MSE 6020: Defects and Microstructure in Materials Spring 2015, Tuesday and Thursday, 3:30-4:45 pm, Rice Hall 032

Transcription

1 MSE 6020: Defects and Microstructure in Materials Spring 2015, Tuesday and Thursday, 3:30-4:45 pm, Rice Hall 032 Contact Information: Instructor: Leonid Zhigilei Office: Wilsdorf Hall 303D Office Hours: Tuesday 10:00 11:00 am & open Telephone: (434) Class web page: Class list:

2 Research in Computational Materials Group: 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 Current projects include: laser-materials interactions melting dislocations ablation structural self-organization & heat transfer in carbon nanotube based materials structure of interfaces CW laser ablation of Al in a shear gas flow with oxidation Oxygen Al vapor

3 Grading: Homework 25% - discussions through list are encouraged 2 mid-term exams 30% Final exam 35% Presentation/discussion of a controversial research problem 10% Discussion of a controversial research problem related to crystal defects: A pair of students debate a controversial/unresolved research issue related to crystal defects. Possible formats include 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

4 Textbooks: Required textbook: none 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, 2 nd ed. (Chapman & Hall, 1992); reprinted by CRC Press in 2003 (2 nd ed.) and 2009 (3 rd 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 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.

5 Crystal Defects and Microstructure in Materials Science microstructure Composition processing Bonding Crystal Structure performance properties Thermomechanical Processing defects are introduced and manipulated Microstructure 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.

6 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

7 Classification of Crystal Defects 0D, Point defects: vacancies and self-interstitials are the only point defects that can exist in pure elemental crystals and are called intrinsic defects interstitial and substitutional impurities are called extrinsic point defects In ordered alloys or compounds atoms occupying sites on a wrong sublattice are called antisite defects The requirement of charge balance in ionic crystals Schottky and Frenkel defects from point to linear defects: vacancy vacancy pair (still a point defect) disc shaped agglomerate of vacancies (dislocation loop - linear defect)

8 Classification of Crystal Defects 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 Ni Manchester Materials Science Center atomistic simulation of work-hardening IBM-LLNL collaboration screw dislocation The Volterra process of introduction of a negative wedge disclination HRTEM image of a disclination dipole in Fe [Murayama, Howe, Hidaka, Takaki, Science 295, 2433, 2002]

9 Classification of Crystal Defects 2D, Planar defects: the interfaces between homogeneous regions of the material (e.g. grain boundaries, stacking faults, external surfaces) C B A stacking fault: ABCACABC disc shaped agglomerate of vacancies (stacking fault outlined by dislocation loop) grain boundary twin boundaries HRTEM image of small angle tilt boundary in Si 3D, Volume defects: Pores, cracks, foreign inclusions,

10 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, ) undeformed LiF Example: Dislocations - not just mechanical properties thermal conductivity: 1 c 3 Mathiessien rule for phonon mean-free path: 1 = l l [Singh, Menon, Sood, PRB 74, ] k 1, 2, 3, 4 are theoretical curves accounting for scattering on dislocation core, static strain field, dynamic effects, and stacking faults individually. = ph ph l v vl defects effect of dislocations on other properties: surfaces University Ch. 17 in J. of Friedel, Virginia, Dislocations MSE 6020: Defects (Pergamon and Microstructure Press, 1964) in Materials, Leonid Zhigilei l 1 plastically deformed

11 Crystal Defects Material Properties Some 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. Energy, ev, Force, ev/å dr repulsion Force F 2 Energy U attraction F r = F r = Distance between atoms, r University of Virginia, MSE 6020: Defects and Microstructure ij,å in Materials, Leonid Zhigilei F r 2 r - du(r r r 12 = r1 r2 ) 2 1 F r 1 what characteristics of interatomic potential curve define bulk modulus thermal expansion melting temperature?

12 Crystal Defects Material Properties Even properties that are normally not affected by defects can become sensitive to microstructure at very high defect densities. Example: melting of nanocrystalline materials laser pulse atomistic simulation of laser melting of nanocrystalline Au 20 nm Au film irradiated by a 200 fs laser pulse 20 ps 50 ps 100 ps Melting starts at grain boundaries, temperature drops (energy goes into ΔH m V l ). Melting continues even after T drops below the equilibrium melting temperature T m at ~30 ps and the last crystalline region disappears at ~250 ps. Lin, Leveugle, Bringa, Zhigilei, J. Phys. Chem. C 114, 5686, 2010

13 Crystal Defects Material Properties Even properties that are normally not affected by defects can become sensitive to microstructure at very high defect densities. Example: melting of nanocrystalline materials ps 100 ps 1.05 T/T m 1 T m = 963 K Time (ps) atoms that belong to the liquid phase are blanked T 1 2γ ΔH m 1 r * SL - temperature of the equilibrium between a = Tm cluster of size r and the surrounding liquid

14 Crystal Defects Material Properties The effect of microstructure on material properties is defined by characteristics of individual defects: Structural - distortion of crystalline atomic arrangements Electronic - local modification of electronic structure Chemical - enhanced reactivity of defect sites Scattering - interaction with phonons, photons, electrons, positrons Thermodynamic - enthalpies and entropies of defects Kinetic - mobility of defects Elastic - defects can be softer or stiffer than perfect crystal etc. and the collective behavior of the totality of crystal defects (microstructure). Crystals are like people: it is the defects in them that make them interesting. -Sir Charles Frank ( )

15 Structural hierarchy, characteristic length- and time-scales modeling of dislocations in semiconductors University from Allen & of Thomas, Virginia, The MSE Structure 6020: Defects of Materials and Microstructure in Materials, V. Bulatov, Leonid LLNL Zhigilei

16 Structural hierarchy, characteristic length- and time-scales Nano-scale: Characteristic length ~ m. Characteristic times ~ s. Atomic level, properties of individual defects (dislocations, vacancies, interstitials, dopants), defect mobility, diffusion, clusters, surface reactions. Micro-scale: Characteristic length ~ m. Characteristic times ~ s. Small ensembles of lattice defects at length scale below the grain size, defect interactions, precipitates, dislocation reactions, microcrack nucleation. Meso-scale: Characteristic length ~ m. Characteristic times ~ 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.

17 Structural hierarchy, characteristic length- and time-scales by Greg Odegard, NASA 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

18 Syllabus: Introduction Classification of defects in crystals Structural hierarchy, characteristic length- and time-scales Nanostructure, microstructure, macrostructure Structure-sensitive and structure-insensitive properties Point defects Equilibrium point defect concentrations Point defects in metals, defect complexes Point defects in semiconductors Point defects in ionic crystals Stoichiometric and nonstoichiometric compounds Diffusion and point defects Diffusion mechanisms Experimental observation and modeling of point defects

19 Syllabus: Line Defects Line defects, dislocations in crystals Burgers circuit, Burgers vector and line direction Strain energy of a dislocation, line tension Forces acting on and between dislocations Movement of dislocations: concept of slip, slip plane and cross slip Dislocation velocity, climb Generation of dislocations, dislocation reactions Low-energy dislocation structures Dislocations in specific crystal structures Partial dislocations and stacking faults Dislocations in FCC crystals, Thompson tetrahedron Dislocations and plastic flow in crystals Strain hardening, obstacle hardening Experimental observation and modeling of dislocations

20 Syllabus: Planar Defects Interfaces in solids Interfacial free energy Twin boundaries Stacking faults Grain boundaries Interphase interfaces Surface energy, shapes and phases Development and stability of microstructure Elements of microstructure in single- and multi-phase materials Structural features and length-scales Kinetics of phase transformations, nucleation and growth Rate of phase transformations Solidification and growth morphologies Recrystallization, grain growth and coarsening Atomistic model of nanocrystalline solid, by Mo Li, JHU Hexagonal symmetry of ice snowflakes by Paul R. Howell