Material Science (MSE 2001) Flashcards

Metal

Chemically characterized by metallic bonding, where positive ion cores are surrounded by a delocalized "sea" of valence electrons.

Ceramics

Chemically characterized by ionic and/or covalent bonding.

Polymers

Chemically characterized by covalent bonds along long-chain molecular backbones with secondary (van der Waals) bonds between chains.

Covalent Bonding

Sharing of valence electrons between atoms with similar electronegativities, related materials- Semicounducts, elementals metal ,hard ceramics, polymers

Metallic Bonding

Positive ionic cores held together by non-directional attraction to delocalized valence electrons. related materials- Pure metals and alloys

van der Waals Bonding

Weak physical bonds caused by fluctuating or permanent dipoles, related materials- Inert gases, molecular solids, and inter-chain bonding in polymers.

Hydrogen Bonding

A strong secondary bond type occurring between a hydrogen atom bound to a highly electronegative atom (N, O, F) and another electronegative atom. related materials-Water/Ice ($\text{H}_2\text{O}$), proteins, DNA, and polymers like Nylon.

Ionic Bonding and related materials

Electrostatic attraction following electron transfer between metallic cations and nonmetallic anions, related materials- Ceramics and salts

Plot of Interatomic Energies vs. Separation Distance

Determining Possible Bonding Types from Chemical Formulas

Metal+Nonmetal:Predominantly Ionic.

Nonmetal+Nonmetal:Covalent.

Metal+Metal/Single Metal Element:Metallic.

Long-chain Hydrocarbons / Organic Molecules: Covalent along the main chain, van der Waals / Hydrogen bonding between chains

Percent Ionic Character Formula

Crystalline vs. Non-Crystalline (Amorphous) Materials

Cystalline Materials:”Atoms are arranged in a periodic, repeating 3D pattern over long-range atomic distances. Upon solidification, they form ordered structures that exhibit sharp melting points.

Non Crystalline Materials: Atoms lack long-range periodic atomic order; only short-range chemical ordering exists

Unit Cell

The smallest repeating structural unit or building block of a crystal lattice that, through translational repetition in 3D space, generates the entire crystal structure.

FCC (Face-centered cubic) Unit CElls

A cube with atoms at all 8 corners and at the center of all 6 faces that results in a face-centered cubic lattice structure, maximizing packing efficiency and coordination number.

Body-Centered Cubic (BCC) Unit Cells

A cube with atoms at all 8 corners and 1 central atom in the interior body center

Hexagonal Close-Packed (HCP): Unit Cells

A hexagonal prism with 12 corner atoms (6 top, 6 bottom), 2 face-centered atoms (1 top, 1 bottom), and 3 inner plane atoms forming a central layer.

Theoretical Density Calculation

APF and Coordination Numbers

Direction Indicates Construction

• Establish a coordinate system origin $(0,0,0)$.

• Identify vector tail and head coordinates.

• Subtract tail coordinates from head coordinates ($x_2-x_1, y_2-y_1, z_2-z_1$) in terms of lattice parameters $a, b, c$.

• Multiply or divide by a factor to reduce values to the smallest integer set $u, v, w$.

• Enclose in square brackets: $[uvw]$ (use overbars for negative integers, e.g., $[\bar{1}10]$)

Miller Indices (hkl) for Planes Construction

• Select a plane that does not pass through the origin $(0,0,0)$ (reposition origin if necessary).

• Determine intercept distances along axes $x, y, z$ in terms of lattice parameters $a, b, c$ (if parallel to an axis, intercept is $\infty$).

• Take reciprocals of these intercepts.

• Multiply by a common factor to clear fractions into the smallest integer set $h, k, l$.

• Enclose in round parentheses: $(hkl)$ (use overbars for negative numbers, e.g., $(1\bar{1}0)$

Drawing Atomic Packing Arrangements on Planes

• Draw the geometry of the plane (e.g., square for $(100)$ in FCC, rectangle for $(110)$ in FCC).

• Calculate side lengths in terms of atomic radius $R$ using unit cell edge relationship $a$.

• Draw atomic cross-sections (circles of radius $R$) centered at their relative crystallographic positions on that plane surface.

Linear and Planar Density Calculations

Stacking of Close-Packed Planes in FCC and HCP

Single Crystals vs. Polycrystals; Isotropy vs. Anisotropy

• Single Crystal: A crystalline solid where the periodic crystal lattice is continuous throughout the entire specimen volume without internal grain boundaries.

• Polycrystalline Material: A solid composed of many small single-crystalline regions (grains) oriented in varying directions and bounded by grain boundaries.

• Anisotropy: Material properties vary depending on the direction of measurement relative to crystallographic axes. (Characteristic of single crystals).

• Isotropy: Material properties are identical in all directions. (Characteristic of randomly oriented polycrystalline materials)

Texture

a state in a polycrystalline material where individual crystalline grains possess a non-random, preferred crystallographic orientation relative to macro axes (often induced by deformation processes like rolling or forging)

Polymorphism

The ability of a solid material/compound to exist in more than one crystal structure depending on ambient temperature and pressure.

Allotropy

• Polymorphism occurring specifically in elemental solids (e.g., Pure Iron shifting from BCC $\alpha$-Fe to FCC $\gamma$-Fe; Carbon existing as Graphite or Diamond).

Grain Boundary

two-dimensional (planar) defect that separates adjacent single-crystal regions (grains) having different crystallographic orientations within a polycrystalline material.

Phenomenon of X-Ray Diffraction

This occurs when a beam of monochromatic X-rays interacts with the periodic planes of atoms in a crystal lattice. When diffracted waves satisfy Bragg's Law, constructive interference occurs, producing characteristic intensity peaks:

Interplanar Spacing for Cubic Crystals

Vacancy Defects

A 0D point defect formed by an unoccupied or missing lattice site within an otherwise perfect crystal structure.

Self-Interstitial Defect

A 0D point defect where a host atom is squeezed into an interstitial site (a small space between lattice positions not normally occupied).

Equilibrium Vacancy Number Calculation

Alloy

a metallic substance composed of two or more elements, where at least one major component is a metal, combined to achieve desired physical/mechanical properties.

Substitutional Solid Solution

Solute atoms replace or substitute host solvent atoms at normal lattice positions.

Interstitial Solid Solution

Small solute atoms fit into empty interstitial spaces between host solvent atoms.

Criteria for Solid Solution Formation

• Hume-Rothery Rules for Complete Substitutional Solubility:

  1. Atomic Size Factor: Atomic radii difference must be less than 15 percent

  2. Crystal Structure: Elements must have identical crystal structures.

  3. Electronegativity: Electronegativities must be similar.

  4. Valence: Valences should match (metals dissolve higher valence metals more readily).

• Criteria for Interstitial Solid Solutions: Solute atoms must be significantly smaller than host solvent atoms (atomic radius ratio $r_{\text{solute}}/R_{\text{host}} less than 0.59$)

Edge Dislocations (1D Defects)

Formed by inserting an extra half-plane of atoms.

• Burgers vector ($\vec{b}$) is perpendicular ($\perp$) to dislocation line ($\vec{l}$).

Screw Dislocation (1D Defects)

Formed by shear stress creating a spiral/helical structural distortion.

• Burgers vector ($\vec{b}$) is parallel ($\parallel$) to dislocation line ($\vec{l}$).

Mixed Dislocation(1D Defects)

• Mixed Dislocation: Exhibits combined edge and screw components.

  • Burgers vector ($\vec{b}$) is neither perpendicular nor parallel to dislocation line ($\vec{l}$).

Atomic Arrangement Near a Twin Boundary

special grain boundary across which there is a specific mirror lattice symmetry. Atoms on one side of the boundary are located in mirror-image positions relative to atoms on the opposite side.

Planar (2D) Defects

• External Surface (high surface energy)

• Grain Boundaries (high-angle & low-angle boundaries)

• Twin Boundaries

• Stacking Faults (disruption in close-packed stacking sequence)

• Phase Boundaries

Microstructure

Structural features of a material (such as grain size, phase distribution, and defects) observable under a microscope within the scale range of $0.1\ \mu\text{m}$ to $100\ \mu\text{m}$.

Microscopy

The discipline and set of experimental techniques using optical, electron, or scanning probe instruments to image and characterize microstructural features.

Specimen Preparation for Optical Microscopy

• Sectioning: Cutting a representative sample to appropriate size.

• Grinding: Sequential grinding using silicon carbide abrasive papers of increasing fineness.

• Polishing: Polishing with fine diamond or alumina suspensions to produce a scratch-free, mirror-like surface.

• Etching: Applying a chemical reagent (e.g., Nital for steels) that preferentially attacks high-energy areas (like grain boundaries or different phases), creating surface relief that reflects light differently under the optical microscope.

Scanning Electron Microscope (SEM)

Scans a focused beam of high-energy electrons across a sample surface. Backscattered or secondary electrons emitted from the surface are detected to form high-resolution, 3D-like topographical images.

Transmission Electron Microscope (TEM)

Transmits an electron beam directly through an ultra-thin specimen ($<100\text{ nm}$). Internal structural features, atomic plane lattices, and dislocation lines are imaged at ultra-high magnification based on beam absorption/diffraction differences.

Atomic Mechanisms of Diffusion & Rate Comparison

• Vacancy Diffusion: Atoms jump into neighboring vacant lattice sites.

• Interstitial Diffusion: Small solute atoms migrate between interstitial spaces within the host lattice.

• Comparison & Reason: Interstitial diffusion occurs much more rapidly than vacancy diffusion. This is because interstitial sites are much more abundant than lattice vacancies, and the activation energy required for interstitial atoms to move between spaces is significantly lower.