Material Science Midterm 1

atomic mass unit (amu)

a measure of atomic mass, 1/12 of the mass of carbon-12

atomic number (Z)

number of protons within the nucleus for an element

atomic weight (A)

weighted average of the atomic masses of an atom’s isotopes. Can be expressed using amu or mass per mole

Bohr atomic model

atomic model in which electrons revolve around the nucleus in discrete orbital

bonding energy

energy required to separate two atoms that are chemically bonded to each other

coulombic force

force between charged particles; the force is attractive when the particles are of opposite charge

covalent bond

bond formed by the sharing of electrons between neighboring atoms

dipole (electric)

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

electronegative

having a tendency to accept valence electrons

electropositive

having a tendency to release valence electrons

hydrogen bond

strong secondary interatomic bond that exists between a bound hydrogen atom and electrons of adjacent atoms

ionic bond

bond between two adjacent and oppositely charged ions

isotope

atoms of the same element but different atomic masses

metallic bond

bond involving nondirectional sharing of nonlocalized valence electrons that are mutually shared by all atoms in the metallic solid

mole

quantity of a substance. 6.022×10^(23) atoms or molecules

polar molecule

molecule that has a permanent electric dipole moment due to asymmetrical distribution of positively and negatively charged regions

primary bonding

interatomic bonds that are relatively strong and have relatively large bonding energies. Three types: ionic, covalent, and metallic

secondary bonding

interatomic and molecular bonds that are relatively weak and have relatively small bonding energies. Two types: van der Waals and hydrogen

valence electron

electrons in the outermost occupied electron shell

van der Waals bond

bond between adjacent molecular dipoles that may be permanent or induced

allotropy

possibility of the existence two or more different crystal structures for a substance

amorphous

having a noncrystalline structure

anisotropic

having different values of a property in different crystallographic directions

atomic packing factor (APF)

fraction of the volume of a unit cell that is occupied by hard-sphere atoms or ions

body-centered cubic (BCC)

atoms located at corner and cell center positions in unit cell

coordination number (CN)

number of atomic or ionic nearest neighbors

crystal system

scheme by which crystal structures are classified according to unit cell geometry

crystal structure

manner which atoms are arrayed in space

crystalline

state of solid material that has periodic and repeating arrays of atoms, ions, or molecules

diffraction (x-ray)

constructive interference of x-ray beams scattered by atoms of a crystal

face-centered cubic (FCC)

atoms located at corner and face-centered positions in a unit cell

grain

individual crystal in a polycrystalline metal or ceramic

grain boundary

interface separating two adjoining grains having different crystallographic orientations 

isotropic

having identical values of a property in all crystallographic directions

lattice

regular geometrical arrangement of points in crystal space

lattice parameters

combination of unit edge lengths and interaxial angles that define the unit cell geometry

Miller indices

set of three integers that designate crystallographic planes

noncrystalline

solid state in which there are no long-range atomic order

polycrystalline

crystalline materials composed of more than one crystal or grain

polymorphism

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

single crystal

crystalline solid that has an uninterrupted periodic and repeated atomic pattern throughout.

unit cell

basic structural unit of a crystal structure

alloy

metal substance composed of two or more elements

Boltzmann’s constant

thermal energy constant. 1.38×10^(-23) J/atom*K

composition

relative content of a particular element within an alloy. Expressed in weight percent or atom percent

edge dislocation

linear defect associated with lattice distortion produced in the vicinity of the end of an extra half-plane of atoms within a crystal

grain size

average grain diameter as determined from a random cross section

imperfection

deviation from perfection

interstitial solid solution

solid solution in which relatively small atoms occupy interstitial positions between solvent and host atoms

microscopy

investigation of microstructural elements using some ype of microscope

microstructure

structural features of an alloy subject to investigation under a microscope

photomicrograph

photograph made by microscope that records a microstructural image

point defect

crystalline defect associated with one or, at most, several atomic sites

scanning electron microscope (SEM)

microscope that produces an image by using an electron beam that scans the surface of a specimen; an image is produced by reflected electron beam

self-interstitial

host atom or ion positioned on a interstitial lattice site (interstitial: a gap)

solid solution

homogenous crystalline phase that contains two or more chemical species

solute

component or element of a solution present in a minor concentration. Dissolved in the solvent

solvent

Component of solution present in the greatest amount. Dissolves the solute

substitution solid solution

solid solution which the solute atoms replace or substitute for the host atoms

transmission electron microscope

microscope that produces an image by using electron beams that pass through the specimen

activation energy (Q)

energy required to initiate a reaction, such as diffusion

carburizing

process by which the surface carbon concentration of a ferrous alloy is increased by diffusion from the surrounding environment

concentration gradient

slope of the concentration profile at a specific position

diffusion

mass transport by atomic motion

diffusion coefficient

constant of proportionality between the diffusion flux and the concentration gradient in Fick's first law

Fick’s First and Second Law

First law: diffusion flux is proportional to the concentration gradient. This relationship is used for steady-state diffusion situations. Second law: time rate of change of concentration is proportional to the second derivative of concentration

interdiffusion (impurity diffusion)

diffusion of atoms of one metal into another metal

interstitial diffusion

diffusion mechanism by which atomic motion is from interstitial site to interstitial site

self-diffusion

atomic migration in pure metals

steady-state diffusion

diffusion condition for which there is no net accumulation or depletion of diffusing species. The diffusion flux is independent of time

vacancy diffusion

diffusion mechanism in which net atomic migration is from a lattice site to an adjacent vacancy

ductility

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

elastic deformation

Deformation that is nonpermanent—that is, totally recovered upon release of an applied stress

engineering stress

instantaneous load applied to a specimen divided by its cross-sectional area before any deformation

engineering strain

change in gauge length of a specimen (in the direction of an applied stress) divided by its original gauge length

hardness

measure of a material's resistance to deformation by surface indentation or by abrasion

modulus of elasticity

ratio of stress to strain when deformation is totally elastic; also a measure of the stiffness of a material

plastic deformation

deformation that is permanent or nonrecoverable after release of the applied load. It is accompanied by permanent atomic displacements

poisson’s ratio

for elastic deformation, the negative ratio of lateral and axial strains that result from an applied axial stress

proportional limit 

the point on a stress-strain curve at which the straight-line proportionality between stress and strain ceases

resilience

capacity of a material to absorb energy when it is elastically deformed

shear

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

tensile strength

maximum engineering stress, in tension, that may be sustained without fracture. Often termed ultimate (tensile) strength

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

yielding

the onset of plastic deformation

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