STRUCTURE OF MATERIALS The Key To Its Properties A .

STRUCTURE OF MATERIALSThe Key to its PropertiesA Multiscale PerspectiveAnandh SubramaniamMaterials and Metallurgical EngineeringINDIAN INSTITUTE OF TECHNOLOGY KANPURKanpur- 208016Email: anandh@iitk.ac.inhttp://home.iitk.ac.in/ anandhJan 2009

OUTLINE Properties of Materials The Scale of “Microstructures” Crystal Structures Defects Microstructure With Examples from the Materials World

PROPERTIESStructure sensitiveE.g. Yield stress, Fracture toughnessStructure InsensitiveE.g. Density, Elastic ModulusStructure Microstructure

What determines the properties? Cannot just be the composition! Few 10s of ppm of Oxygen in Cu can degrade its conductivity Cannot just be the amount of phases present! A small amount of cementite along grain boundaries can cause thematerial to have poor impact toughness Cannot just be the distribution of phases! Dislocations can severely weaken a crystal[1] Cannot just be the defect structure in the phases present! The presence of surface compressive stress toughens glassCompositionPhases &TheirDistributionResidual StressDefect Structure[1] Metals Handbook, Vol.8, 8th Edition, ASM, 1973

Length Scales ngstromsDislocation Stress fields NanometersMicronsCentimetersUnit Cell*Crystalline DefectsMicrostructureComponentDefectsGrain Size*Simple Unit Cells

ctromagneticMicrostructure Casting Metal Forming Welding Powder Processing MachiningComponent Avoid Stress Concentrators Good Surface FinishPhases Crystalline Quasicrystalline AmorphousStructurebased Ferromagnetic Ferroelectric SuperconductingPropertybased Defects Vacancies DislocationsResidual Twins Stress Stacking Faults Grain Boundaries Voids CracksProcessing determines shape and microstructure of a component

ctromagneticMicrostructure Casting Metal Forming Welding Powder Processing MachiningComponent Avoid Stress Concentrators Good Surface FinishPhases Defects& their distribution Crystalline Quasicrystalline AmorphousStructurebased Ferromagnetic Ferroelectric SuperconductingPropertybased Vacancies DislocationsResidual Twins Stress Stacking Faults Grain Boundaries Voids CracksProcessing determines shape and microstructure of a component

METALSEMI-METALBAND STRUCTURESEMI-CONDUCTORINSULATORATOMICSTATE / NANO-QUASICRYSTALSNANOCRYSTALS

magneticWhy is BCC Iron the stable form of Iron at room temperature and not the FCCform of Iron?1 AtmG vs T showing regions of stability of FCC andBCC Iron(Computed using thermo-calc software and database developed atthe Royal Institute of Technology, Stockholm)The Structure of Materials, S.M. Allen & E.L. Thomas, John Wiley & Sons, Inc. New York, 1999.

MicrostructureThis functional definitionof “microstructure”includes all lengthscalesPhases Defects Vacancies DislocationsResidual Twins Stress Stacking Faults Grain Boundaries Voids Cracks

MicrostructurePhases DefectsResidual Stress Vacancies Dislocations Twins Stacking Faults Grain Boundaries Voids Cracks

MicrostructurePhases Defects Vacancies Dislocations Twins Stacking Faults Grain Boundaries Voids CracksResidualStressDefects (Crystalline)MicrostructuralComponent Vacancies Dislocations Phase Transformation Voids Cracks Stress corrosion cracking Residual SurfaceCompressive Stress

CRYSTAL STRUCTURES

Crystal Lattice (Where to repeat) Motif (What to repeat) Unit cell of BCC latticeCrystal Space group (how to repeat) Asymmetric unit (Motif’: what to repeat)

Progressive lowering of symmetry amongst the 7 crystal systemsCubicIncreasing oclinicTriclinicArrow marks lead from supergroups to subgroups

1. Cubic Crystalsa b c 90º Simple Cubic (P)Body Centred Cubic (I) – BCCFace Centred Cubic (F) - FCC4 2Point groups 23, 4 3m, m 3 , 432,3m m[2]Vapor grown NiO crystalTetrakaidecahedron(Truncated tDodecahedron[1][1] 2] L.E. Muir, Interfacial Phenomenon in Metals, Addison-Wesley Publ. co.

Crystals and Properties (The symmetry of) Any physical property of a crystal has at least thesymmetry of the crystal Crystals are anisotropic with respect to most properties The growth shape of a (well grown) crystal has the internal symmetry ofthe crystal Polycrystalline materials or aggregates of crystals may have isotropicproperties (due to averaging of may randomly oriented grains) The properties of a crystal can be drastically altered in the presence ofdefects (starting with crystal defects)

CLASSIFICATION OF DEFECTS BASED ON DIMENSIONALITY0D(Point defects)1D(Line defects)2D(Surface / Interface)3D(Volume nTwinboundaryVoids ndaries

0D(Point defects)

VacancyNon-ioniccrystals0D(Point ionalFrenkel defectOther Schottky defect Imperfect point-like regions in the crystal about the size of 1-2 atomicdiameters

Vacancy Missing atom from an atomic site Atoms around the vacancy displaced Tensile stress field produced in the vicinityTensile StressFields

uritySubstitutionalCompressive stressfields SUBSTITUTIONAL IMPURITY Foreign atom replacing the parent atom in the crystal E.g. Cu sitting in the lattice site of FCC-Ni INTERSTITIAL IMPURITY Foreign atom sitting in the void of a crystal E.g. C sitting in the octahedral void in HT FCC-FeTensile StressFields

Interstitial C sitting in the octahedral void in HT FCC-Fe rOctahedral void / rFCC atom 0.414 rFe-FCC 1.29 Å rOctahedral void 0.414 x 1.29 0.53 Å rC 0.71 Å Compressive strains around the C atom Solubility limited to 2 wt% (9.3 at%)Interstitial C sitting in the octahedral void in LT BCC-Fe rTetrahedral void / rBCC atom 0.29 rC 0.71 Å rFe-BCC 1.258 Å rTetrahedral void 0.29 x 1.258 0.364 Å But C sits in smaller octahedral void- displaces fewer atoms Severe compressive strains around the C atom Solubility limited to 0.008 wt% (0.037 at%)

Equilibrium Concentration of Vacancies Formation of a vacancy leads to missing bonds and distortion of thelattice The potential energy (Enthalpy) of the system increases Work required for the formaion of a point defect Enthalpy of formation ( Hf) [kJ/mol or eV/defect] Though it costs energy to form a vacancy its formation leads toincrease in configurational entropy above zero Kelvin there is an equilibrium number of vacanciesSConfig k ln( ) G H T SCrystalKrCdPbZnMgAlAgCuNikJ / 41.74eV / vacancy 0.08

G (Gibbs free energy)G (perfect crystal) Gm inEquilibriumconcentration HfT (ºC)n/N5001 x 10 1010001 x 10 515005 x 10 420003 x 10 3 1 eV/vacancy 0.16 x 10 18 J/vacancyn (number of vacancies) Certain equilibrium number of vacancies are preferred at T 0K Vacancies play a role in: Diffusion Climb Electrical conductivity Creep etc.

1D(Line defects)

The shear modulus of metals is in the range 20 – 150 GPaG m 2 The theoretical shear stress will bein the range 3 – 30 GPa Actual shear stress is 0.5 – 10 MPa I.e. (Shear stress)theoretical 100 * (Shear stress)experimental !!!!DISLOCATIONSDislocations weaken the crystal

DISLOCATIONSEDGEMIXEDSCREW Usually dislocations have a mixed character and Edge and Screwdislocations are the ideal extremes

Conservative(Glide)Motion of dislocationsOn the slip planeNon-conservative(Climb)Motion of dislocation to the slip planeMotion ofEdgedislocation For edge dislocation: as b t they define a plane the slip plane Climb involves addition or subtraction of a row of atoms below thehalf plane ve climb climb up removal of a plane of atoms ve climb climb down addition of a plane of atoms

Mixed dislocations b bPure screwPure Edge t

Role of eStructuralIncoherent TwinGrain boundary(low angle)Semicoherent InterfacesCross-slipCreepmechanisms incrystallinematerialsDislocation climbVacancy diffusionGrain boundary slidingDisc of vacancies edge dislocation

2D(Surface / Interface)

Grain Boundary The grain boundary region may be distorted with atoms belonging toneither crystal The thickness may be of the order of few atomic diameters The crystal orientation changes abruptly at the grain boundary In an low angle boundary the orientation difference is 10º In the low angle boundary the distortion is not so drastic as thehigh-angle boundary can be described as an array ofdislocations Grain boundary energy is responsible for grain growth on heating ( 0.5Tm) Large grains grow at the expense of smaller ones The average no. of nearest neighbours for an atom in the grainboundary of a close packed crystal is 11

Energy (J/m2)Type of boundaryGrain boundary between BCC crystals0.89Grain boundary between FCC crystals0.85Interface between BCC and FCC crystals0.63Grain boundaries inSrTiO3

Twin Boundary The atomic arrangement on one side of the twin boundary is related tothe other side by a symmetry operation (usually a mirror) Mirror twin boundaries usually occur in pairs such that the orientationdifference introduced by one is restored by the other The region between the regions is called the twinned regionAnnealing twins (formed during recrystallization)Annealing twins in Austenitic StainlessSteelTwin[1]Deformation twins (formed during plastic deformation)[1] Transformations in Metals, Paul G. Shewmon,McGraw-Hill Book Company, New York, 1969.

Twin boundary in Fe doped SrTiO3 bicrystals (artificially prepared)High-resolution micrographMirror relatedvariantsTwin plane[1] S. Hutt, O. Kienzle, F. Ernst and M. Rühle, Z Metallkd, 92 (2001) 2

Grain size and strengthk y i d y Yield stress i Stress to move a dislocation in single crystal k Locking parameter (measure of the relativeHall-Petch Relation d Grain diameterhardening contribution of grain boundaries)

Defects:Further Enquiry

DEFECTSBased onoriginRandomStructural Vacancies Dislocations Ledges The role played by a random defect is very different from the role played by astructural defect in various phenomenon

Low Angle Grain Boundaries b 22h

8º TILT BOUNDARY IN SrTiO3 POLYCRYSTAL2.761 ÅNo visibleGrainBoundaryFourier filtered imageDislocationstructures atthe Grainboundary

DEFECTSBased onpositionRandomOrdered Vacancies Stacking Faults Ordered defects become part of the structure and hence affect the basic symmetryof the structure

Crystal with vacanciesVacancy orderingE.g. V6C5, V8C7

Yield Point Phenomenony (Å) Effect of Atomic Level Residual Stress(GPa)x (Å) Interaction of the stress fields ofdislocations’ with Interstitial atoms’

3D(Volume defects)&MICROSTRUCTURES

Bright field TEM micrograph of an Al3.3% Cu alloy, aged at room temperaturefor 100 days, showing the GP-I zones.HAADF micrographs of the GP zones:(a) Intercalated monatomic Cu layers several nm in width are clearly resolved,(b) a GP-zone two Cu layers thick can ‘chemically’ be identified.T.J. Konno, K. Hiraga and M. Kawasaki, Scripta mater. 44 (2001) 2303–2307

Precipitate particlebbIncrease in surface area due to particle shearingHardening effectPart of the dislocation line segment (inside theprecipitate) could face a higher PN stress

Pinning effect of the precipitateCan act like a Frank-Reed sourceGb 2r

Controlling microstructures:Examples

Fe-Cementite diagramPeritecticL EutecticL Fe3CL1493ºC L Fe3C1147ºC2.06 Fe3CEutectoid Fe3C723ºC [1]T Austenite ( ) FCC Ferrite ( ) BCC Cementite (Fe3C) OrthorhombicFe0.16 0.84.3%C Fe3C6.7[1] rruff.geo.arizona.edu/doclib/zk/vol74/ZK74 534.pdf

Time- Temperature-Transformation (TTT) Curves – Isothermal TransformationEutectoid steel (0.8%C)800723Eutectoid te BainiteT Metals Handbook, Vol.8, 8th edition, ASM, Metals Park, .1[1]348 C110210t (s) [2] Introduction to Physical Metallurgy, S.H. Avner, McGraw-Hill Book Company, 1974[2]103104278 C105[1][1] Physical Metallurgy for Engineers, D S Clark and W R Varney, Affiliated EastWest Press Pvt. Ltd., New Delhi, 1962

Time- Temperature-Transformation (TTT) Curves – Isothermal TransformationEutectoid steel (0.8%C)800723Eutectoid temperatureAustenitePearlite600 Fe3CT 500Pearlite Bainite400Bainite300200100MsMfMartensite0.1110210t (s) 103104105

Different cooling treatmentsEutectoid steel (0.8%C)800723M Martensite 600M0.11Coarse PFine PM P10210t (s) T lnea100nll a200FuchuenlqOi300gzinalirmNo400enchWater qu500P Pearlite103104105