Exam 1 key terms Materials engineering
atomic number (Z): For a chemical element, the number of protons within the atomic nucleus.
isotope: Atoms of the same element that have different atomic masses.
atomic weight (A): The weighted average of the atomic masses of an atom's naturally occurring
isotopes. It may be expressed in terms of atomic mass units (on an atomic basis), or the mass per
mole of atoms.
atomic mass unit (amu): A measure of atomic mass; 1/12 of the mass of an atom of 12C.
mole: The quantity of a substance corresponding to 6.022 × 1023 atoms or molecules.
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quantum mechanics: A branch of physics that deals with atomic and subatomic systems; it
allows only discrete values of energy. By contrast, for classical mechanics, continuous energy
values are permissible.
Bohr atomic model: An early atomic model in which electrons are assumed to revolve around
the nucleus in discrete orbitals.
wave mechanical model: An atomic model in which electrons are treated as being wavelike.
quantum number: A set of four numbers, the values of which are used to label possible electron
states. Three of the quantum numbers are integers that specify the size, shape, and spatial
orientation of an electron's probability density; the fourth number designates spin orientation.
electron state (level): One of a set of discrete, quantized energies that are allowed for electrons.
In the atomic case, each state is specified by four quantum numbers.
Pauli exclusion principle: The postulate that for an individual atom, at most two electrons,
which necessarily have opposite spins, can occupy the same state.
ground state: A normally filled electron energy state from which an electron excitation may
occur.
electron configuration: For an atom, the manner in which possible electron states are filled with
electrons.
valence electron: The electrons in the outermost occupied electron shell, which participate in
interatomic bonding.
periodic table: The arrangement of the chemical elements with increasing atomic number
according to the periodic variation in electron structure. Nonmetallic elements are positioned at
the far right-hand side of the table.
electropositive: For an atom, having a tendency to release valence electrons. Also used to
describe metallic elements.
electronegative: For an atom, having a tendency to accept valence electrons. Also used to
describe nonmetallic elements.
bonding energy: The energy required to separate two atoms that are chemically bonded to each
other. It may be expressed on a per-atom basis or per mole of atoms.
primary bonds: Interatomic bonds that are relatively strong and for which bonding energies are
relatively large. Primary bonding types are ionic, covalent, and metallic.
ionic bond: A coulombic interatomic bond that exists between two adjacent and oppositely
charged ions.
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coulombic force: A force between charged particles such as ions; the force is attractive when the
particles are of opposite charge.
covalent bond: A primary interatomic bond that is formed by the sharing of electrons between
neighboring atoms.
metallic bond: A primary interatomic bond involving the nondirectional sharing of nonlocalized
valence electrons (“sea of electrons”) that are mutually shared by all the atoms in the metallic
solid.
secondary bonds: Interatomic and intermolecular bonds that are relatively weak and for which
bonding energies are relatively small. Normally, atomic or molecular dipoles are involved.
Examples of secondary bonding types are van der Waals forces and hydrogen bonding.
van der Waals bond: A secondary interatomic bond between adjacent molecular dipoles that
may be permanent or induced.
dipole (electric): A pair of equal and opposite electrical charges separated by a small distance.
hydrogen bond: A strong secondary interatomic bond that exists between a bound hydrogen
atom (its unscreened proton) and the electrons of adjacent atoms.
polar molecule: A molecule in which there exists a permanent electric dipole moment by virtue
of the asymmetrical distribution of positively and negatively charged regions.
crystalline: The state of a solid material characterized by a periodic and repeating three-
dimensional array of atoms, ions, or molecules.
crystal structure: For crystalline materials, the manner in which atoms or ions are arrayed in
space. It is defined in terms of the unit cell geometry and the atom positions within the unit cell.
lattice: The regular geometrical arrangement of points in crystal space.
unit cell: The basic structural unit of a crystal structure. It is generally defined in terms of atom
(or ion) positions within a parallelepiped volume.
face-centered cubic (FCC): A crystal structure found in some common elemental metals.
Within the cubic unit cell, atoms are located at all corner and face-centered positions.
coordination number: The number of atomic or ionic nearest neighbors.
atomic packing factor (APF): The fraction of the volume of a unit cell that is occupied by
hard-sphere atoms or ions.
body-centered cubic (BCC): A common crystal structure found in some elemental metals.
Within the cubic unit cell, atoms are located at corner and cell center positions.
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hexagonal close-packed (HCP): A crystal structure found for some metals. The HCP unit cell is
of hexagonal geometry and is generated by the stacking of close-packed planes of atoms.
polymorphism: The ability of a solid material to exist in more than one form or crystal
structure.
allotropy: The possibility of the existence of two or more different crystal structures for a
substance (generally an elemental solid).
lattice parameters: The combination of unit cell edge lengths and interaxial angles that defines
the unit cell geometry.
crystal system: A scheme by which crystal structures are classified according to unit cell
geometry. This geometry is specified in terms of the relationships between edge lengths and
interaxial angles. There are seven different crystal systems.
Miller indices: A set of three integers (four for hexagonal) that designate crystallographic
planes, as determined from reciprocals of fractional axial intercepts.
single crystal: A crystalline solid for which the periodic and repeated atomic pattern extends
throughout its entirety without interruption.
grain: An individual crystal in a polycrystalline metal or ceramic.
polycrystalline: Crystalline materials composed of more than one crystal or grain.
grain boundary: The interface separating two adjoining grains having different crystallographic
orientations.
anisotropic: Exhibiting different values of a property in different crystallographic directions.
isotropic: Having identical values of a property in all crystallographic directions.
diffraction (x-ray): Constructive interference of x-ray beams scattered by atoms of a crystal.
Bragg's law: A relationship that stipulates the condition for diffraction by a set of
crystallographic planes.
noncrystalline: The solid state in which there is no long-range atomic order. Sometimes the
terms amorphous, glassy, and vitreous are used synonymously.
amorphous: a material with a noncrystalline structure (showing no long-range order).
vacancy: A normally occupied lattice site from which an atom or ion is missing.
self-interstitial: A host atom or ion positioned on an interstitial lattice site.
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alloy: A metallic substance that is composed of two or more elements.
solid solution: A homogeneous crystalline phase that contains two or more chemical species.
Both substitutional and interstitial solid solutions are possible.
solute: One component or element of a solution present in a minor concentration. It is dissolved
in the solvent.
solvent: The component of a solution present in the greatest amount. It is the component that
dissolves a solute.
substitutional solid solution: A solid solution in which the solute atoms replace or substitute for
the host atoms.
interstitial solid solution: A solid solution in which relatively small solute atoms occupy
interstitial positions between the solvent or host atoms.
weight percent (wt%): A concentration specification on the basis of weight (or mass) of a
particular element relative to the total alloy weight (or mass).
atom percent (at%): A concentration specification on the basis of the number of moles (or
atoms) of a particular element relative to the total number of moles (or atoms) of all elements
within an alloy.
edge dislocation: A linear crystalline defect associated with the lattice distortion produced in the
vicinity of the end of an extra half-plane of atoms within a crystal. The Burgers vector is
perpendicular to the dislocation line.
dislocation line: The line that extends along the end of the extra half-plane of atoms for an edge
dislocation and along the center of the spiral of a screw dislocation.
screw dislocation: A linear crystalline defect associated with the lattice distortion created when
normally parallel planes are joined together to form a helical ramp. The Burgers vector is parallel
to the dislocation line.
mixed dislocation: A dislocation that has both edge and screw components.
Burgers vector (b): A vector that denotes the magnitude and direction of lattice distortion
associated with a dislocation.
microstructure: The structural features of an alloy (e.g., grain and phase structure) subject to
observation under a microscope.
microscopy: The investigation of microstructural elements using some type of microscope.
photomicrograph: A photograph made with a microscope that records a microstructural image.
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transmission electron microscope (TEM): A microscope that produces an image by using
electron beams that are transmitted (pass through) the specimen. Examination of internal features
at high magnifications is possible.
scanning electron microscope (SEM): A microscope that produces an image by using an
electron beam that scans the surface of a specimen; an image is produced by reflected electron
beams. Examination of surface and/or microstructural features at high magnifications is possible.
scanning probe microscope (SPM): A microscope that does not produce an image using light
radiation. Rather, a very small and sharp probe raster scans across the specimen surface; out-of-
surface plane deflections in response to electronic or other interactions with the probe are
monitored, from which a topographical map of the specimen surface (on a nanometer scale) is
produced.
atomic force microscopy (AFM): AFM is a very-high-resolution type of scanning probe
microscopy in which the solid stylus probe is rastered across the surface (in either non-contact
and contact mode), with demonstrated resolution on the order of fractions of a nanometer, more
than 1000 times better than the optical diffraction limit.
grain size: The average grain diameter as determined from a random cross section.
engineering stress: The instantaneous load applied to a specimen divided by its cross-sectional
area before any deformation.
engineering strain: The change in gauge length of a specimen (in the direction of an applied
stress) divided by its original gauge length.
shear: A force applied so as to cause or tend to cause two adjacent parts of the same body to
slide relative to each other in a direction parallel to their plane of contact.
modulus of elasticity (E): The ratio of stress to strain when deformation is totally elastic; also a
measure of the stiffness of a material.
elastic deformation: Deformation that is nonpermanent—that is, totally recovered upon release
of an applied stress.
anelastic deformation: Time-dependent elastic (nonpermanent) deformation.
Poisson's ratio: For elastic deformation, the negative ratio of lateral and axial strains that result
from an applied axial stress.
plastic deformation: Deformation that is permanent or nonrecoverable after release of the
applied load. It is accompanied by permanent atomic displacements.
yielding: The onset of plastic deformation.
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proportional limit: The point on a stress-strain curve at which the straight-line proportionality
between stress and strain ceases.
yield strength: The stress required to produce a very slight yet specified amount of plastic
strain; a strain offset of 0.002 is commonly used.
tensile strength: The maximum engineering stress, in tension, that may be sustained without
fracture. Often termed ultimate (tensile) strength.
ductility: A measure of a material's ability to undergo appreciable plastic deformation before
fracture; it may be expressed as percent elongation (%EL) or percent reduction in area (%RA)
from a tensile test.
resilience: The capacity of a material to absorb energy when it is elastically deformed.
toughness: A mechanical characteristic that may be expressed in three contexts: (1) the measure
of a material's resistance to fracture when a crack (or other stress-concentrating defect) is
present; (2) the ability of a material to absorb energy and plastically deform before fracturing;
and (3) the total area under the material's tensile engineering stress-strain curve taken to fracture.
true stress: The instantaneous applied load divided by the instantaneous cross-sectional area of a
specimen.
true strain: The natural logarithm of the ratio of instantaneous gauge length to original gauge
length of a specimen being deformed by a uniaxial force.
hardness: The measure of a material's resistance to deformation by surface indentation or by
abrasion.
design stress: Product of the calculated stress level (based on estimated maximum load) and a
design factor (which has a value greater than unity). Used to protect against unanticipated failure.
safe stress: A stress used for design purposes; for ductile metals, it is the yield strength divided
by a factor of safety.