Phase Transformations & Diffusion in Materials MSN 504 Phase Transformations & Diffusion in Materials Assist. Prof. Bilge Imer

Institute of Materials Science and Nanotechnology Phase A phase is a physically distinct, homogeneous portion of a thermodynamic system delineated in space by a bounding surface, the interphase interface, and distinguished by its state of aggregation (solid, liquid or gas), its crystal structure, composition and/or degree of order. Each phase generally exhibits a characteristic set of physical, mechanical and chemical properties and is conceivably mechanically separable from the whole. Bilkent University Institute of Materials Science and Nanotechnology

Institute of Materials Science and Nanotechnology Phase Transformation A Phase transformation is a change in the state of an assembly of interacting particles (atoms, molecules, electrons, etc.) as indicated by qualitative changes in the physical, mechanical and chemical properties induced by small quantitative changes in the thermodynamic variables such as T, P, E (electric field), H (magnetic field), etc. The rearrangement of the constituent particles carries the system from one configuration to another of lower free energy which can be described generally by one or several so-called order parameters which define the particular state of the system. Bilkent University Institute of Materials Science and Nanotechnology

Institute of Materials Science and Nanotechnology Diffusion It is a form of mass transport. In liquids and gases mass transport occurs in the form of convection and diffusion while in solids it only occurs with diffusion. It can be said that diffusion is the movement of particles/atoms/electrons/defects in a matter from high to low concentration in the presence of gradient until equilibrium is reached. Bilkent University Institute of Materials Science and Nanotechnology

Institute of Materials Science and Nanotechnology Nanomaterials Bilkent University Institute of Materials Science and Nanotechnology

Institute of Materials Science and Nanotechnology Types of Materials Materials can be classified according to structural, physical, electrical, optical and magnetic properties, area of use, etc. All these properties are closely related with bonding type and energies between atoms.   However if a group of material shows close resemblance in all properties we can classify them in one category. So according to this: Metals, Polymers, Ceramics and Composites can be the general classification of materials. Materials Metals Polymers Ceramics Composites Bilkent University Institute of Materials Science and Nanotechnology

Institute of Materials Science and Nanotechnology Periodic Table Bilkent University Institute of Materials Science and Nanotechnology

Courtesy of Prof. Erman Bengu, CHEM 201 Atomic Configuration • Most elements: Electron configuration not stable. Adapted from Table 2.2, Callister 7e. • Valance electrons determine chemical, electrical, thermal and optical properties, and they are responsible for bonding 5

Courtesy of Prof. Erman Bengu, CHEM 201 THE PERIODIC TABLE • Columns: Similar Valence Structure, Similar Properties Electropositive elements: Readily give up electrons to become + ions. Electronegative elements: Readily acquire electrons to become - ions.

Institute of Materials Science and Nanotechnology Bonding types BONDS PRIMARY METALLIC IONIC COVALENT SECONDARY (van der waals) Fluctuating induced dipole, polar molecule induced dipole, permanent dipole (hydrogen bond) Bilkent University Institute of Materials Science and Nanotechnology

Atomic Bonding in Solids Courtesy of Prof. Erman Bengu, CHEM 201 Atomic Bonding in Solids Start with two atoms infinitely separated Attractive component is due to nature of the bonding (minimize energy thru electronic configuration) Repulsive component is due to Pauli exclusion principle; electron clouds tend to overlap Essentially atoms either want to give up (transfer) or acquire (share) electrons to complete electron configurations; minimize their energy Transfer of electrons => ionic bond Sharing of electrons => covalent Metallic bond => sea of electons r

Courtesy of Prof. Erman Bengu, CHEM 201 METALLIC BONDING • Arises from a sea of donated valence electrons (1, 2, or 3 from each atom). Ion cores in the “sea of electrons”. Valance electrons belong no one particular atom but drift throughout the entire metal. “Free electrons” shield +’ly charged ions from repelling each other… Adapted from Fig. 2.11, Callister 6e. • Primary bond for metals and their alloys

Courtesy of Prof. Erman Bengu, CHEM 201 IONIC BONDING • Occurs between + and – ions (anion and cation). • Requires electron transfer. • Large difference in electronegativity required. • Example: Na+ Cl-

Courtesy of Prof. Erman Bengu, CHEM 201 COVALENT BONDING • Requires shared electrons • Example: CH4 C: has 4 valence e, needs 4 more H: has 1 valence e, needs 1 more Electronegativities are comparable. Adapted from Fig. 2.10, Callister 6e.

Summary: Primary Bonds Courtesy of Prof. Erman Bengu, CHEM 201 Summary: Primary Bonds Ceramics Large bond energy large Tm large E small a (Ionic & covalent bonding): Metals Variable bond energy moderate Tm moderate E moderate a (Metallic bonding): Polymers Directional Properties Secondary bonding dominates small Tm small E large a (Covalent & Secondary): secondary bonding

Courtesy of Prof. Erman Bengu, CHEM 201 SECONDARY BONDING Arises from interaction between dipoles • Fluctuating dipoles asymmetric electron clouds + - secondary bonding H 2 ex: liquid H Adapted from Fig. 2.13, Callister 7e. • Permanent dipoles-molecule induced + - -general case: secondary bonding Adapted from Fig. 2.14, Callister 7e. Cl Cl -ex: liquid HCl secondary H H bonding secondary bonding -ex: polymer secondary bonding

Courtesy of Prof. Erman Bengu, CHEM 201 Summary: Bonding Type Bond Energy Comments Ionic Large! Nondirectional (ceramics) Covalent Variable Directional (semiconductors, ceramics polymer chains) large-Diamond small-Bismuth Metallic Variable large-Tungsten Nondirectional (metals) small-Mercury Secondary smallest Directional inter-chain (polymer) inter-molecular

Courtesy of Prof. Erman Bengu, CHEM 201 Energy and Packing • Non dense, random packing Energy r typical neighbor bond length bond energy COOLING • Dense, ordered packing Energy r typical neighbor bond length bond energy Dense, ordered packed structures tend to have lower energies.

Courtesy of Prof. Erman Bengu, CHEM 201 MATERIALS AND PACKING Crystalline materials... • atoms pack in periodic, 3D arrays • typical of: -metals -many ceramics -some polymers crystalline SiO2 Adapted from Fig. 3.18(a), Callister 6e. LONG RANGE ORDER Noncrystalline materials... • atoms have no periodic packing • occurs for: -complex structures -rapid cooling "Amorphous" = Noncrystalline noncrystalline SiO2 Adapted from Fig. 3.18(b), Callister 6e. SHORT RANGE ORDER

SIMPLE CUBIC STRUCTURE (SC) Courtesy of Prof. Erman Bengu, CHEM 201 SIMPLE CUBIC STRUCTURE (SC) • Rare due to poor packing (only Po has this structure) • Close-packed directions are cube edges. Closed packed direction is where the atoms touch each other • Coordination # = 6 (# nearest neighbors) (Courtesy P.M. Anderson)

BODY CENTERED CUBIC STRUCTURE (BCC) Courtesy of Prof. Erman Bengu, CHEM 201 BODY CENTERED CUBIC STRUCTURE (BCC) • Close packed directions are cube diagonals. --Note: All atoms are identical; the center atom is shaded differently only for ease of viewing. ex: Cr, W, Fe (), Tantalum, Molybdenum • Coordination # = 8 2 atoms/unit cell: 1 center + 8 corners x 1/8 (Courtesy P.M. Anderson)

FACE CENTERED CUBIC STRUCTURE (FCC) Courtesy of Prof. Erman Bengu, CHEM 201 FACE CENTERED CUBIC STRUCTURE (FCC) • Close packed directions are face diagonals. --Note: All atoms are identical; the face-centered atoms are shaded differently only for ease of viewing. ex: Al, Cu, Au, Pb, Ni, Pt, Ag • Coordination # = 12 Adapted from Fig. 3.1, Callister 7e. 4 atoms/unit cell: 6 face x 1/2 + 8 corners x 1/8 (Courtesy P.M. Anderson)

Courtesy of Prof. Erman Bengu, CHEM 201 FCC STACKING SEQUENCE • ABCABC... Stacking Sequence • 2D Projection • FCC Unit Cell

HEXAGONAL CLOSE-PACKED STRUCTURE (HCP) Courtesy of Prof. Erman Bengu, CHEM 201 HEXAGONAL CLOSE-PACKED STRUCTURE (HCP) • ABAB... Stacking Sequence • 3D Projection • 2D Projection Adapted from Fig. 3.3, Callister 6e. • Coordination # = 12 6 atoms/unit cell • APF = 0.74 ex: Cd, Mg, Ti, Zn • c/a = 1.633

COORDINATION # AND IONIC RADII Courtesy of Prof. Erman Bengu, CHEM 201 COORDINATION # AND IONIC RADII • Coordination # increases with Adapted from Fig. 12.4, Callister 6e. Adapted from Fig. 12.2, Callister 6e. Adapted from Fig. 12.3, Callister 6e. Adapted from Table 12.2, Callister 6e.

Imperfections in Solids Courtesy of Prof. Erman Bengu, CHEM 201 Imperfections in Solids Is it enough to know bonding and structure of materials to estimate their macro properties ? BONDING + STRUCTURE DEFECTS PROPERTIES Defects do have a significant impact on the properties of materials

Imperfections in Solids Courtesy of Prof. Erman Bengu, CHEM 201 Imperfections in Solids Defects in Solids 0-D, Point defects Vacancy Interstitial Substitutional 1-D, Line Defects / Dislocations Edge Screw 2-D, Area Defects / Grain boundaries Tilt Twist 3-D, Bulk or Volume defects Crack, pore Secondary Phase Crystals in nature are never perfect, they have defects ! Atoms in irregular positions MATERIALS PROPERTIES Planes or groups of atoms in irregular positions Interfaces between homogeneous regions of atoms

Imperfections in Solids Courtesy of Prof. Erman Bengu, CHEM 201 Imperfections in Solids Atomic Composition Bonding Microstructure: Materials properties Thermo-Mechanical Processing X’tal Structure Addition and manipulation of defects

Courtesy of Prof. Erman Bengu, CHEM 201 POINT DEFECTS • Vacancies: -vacant atomic/lattice sites in a structure. • Self-Interstitials: -"extra" atoms positioned between atomic sites.

Point Defects: Vacancies & Interstitials Courtesy of Prof. Erman Bengu, CHEM 201 Point Defects: Vacancies & Interstitials Most common defects in crystalline solids are point defects. At high temperatures, atoms frequently and randomly change their positions leaving behind empty lattice sites. In general, diffusion (mass transport by atomic motion) - can only occur because of vacancies.

Point Defects: Vacancies & Interstitials Courtesy of Prof. Erman Bengu, CHEM 201 Point Defects: Vacancies & Interstitials Schematic representation of a variety of point defects: (1) vacancy; (2) self-interstitial; (3) interstitial impurity; (4,5) substitutional impurities The arrows represent the local stresses introduced by the point defects. less distortion caused