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.

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 × 10^23 atoms or molecules.

quantum mechanics

A branch of physics that deals with atomic and subatomic systems, allowing only discrete values of energy.

Bohr atomic model

An early atomic model where electrons revolve around the nucleus in discrete orbitals.

wave mechanical model

An atomic model treating electrons as being wavelike.

quantum number

A set of four numbers used to label possible electron states.

electron state (level)

One of a set of discrete, quantized energies that are allowed for electrons.

Pauli exclusion principle

At most two electrons, with opposite spins, can occupy the same state within an atom.

ground state

A normally filled electron energy state from which an electron excitation may occur.

electron configuration

The manner in which possible electron states are filled with electrons in an atom.

valence electron

The electrons in the outermost occupied electron shell participating in interatomic bonding.

periodic table

The arrangement of chemical elements with increasing atomic number according to periodic variation in electron structure.

electropositive

Describing an atom that tends to release valence electrons.

electronegative

Describing an atom that tends to accept valence electrons.

bonding energy

The energy required to separate two atoms chemically bonded to each other.

primary bonds

Interatomic bonds that are relatively strong; includes ionic, covalent, and metallic bonding.

ionic bond

A coulombic interatomic bond existing between two adjacent and oppositely charged ions.

covalent bond

A primary interatomic bond formed by the sharing of electrons between neighboring atoms.

metallic bond

A primary interatomic bond involving the nondirectional sharing of nonlocalized valence electrons.

secondary bonds

Interatomic and intermolecular bonds that are relatively weak.

van der Waals bond

A secondary interatomic bond between adjacent molecular dipoles.

dipole (electric)

A pair of equal and opposite electrical charges separated by a small distance.

hydrogen bond

A strong secondary interatomic bond between a hydrogen atom and the electrons of adjacent atoms.

polar molecule

A molecule with a permanent electric dipole moment due to asymmetrical charge distribution.

crystalline

The state of a solid characterized by a periodic and repeating three-dimensional atomic array.

crystal structure

The manner in which atoms or ions are arranged in space for crystalline materials.

lattice

The regular geometrical arrangement of points in crystal space.

unit cell

The basic structural unit of a crystal structure, defined by atom positions within a parallelepiped volume.

face-centered cubic (FCC)

A crystal structure where atoms are located at all corner and face-centered positions of the cubic unit cell.

coordination number

The number of atomic or ionic nearest neighbors.

atomic packing factor (APF)

The fraction of the volume of a unit cell occupied by atoms or ions.

body-centered cubic (BCC)

A crystal structure with atoms located at corner and cell center positions.

hexagonal close-packed (HCP)

A crystal structure with hexagonal geometry formed by stacking close-packed planes of atoms.

polymorphism

The ability of a solid material to exist in more than one form or crystal structure.

allotropy

The existence of two or more different crystal structures for a substance.

lattice parameters

Unit cell edge lengths and interaxial angles defining the unit cell geometry.

crystal system

A classification scheme for crystal structures based on unit cell geometry.

Miller indices

A set of integers designating crystallographic planes, determined from reciprocal references.

single crystal

A crystalline solid with a periodic atomic pattern extending throughout its entirety.

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 with 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 crystal atoms.

Bragg's law

A relationship stipulating the condition for diffraction by crystallographic planes.

noncrystalline

A solid state with no long-range atomic order.

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.

alloy

A metallic substance composed of two or more elements.

solid solution

A homogeneous crystalline phase containing two or more chemical species.

solute

A minor concentration component of a solution that is dissolved in a solvent.

solvent

The component of a solution present in the greatest amount.

substitutional solid solution

A solid solution where solute atoms replace host atoms.

interstitial solid solution

A solid solution where small solute atoms occupy interstitial positions.

weight percent (wt%)

A concentration specification based on weight of an element relative to total alloy weight.

atom percent (at%)

A concentration specification based on number of moles of an element relative to total moles.

edge dislocation

A linear crystalline defect associated with lattice distortion from an extra half-plane of atoms.

dislocation line

The line extending along the end of the extra half-plane of atoms in an edge dislocation.

screw dislocation

A linear crystalline defect associated with lattice distortion forming a helical ramp.

mixed dislocation

A dislocation that contains both edge and screw components.

Burgers vector (b)

A vector denoting the magnitude and direction of lattice distortion associated with dislocation.

microstructure

The structural features of an alloy subject to observation under a microscope.

microscopy

The investigation of microstructural elements using a microscope.

photomicrograph

A photograph made with a microscope recording a microstructural image.

transmission electron microscope (TEM)

A microscope using electron beams transmitted through the specimen for internal features examination.

scanning electron microscope (SEM)

A microscope using electron beams that scan the surface of a specimen to produce an image.

scanning probe microscope (SPM)

A microscope that uses a sharp probe to produce a topographical map of a specimen surface.

atomic force microscopy (AFM)

A high-resolution type of scanning probe microscopy demonstrating resolution on the order of nanometers.

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 divided by its original gauge length.

shear

A force causing adjacent parts of the same body to slide relative to each other.

modulus of elasticity (E)

The ratio of stress to strain when deformation is totally elastic.

elastic deformation

Nonpermanent deformation that recovers upon release of applied stress.

anelastic deformation

Time-dependent elastic (nonpermanent) deformation.

Poisson's ratio

The negative ratio of lateral and axial strains resulting from applied axial stress.

plastic deformation

Permanent deformation accompanied by atomic displacements.

yielding

The onset of plastic deformation.

proportional limit

The point on a stress-strain curve where linear proportionality ceases.

yield strength

Stress required to produce a specified amount of plastic strain.

tensile strength

Maximum engineering stress that may be sustained without fracture.

ductility

A measure of material's ability to undergo plastic deformation before fracture.

resilience

Material's capacity to absorb energy elastically.

toughness

A measure of resistance to fracture and energy absorption capacity when plastically deforming.

true stress

Instantaneous applied load divided by instantaneous cross-sectional area of a specimen.

true strain

Natural logarithm of the ratio of instantaneous to original gauge length of a specimen.

hardness

Measure of a material's resistance to deformation by indentation or abrasion.

design stress

Calculated stress level multiplied by a design factor greater than unity.

safe stress

Stress used for design; for ductile metals, yield strength divided by a factor of safety.