

Lin, Leveugle, Bringa, Zhigilei, J. Phys. Chem. C 114, 5686, 2010

laser pulse20 ps 100 ps50 ps

Melting starts at grain boundaries, temperature drops (energy goes into ΔHm×Vl). Melting continues even after T drops below the equilibrium melting temperature Tm at ~30 ps and the last crystalline region disappears at ~250 ps.



Even properties that are normally not affected by defects can become sensitive to microstructure at very high defect densities.Example: melting of nanocrystalline materials

Time (ps)

T/T m

0 100 200 300 400 5000.9

0.95

1

1.05

1.1

Tm = 963 K

⎥⎦

⎤⎢⎣

⎡Δγ

−=rH

TTm

SLm

121* - temperature of the equilibrium between a

cluster of size r and the surrounding liquid

100 ps50 ps

atoms that belong to the liquid phase are blanked



The effect of microstructure on material properties is defined by characteristics of individual defects:

Structural - distortion of crystalline atomic arrangementsElectronic - local modification of electronic structureChemical - enhanced reactivity of defect sitesScattering - interaction with phonons, photons, electrons, positronsThermodynamic - enthalpies and entropies of defectsKinetic - mobility of defectsElastic - defects can be softer or stiffer than perfect crystal… etc.

“Crystals are like people: it is the defects in them that make them interesting.”-Sir Charles Frank (1911-1998)

and the collective behavior of the totality of crystal defects (microstructure).


Structural hierarchy, characteristic length- and time-scales

from Allen & Thomas, The Structure of Materialsmodeling of dislocations in semiconductors

V. Bulatov, LLNL


Nano-scale: Characteristic length ~10-9 - 10-7 m. Characteristic times ~10-14 -10-10 s. Atomic level, properties of individual defects (dislocations, vacancies, interstitials, dopants), defect mobility, diffusion, clusters, surface reactions.

Micro-scale: Characteristic length ~10-8 - 10-6 m. Characteristic times ~10-11 –10-8 s. Small ensembles of lattice defects at length scale below the grain size, defect interactions, precipitates, dislocation reactions, microcrack nucleation.

Meso-scale: Characteristic length ~10-7 - 10-4 m. Characteristic times ~10-9 -10-3 s. Ensembles of lattice defects at length scale of the grain size, shear bands, dislocation walls, disclinations, collective dynamics of microstructure, interface diffusion, grain coarsening, recrystallization, crack growth, fracture.

Macro-scale: Characteristic length ≥10-3 m. Characteristic times ≥10-3 s. Sample geometry, mechanics, plasticity of polycrystalline materials, temperature fields, hydrodynamic motion, microstructure homogenization etc.



An emerging understanding of the connections between the structure and properties of materials has lead to a remarkable progress in the design of new advanced materials.

from M. A. White, Properties of Materials


by Greg Odegard, NASA


Syllabus:

• IntroductionClassification of defects in crystalsStructural hierarchy, characteristic length- and time-scales Nanostructure, microstructure, macrostructure Structure-sensitive and structure-insensitive properties

• Point defectsEquilibrium point defect concentrationsPoint defects in metals, defect complexesPoint defects in semiconductorsPoint defects in ionic crystalsStoichiometric and nonstoichiometric compoundsDiffusion and point defectsDiffusion mechanismsExperimental observation and modeling of point defects


Syllabus:

• Line DefectsLine defects, dislocations in crystalsBurgers circuit, Burgers vector and line directionStrain energy of a dislocation, line tensionForces acting on and between dislocationsMovement of dislocations: concept of slip, slip plane and cross slipDislocation velocity, climb Generation of dislocations, dislocation reactionsLow-energy dislocation structuresDislocations in specific crystal structuresPartial dislocations and stacking faultsDislocations in FCC crystals, Thompson tetrahedronDislocations and plastic flow in crystalsStrain hardening, obstacle hardeningExperimental observation and modeling of dislocations

http://zig.onera.fr/DisGallery/3D.html


Syllabus:

• Planar DefectsInterfaces in solidsInterfacial free energyTwin boundariesStacking faultsGrain boundariesInterphase interfacesSurface energy, shapes and phases

• Development and stability of microstructureElements of microstructure in single- and multi-phase materialsStructural features and length-scalesKinetics of phase transformations, nucleation and growthRate of phase transformationsSolidification and growth morphologiesRecrystallization, grain growth and coarsening

Atomistic model of nanocrystalline solid, by Mo Li, JHU

Hexagonal symmetry of ice snowflakes by Paul R. Howell

mse 6020: defects and microstructure in materials...

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