Chapter 1-5 Definitions (Review)

Front 1

Alloy

Back 1

A metallic material that is obtained by chemical combinations of different elements (e.g., steel is made from iron and carbon). Typically, alloys have better mechanical properties than pure metals.

Front 2

Ceramics

Back 2

A group of crystalline inorganic materials characterized by good strength, especially in compression, and high melting temperatures. Many ceramics have very good electrical and thermal insulation behavior.

Front 3

Composites

Back 3

A group of materials formed from mixtures of metals, ceramics, or polymers in such a manner that unusual combinations of properties are obtained (e.g., fiberglass).

Front 4

Composition

Back 4

The chemical make-up of a material.

Front 5

Crystalline material

Back 5

A material composed of one or many crystals. In each crystal, atoms or ions show a long-range periodic arrangement.

Front 6

Density

Back 6

Mass per unit volume of a material, usually expressed in units of g/cm3 or lb/in.3

Front 7

Fatigue failure

Back 7

Failure of a material due to repeated loading and unloading.

Front 8

Glass

Back 8

An amorphous material derived from the molten state, typically, but not always, based on silica.

Front 9

Glass-ceramics

Back 9

A special class of materials obtained by forming a glass and then heat treating it to form small crystals.

Front 10

Grain boundaries

Back 10

Regions between grains of a polycrystalline material.

Front 11

Grains

Back 11

Crystals in a polycrystalline material.

Front 12

Materials engineering

Back 12

An engineering oriented field that focuses on how to transform materials into a useful device or structure.

Front 13

Materials science

Back 13

A field of science that emphasizes studies of relationships between the microstructure, synthesis and processing, and properties of materials.

Front 14

Materials science and engineering (MSE)

Back 14

An interdisciplinary field concerned with inventing new materials and improving previously known materials by developing a deeper understanding of the microstructure-composition-synthesis-processing relationships between different materials.

Front 15

Materials science and engineering tetrahedron

Back 15

A tetrahedron diagram showing how the performance-to-cost ratio of materials depends upon the composition, microstructure, synthesis, and processing.

Front 16

Mechanical properties

Back 16

Properties of a material, such as strength, that describe how well a material withstands applied forces, including tensile or compressive forces, impact forces, cyclical or fatigue forces, or forces at high temperatures.

Front 17

Metal

Back 17

An element that has metallic bonding and generally good ductility, strength, and electrical conductivity.

Front 18

Microstructure

Back 18

The structure of a material at the microscopic length scale.

Front 19

Physical properties

Back 19

Characteristics such as color, elasticity, electrical or thermal conductivity, magnetism, and optical behavior that generally are not significantly influenced by forces acting on a material.

Front 20

Plastics

Back 20

Polymers containing other additives.

Front 21

Polycrystalline material

Back 21

A material composed of many crystals (as opposed to a single-crystal material that has only one crystal).

Front 22

Polymerization

Back 22

The process by which organic molecules are joined into giant molecules, or polymers.

Front 23

Polymers

Back 23

A group of materials normally obtained by joining organic molecules into giant molecular chains or networks. Polymers are characterized by low strengths, low melting temperatures, and poor electrical conductivity.

Front 24

Processing

Back 24

Different ways for shaping materials into useful components or changing their properties.

Front 25

Semiconductors

Back 25

A group of materials having electrical conductivity between metals and typical ceramics (e.g., Si, GaAs).

Front 26

Single crystal

Back 26

A crystalline material that is made of only one crystal (there are no grain boundaries).

Front 27

Smart material

Back 27

A material that can sense and respond to an external stimulus such as change in temperature, application of a stress, or change in humidity or chemical environment.

Front 28

Strength-to-weight ratio

Back 28

The strength of a material divided by its density; materials with a high strength-to-weight ratio are strong but lightweight.

Front 29

Structure

Back 29

Description of the arrangements of atoms or ions in a material. The structure of materials has a profound influence on many properties of materials, even if the overall composition does not change.

Front 30

Synthesis

Back 30

The process by which materials are made from naturally occurring or other chemicals.

Front 31

Thermoplastics

Back 31

A special group of polymers in which molecular chains are entangled but not interconnected. They can be easily melted and formed into useful shapes. Normally, these polymers have a chainlike structure (e.g., polyethylene).

Front 32

Thermosets

Back 32

A special group of polymers that decompose rather than melt upon heating. They are normally quite brittle due to a relatively rigid, three-dimensional network structure (e.g., polyurethane).

Front 33

Allotropy

Back 33

The characteristic of an element being able to exist in more than one crystal structure, depending on temperature and pressure.

Front 34

Amorphous material

Back 34

A material that does not have long-range order for the arrangement of its atoms (e.g., silica glass).

Front 35

Anion

Back 35

A negatively charged ion produced when an atom, usually of a non-metal, accepts one or more electrons.

Front 36

Atomic mass

Back 36

The mass of the Avogadro constant of atoms, g/mol. Normally, this is the average number of protons and neutrons in the atom. Also called the atomic weight.

Front 37

Atomic mass unit

Back 37

The mass of an atom expressed as 1 12 the mass of a carbon atom with twelve nucleons.

Front 38

Atomic number

Back 38

The number of protons in an atom.

Front 39

Aufbau Principle

Back 39

A graphical device used to determine the order in which the energy levels of quantum shells are filled by electrons.

Front 40

Avogadro constant

Back 40

The number of atoms or molecules in a mole. The Avogadro constant is 6.022 * 10^23 per mole.

Front 41

Azimuthal quantum number

Back 41

A quantum number that designates different energy levels in principal shells. Also called the secondary quantum number.

Front 42

Binding energy

Back 42

The energy required to separate two atoms from their equilibrium spacing to an infinite distance apart. The binding energy is a measure of the strength of the bond between two atoms.

Front 43

Cation

Back 43

A positively charged ion produced when an atom, usually of a metal, gives up its valence electrons.

Front 44

Coefficient of thermal expansion (CTE)

Back 44

The fractional change in linear dimension of a material per degree of temperature. A material with a low coefficient of thermal expansion tends to retain its dimensions when the temperature changes.

Front 45

Composition

Back 45

The chemical make-up of a material.

Front 46

Covalent bond

Back 46

The bond formed between two atoms when the atoms share their valence electrons.

Front 47

Crystalline materials

Back 47

Materials in which atoms are arranged in a periodic fashion exhibiting a long-range order.

Front 48

Debye interactions

Back 48

Van der Waals forces that occur between two molecules, with only one molecule having a permanent dipole moment.

Front 49

Directional relationship

Back 49

The bonds between atoms in covalently bonded materials form specific angles, depending on the material.

Front 50

Ductility

Back 50

The ability of materials to be permanently stretched or bent without breaking.

Front 51

Electronegativity

Back 51

The relative tendency of an atom to accept an electron and become an anion. Strongly electronegative atoms readily accept electrons.

Front 52

Electropositive

Back 52

The tendency for atoms to donate electrons, thus being highly reactive.

Front 53

Glass transition temperature

Back 53

A temperature above which many polymers and inorganic glasses no longer behave as brittle materials. They gain a considerable amount of ductility above the glass transition temperature.

Front 54

Hydrogen bond

Back 54

A Keesom interaction (a type of van der Waals bond) between molecules in which a hydrogen atom is involved (e.g., bonds between water molecules).

Front 55

Interatomic spacing

Back 55

The equilibrium spacing between the centers of two atoms. In solid elements, the interatomic spacing equals the apparent diameter of the atom.

Front 56

Intermetallic compound

Back 56

A compound such as Al3V formed by two or more metallic atoms; bonding is typically a combination of metallic and ionic bonds.

Front 57

Ionic bond

Back 57

The bond formed between two different atom species when one atom (the cation) donates its valence electrons to the second atom (the anion). An electrostatic attraction binds the ions together.

Front 58

Keesom interactions

Back 58

Van der Waals forces that occur between molecules that have permanent dipole moments.

Front 59

Length scale

Back 59

A relative distance or range of distances used to describe materials-related structure, properties or phenomena.

Front 60

London forces

Back 60

Van der Waals forces that occur between molecules that do not have permanent dipole moments.

Front 61

Long-range atomic arrangements

Back 61

Repetitive three-dimensional patterns with which atoms or ions are arranged in crystalline materials.

Front 62

Macrostructure

Back 62

Structure of a material at a macroscopic level. The length scale is 100,000 nm. Typical features include porosity, surface coatings, and internal or external microcracks.

Front 63

Magnetic quantum number

Back 63

A quantum number that describes the orbitals for each azimuthal quantum number.

Front 64

Metallic bond

Back 64

The electrostatic attraction between the valence electrons and the positively charged ion cores.

Front 65

Micro-electro-mechanical systems (MEMS)

Back 65

These consist of miniaturized devices typically prepared by micromachining.

Front 66

Microstructure

Back 66

Structure of a material at a length scale of 100 to 100,000 nm.

Front 67

Modulus of elasticity

Back 67

The slope of the stress-strain curve in the elastic region (E). Also known as Young’s modulus.

Front 68

Nanoscale

Back 68

A length scale of 1–100 nm.

Front 69

Nanostructure

Back 69

Structure of a material at the nanoscale ( length-scale 1–100 nm).

Front 70

Nanotechnology

Back 70

An emerging set of technologies based on nanoscale devices, phenomena, and materials.

Front 71

Nucleon

Back 71

A proton or neutron.

Front 72

Pauli exclusion principle

Back 72

No more than two electrons in a material can have the same energy. The two electrons have opposite magnetic spins.

Front 73

Polarized molecules

Back 73

Molecules that have developed a dipole moment by virtue of an internal or external electric field.

Front 74

Primary bonds

Back 74

Strong bonds between adjacent atoms resulting from the transfer or sharing of outer orbital electrons.

Front 75

Quantum numbers

Back 75

The numbers that assign electrons in an atom to discrete energy levels. The four quantum numbers are the principal quantum number n, the azimuthal quantum number l, the magnetic quantum number ml, and the spin quantum number ms.

Front 76

Quantum shell

Back 76

A set of fixed energy levels to which electrons belong. Each electron in the shell is designated by four quantum numbers.

Front 77

Secondary bond

Back 77

Weak bonds, such as van der Waals bonds, that typically join molecules to one another.

Front 78

Short-range atomic arrangements

Back 78

Atomic arrangements up to a distance of a few nm.

Front 79

Spectroscopy

Back 79

The science that analyzes the emission and absorption of electromagnetic radiation.

Front 80

Spin quantum number

Back 80

A quantum number that indicates the spin of an electron.

Front 81

Structure

Back 81

Description of spatial arrangements of atoms or ions in a material.

Front 82

Transition elements

Back 82

A set of elements with partially filled d and f orbitals. These elements usually exhibit multiple valence and are useful for electronic, magnetic, and optical applications.

Front 83

III–V semiconductor

Back 83

A semiconductor that is based on Group 3B and 5B elements (e.g., GaAs).

Front 84

II–VI semiconductor

Back 84

A semiconductor that is based on Group 2B and 6B elements (e.g., CdSe).

Front 85

Valence

Back 85

The number of electrons in an atom that participate in bonding or chemical reactions. Usually, the valence is the number of electrons in the outer s and p energy levels.

Front 86

Van der Waals bond

Back 86

A secondary bond developed between atoms and molecules as a result of interactions between dipoles that are induced or permanent.

Front 87

Yield strength

Back 87

The level of stress above which a material permanently deforms.

Front 88

Allotropy

Back 88

The characteristic of an element being able to exist in more than one crystal structure, depending on temperature and pressure.

Front 89

Amorphous materials

Back 89

Materials, including glasses, that have no long-range order or crystal structure.

Front 90

Anisotropic

Back 90

Having different properties in different directions.

Front 91

Atomic level defects

Back 91

Defects such as vacancies, dislocations, etc., occurring over a length scale comparable to a few interatomic distances.

Front 92

Atomic radius

Back 92

The apparent radius of an atom, typically calculated from the dimensions of the unit cell, using close-packed directions (depends upon coordination number).

Front 93

Basal plane

Back 93

The special name given to the close-packed plane in hexagonal close-packed unit cells.

Front 94

Basis

Back 94

A group of atoms associated with a lattice point (same as motif).

Front 95

Blow-stretch forming

Back 95

A process used to form plastic bottles.

Front 96

Bragg’s law

Back 96

The relationship describing the angle at which a beam of x-rays of a particular wavelength diffracts from crystallographic planes of a given interplanar spacing.

Front 97

Bravais lattices

Back 97

The fourteen possible lattices that can be created in three dimensions using lattice points.

Front 98

Close-packed directions

Back 98

Directions in a crystal along which atoms are in contact.

Front 99

Close-packed (CP) structure

Back 99

Structures showing a packing fraction of 0.74 (FCC and HCP).

Front 100

Coordination number

Back 100

The number of nearest neighbors to an atom in its atomic arrangement.

Front 101

Crystal structure

Back 101

The arrangement of the atoms in a material into a regular repeatable lattice. A crystal structure is fully described by a lattice and a basis.

Front 102

Crystal systems

Back 102

Cubic, tetragonal, orthorhombic, hexagonal, monoclinic, rhombohedral and triclinic arrangements of points in space that lead to fourteen Bravais lattices and hundreds of crystal structures.

Front 103

Crystallography

Back 103

The formal study of the arrangements of atoms in solids.

Front 104

Crystalline materials

Back 104

Materials comprising one or many small crystals or grains.

Front 105

Crystallization

Back 105

The process responsible for the formation of crystals, typically in an amorphous material.

Front 106

Cubic site

Back 106

An interstitial position that has a coordination number of eight. An atom or ion in the cubic site has eight nearest neighbor atoms or ions.

Front 107

Defect

Back 107

A microstructural feature representing a disruption in the perfect periodic arrangement of atoms ions in a crystalline material. This term is not used to convey the presence of a flaw in the material.

Front 108

Density

Back 108

Mass per unit volume of a material, usually in units of g cm3.

Front 109

Diamond cubic (DC)

Back 109

The crystal structure of carbon, silicon, and other covalently bonded materials.

Front 110

Diffraction

Back 110

The constructive interference, or reinforcement, of a beam of x-rays or electrons interacting with a material. The diffracted beam provides useful information concerning the structure of the material.

Front 111

Directions of a form or directions of a family

Back 111

Crystallographic directions that all have the same characteristics. Denoted by brackets.

Front 112

Electron diffraction

Back 112

A method to determine the level of crystallinity at relatively small length scales. Usually conducted in a transmission electron microscope.

Front 113

Glass-ceramics

Back 113

A family of materials typically derived from molten inorganic glasses and processed into crystalline materials with very fine grain size and improved mechanical properties.

Front 114

Glasses

Back 114

Solid, non-crystalline materials (typically derived from the molten state) that have only short-range atomic order.

Front 115

Grain

Back 115

A small crystal in a polycrystalline material.

Front 116

Grain boundaries

Back 116

Regions between grains of a polycrystalline material.

Front 117

Interplanar spacing

Back 117

Distance between two adjacent parallel planes with the same Miller indices.

Front 118

Interstitial sites

Back 118

Locations between the “normal” atoms or ions in a crystal into which another—usually different—atom or ion is placed. Typically, the size of this interstitial location is smaller than the atom or ion that is to be introduced.

Front 119

Isotropic

Back 119

Having the same properties in all directions.

Front 120

Kepler’s conjecture

Back 120

A conjecture made by Johannes Kepler in 1611 that stated that the maximum packing fraction with spheres of uniform size could not exceed . In 1998, Thomas Hales proved this to be true.

Front 121

Lattice

Back 121

A collection of points that divide space into smaller equally sized segments.

Front 122

Lattice parameters

Back 122

The lengths of the sides of the unit cell and the angles between those sides. The lattice parameters describe the size and shape of the unit cell.

Front 123

Lattice points

Back 123

Points that make up the lattice. The surroundings of each lattice point are identical.

Front 124

Linear density

Back 124

The number of lattice points per unit length along a direction.

Front 125

Liquid crystals (LCs)

Back 125

Polymeric materials that are typically amorphous but can become partially crystalline when an external electric field is applied. The effect of the electric field is reversible. Such materials are used in liquid crystal displays.

Front 126

Long-range order (LRO)

Back 126

A regular repetitive arrangement of atoms in a solid which extends over a very large distance.

Front 127

Metallic glass

Back 127

Amorphous metals or alloys obtained using rapid solidification.

Front 128

Miller-Bravais indices

Back 128

A special shorthand notation to describe the crystallographic planes in hexagonal close-packed unit cells.

Front 129

Miller indices

Back 129

A shorthand notation to describe certain crystallographic directions and planes in a material. A negative number is represented by a bar over the number.

Front 130

Motif

Back 130

A group of atoms affiliated with a lattice point (same as basis).

Front 131

Octahedral site

Back 131

An interstitial position that has a coordination number of six. An atom or ion in the octahedral site has six nearest neighbor atoms or ions.

Front 132

Packing factor

Back 132

The fraction of space in a unit cell occupied by atoms.

Front 133

Packing fraction

Back 133

The fraction of a direction (linear-packing fraction) or a plane (planar packing factor) that is actually covered by atoms or ions. When one atom is located at each lattice point, the linear packing fraction along a direction is the product of the linear density and twice the atomic radius.

Front 134

Planar density

Back 134

The number of atoms per unit area whose centers lie on the plane.

Front 135

Planes of a form or planes of a family

Back 135

Crystallographic planes that all have the same characteristics, although their orientations are different. Denoted by { } braces.

Front 136

Polycrystalline material

Back 136

A material comprising many grains.

Front 137

Polymorphism

Back 137

Compounds exhibiting more than one type of crystal structure.

Front 138

Rapid solidification

Back 138

A technique used to cool metals and alloys very quickly.

Front 139

Repeat distance

Back 139

The distance from one lattice point to the adjacent lattice point along a direction.

Front 140

Short-range order

Back 140

The regular and predictable arrangement of the atoms over a short distance—usually one or two atom spacings.

Front 141

Stacking sequence

Back 141

The sequence in which close-packed planes are stacked. If the sequence is ABABAB, a hexagonal close-packed unit cell is produced; if the sequence is ABCABCABC, a facecentered cubic structure is produced.

Front 142

Stress-induced crystallization

Back 142

The process of forming crystals by the application of an external stress. Typically, a significant fraction of many amorphous plastics can be crystallized in this fashion, making them stronger.

Front 143

Tetrahedral site

Back 143

An interstitial position that has a coordination number of four. An atom or ion in the tetrahedral site has four nearest neighbor atoms or ions.

Front 144

Tetrahedron

Back 144

The structure produced when atoms are packed together with a four-fold coordination.

Front 145

Transmission electron microscopy (TEM)

Back 145

A technique for imaging and analysis of microstructures using a high energy electron beam.

Front 146

Unit cell

Back 146

A subdivision of the lattice that still retains the overall characteristics of the entire lattice.

Front 147

X-ray diffraction (XRD)

Back 147

A technique for analysis of crystalline materials using a beam of x-rays.

Front 148

Annealing

Back 148

A heat treatment that typically involves heating a metallic material to a high temperature for an extended period of time in order to lower the dislocation density and hence impart ductility.

Front 149

ASTM

Back 149

American Society for Testing and Materials.

Front 150

ASTM grain size number (n)

Back 150

A measure of the size of the grains in a crystalline material obtained by counting the number of grains per square inch at a magnification of 100.

Front 151

Burgers vector

Back 151

The direction and distance that a dislocation moves in each step, also known asthe slip vector.

Front 152

Critical resolved shear stress

Back 152

The shear stress required to cause a dislocation to move and cause slip.

Front 153

Cross-slip

Back 153

A change in the slip system of a dislocation.

Front 154

Defect chemical reactions

Back 154

Reactions written using the Kröger-Vink notation to describe defect chemistry. The reactions must be written in such a way that mass and electrical charges are balanced and stoichiometry of sites is maintained. The existence of defects predicted by such reactions needs to be verified experimentally.

Front 155

Dislocation

Back 155

A line imperfection in a crystalline material. Movement of dislocations helps explain how metallic materials deform. Interference with the movement of dislocations helps explain how metallic materials are strengthened.

Front 156

Dislocation density

Back 156

The total length of dislocation line per cubic centimeter in a material.

Front 157

Domain

Back 157

A small region of a ferroelectric, ferromagnetic, or ferrimagnetic material in which the direction of dielectric polarization (for ferroelectric) or magnetization (for ferromagnetic or ferrimagnetic) remains the same.

Front 158

Dopants

Back 158

Elements or compounds typically added, in known concentrations and appearing at specific places within the microstructure, to enhance the properties or processing of a material.

Front 159

Edge dislocation

Back 159

A dislocation introduced into the crystal by adding an “extra half plane” of atoms.

Front 160

Elastic deformation

Back 160

Deformation that is fully recovered when the stress causing it is removed.

Front 161

Etch pits

Back 161

Holes created at locations where dislocations meet the surface. These are used to examine the presence and density of dislocations.

Front 162

Extended defects

Back 162

Defects that involve several atoms ions and thus occur over a finite volume of the crystalline material (e.g., dislocations, stacking faults, etc.).

Front 163

Ferroelectric

Back 163

A dielectric material that develops a spontaneous and reversible electric polarization (e.g., PZT, BaTiO3).

Front 164

Frenkel defect

Back 164

A pair of point defects produced when an ion moves to create an interstitial site, leaving behind a vacancy.

Front 165

Grain

Back 165

One of the crystals present in a polycrystalline material.

Front 166

Grain boundary

Back 166

A surface defect representing the boundary between two grains. The crystal has a different orientation on either side of the grain boundary.

Front 167

Grain-size strengthening

Back 167

Strengthening of a material by decreasing the grain size and therefore increasing the grain boundary area. Grain boundaries resist dislocation motion, and thus, increasing the grain boundary area leads to increased strength.

Front 168

Hall-Petch equation

Back 168

The relationship between yield strength and grain size in a metallic material— that is, .

Front 169

Image analysis

Back 169

A technique that is used to analyze images of microstructures to obtain quantitative information on grain size, shape, grain size distribution, etc.

Front 170

Impurities

Back 170

Elements or compounds that find their way into a material, often originating from processing or raw materials and typically having a deleterious effect on the properties or processing of a material.

Front 171

Interstitial defect

Back 171

A point defect produced when an atom is placed into the crystal at a site that is normally not a lattice point.

Front 172

Interstitialcy

Back 172

A point defect caused when a “normal” atom occupies an interstitial site in the crystal.

Front 173

Kröger-Vink notation

Back 173

A system used to indicate point defects in materials. The main body of the notation indicates the type of defect or the element involved. The subscript indicates the location of the point defect, and the superscript indicates the effective positive () or negative () charge.

Front 174

Metallography

Back 174

Preparation of a metallic sample of a material by polishing and etching so that the structure can be examined using a microscope.

Front 175

Mixed dislocation

Back 175

A dislocation that contains partly edge components and partly screw components.

Front 176

Peierls-Nabarro stress

Back 176

The shear stress, which depends on the Burgers vector and the interplanar spacing, required to cause a dislocation to move—that is, .

Front 177

Plastic deformation

Back 177

Permanent deformation of a material when a load is applied, then removed.

Front 178

Point defects

Back 178

Imperfections, such as vacancies, that are located typically at one (in some cases a few) sites in the crystal.

Front 179

Precipitation strengthening

Back 179

Strengthening of metals and alloys by formation of precipitates inside the grains. The small precipitates resist dislocation motion.

Front 180

Schmid’s law

Back 180

The relationship between shear stress, the applied stress, and the orientation of the slip system—that is, .

Front 181

Schottky defect

Back 181

A point defect in ionically bonded materials. In order to maintain a neutral charge, a stoichiometric number of cation and anion vacancies must form.

Front 182

Screw dislocation

Back 182

A dislocation produced by skewing a crystal by one atomic spacing so that a spiral ramp is produced.

Front 183

Second-phase strengthening

Back 183

A mechanism by which grains of an additional compound or phase are introduced in a polycrystalline material. These second phase crystals resist dislocation motion, thereby causing an increase in the strength of a metallic material.

Front 184

Sintering

Back 184

A process for forming a dense mass by heating compacted powders.

Front 185

Slip

Back 185

Deformation of a metallic material by the movement of dislocations through the crystal.

Front 186

Slip band

Back 186

Collection of many slip lines, often easily visible.

Front 187

Slip direction

Back 187

The direction in the crystal in which the dislocation moves. The slip direction is the same as the direction of the Burgers vector.

Front 188

Slip line

Back 188

A visible line produced at the surface of a metallic material by the presence of several thousand dislocations.

Front 189

Slip plane

Back 189

The plane swept out by the dislocation line during slip. Normally, the slip plane is a close-packed plane, if one exists in the crystal structure.

Front 190

Slip system

Back 190

The combination of the slip plane and the slip direction.

Front 191

Small angle grain boundary

Back 191

An array of dislocations causing a small misorientation of the crystal across the surface of the imperfection.

Front 192

Stacking fault

Back 192

A surface defect in metals caused by the improper stacking sequence of closepacked planes.

Front 193

Strain hardening

Back 193

Strengthening of a material by increasing the number of dislocations by deformation, or cold working. Also known as “work hardening.”

Front 194

Substitutional defect

Back 194

A point defect produced when an atom is removed from a regular lattice point and replaced with a different atom, usually of a different size.

Front 195

Surface defects

Back 195

Imperfections, such as grain boundaries, that form a two-dimensional plane within the crystal.

Front 196

Thermal grooving

Back 196

A technique used for observing microstructures in ceramic materials that involves heating a polished sample to a temperature slightly below the sintering temperature for a short time.

Front 197

Tilt boundary

Back 197

A small angle grain boundary composed of an array of edge dislocations.

Front 198

Transmission electron microscope (TEM)

Back 198

An instrument that, by passing an electron beam through a material, can detect microscopic structural features.

Front 199

Twin boundary

Back 199

A surface defect across which there is a mirror image misorientation of the crystal structure. Twin boundaries can also move and cause deformation of the material.

Front 200

Twist boundary

Back 200

A small angle grain boundary composed of an array of screw dislocations.

Front 201

Vacancy

Back 201

An atom or an ion missing from its regular crystallographic site.

Front 202

Yield strength

Back 202

The level of stress above which a material begins to show permanent deformation.

Front 203

Abnormal grain growth

Back 203

A type of grain growth observed in metals and ceramics. In this mode of grain growth, a bimodal grain size distribution usually emerges as some grains become very large at the expense of smaller grains. See “Grain growth” and “Normal grain growth.”

Front 204

Activation energy

Back 204

The energy required to cause a particular reaction to occur. In diffusion, the activation energy is related to the energy required to move an atom from one lattice site to another.

Front 205

Carburization

Back 205

A heat treatment for steels to harden the surface using a gaseous or solid source of carbon. The carbon diffusing into the surface makes the surface harder and more abrasion resistant.

Front 206

Concentration gradient

Back 206

The rate of change of composition with distance in a nonuniform material, typically expressed as or .

Front 207

Conductive ceramics

Back 207

Ceramic materials that are good conductors of electricity as a result of their ionic and electronic charge carriers (electrons, holes, or ions). Examples of such materials are stabilized zirconia and indium tin oxide.

Front 208

Diffusion

Back 208

The net flux of atoms, ions, or other species within a material caused by temperature and a concentration gradient.

Front 209

Diffusion bonding

Back 209

A joining technique in which two surfaces are pressed together at high pressures and temperatures. Diffusion of atoms to the interface fills in voids and produces a strong bond between the surfaces.

Front 210

Diffusion coefficient (D)

Back 210

A temperature-dependent coefficient related to the rate at which atoms, ions, or other species diffuse. The diffusion coefficient depends on temperature, the composition and microstructure of the host material and also the concentration of the diffusing species.

Front 211

Diffusion couple

Back 211

A combination of elements involved in diffusion studies (e.g., if we are considering diffusion of Al in Si, then Al-Si is a diffusion couple).

Front 212

Diffusion distance

Back 212

The maximum or desired distance that atoms must diffuse; often, the distance between the locations of the maximum and minimum concentrations of the diffusing atom.

Front 213

Diffusivity

Back 213

Another term for the diffusion coefficient (D).

Front 214

Driving force

Back 214

A cause that induces an effect. For example, an increased gradient in composition enhances diffusion; similarly reduction in surface area of powder particles is the driving force for sintering.

Front 215

Fick’s first law

Back 215

The equation relating the flux of atoms by diffusion to the diffusion coefficient and the concentration gradient.

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Fick’s second law

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The partial differential equation that describes the rate at which atoms are redistributed in a material by diffusion. Many solutions exist to Fick’s second law; Equation 5-7 is one possible solution.

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Flux

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The number of atoms or other diffusing species passing through a plane of unit area per unit time. This is related to the rate at which mass is transported by diffusion in a solid.

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Grain boundary diffusion

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Diffusion of atoms along grain boundaries. This is faster than volume diffusion, because the atoms are less closely packed in grain boundaries.

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Grain growth

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Movement of grain boundaries by diffusion in order to reduce the amount of grain boundary area. As a result, small grains shrink and disappear and other grains become larger, similar to how some bubbles in soap froth become larger at the expense of smaller bubbles. In many situations, grain growth is not desirable.

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Hot isostatic pressing

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A sintering process in which a uniform pressure is applied in all directions during sintering. This process is used for obtaining very high densities and isotropic properties.

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Hot pressing

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A sintering process conducted under uniaxial pressure, used for achieving higher densities.

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Interdiffusion

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Diffusion of different atoms in opposite directions. Interdiffusion may eventually produce an equilibrium concentration of atoms within the material.

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Interstitial diffusion

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Diffusion of small atoms from one interstitial position to another in the crystal structure.

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Liquid phase sintering

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A sintering process in which a liquid phase forms. Since diffusion is faster in liquids, if the liquid can wet the grains, it can accelerate the sintering process.

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Nitriding

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A process in which nitrogen is diffused into the surface of a material, such as a steel, leading to increased hardness and wear resistance.

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Normal grain growth

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Grain growth that occurs in an effort to reduce grain boundary area. This type of grain growth is to be distinguished from abnormal grain growth in that the grain size distribution remains unimodal but the average grain size increases steadily.

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Permeability

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A relative measure of the diffusion rate in materials, often applied to plastics and coatings. It is often used as an engineering design parameter that describes the effectiveness of a particular material to serve as a barrier against diffusion.

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Powder metallurgy

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A method for producing monolithic metallic parts; metal powders are compacted into a desired shape, which is then heated to allow diffusion and sintering to join the powders into a solid mass.

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Self-diffusion

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The random movement of atoms within an essentially pure material. No net change in composition results.

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Sintering

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A high-temperature treatment used to join small particles. Diffusion of atoms to points of contact causes bridges to form between the particles. Further diffusion eventually fills in any remaining voids. The driving force for sintering is a reduction in total surface area of the powder particles.

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Surface diffusion

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Diffusion of atoms along surfaces, such as cracks or particle surfaces.

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Thermal barrier coatings (TBC)

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Coatings used to protect a component from heat. For example, some of the turbine blades in an aircraft engine are made from nickel-based superalloys and are coated with yttria stabilized zirconia (YSZ).

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Vacancy diffusion

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Diffusion of atoms when an atom leaves a regular lattice position to fill a vacancy in the crystal. This process creates a new vacancy, and the process continues.

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Volume diffusion

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Diffusion of atoms through the interior of grains.