181 terms

Material Science

Sophomore Level Course for Mechanical Engineers
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eutectic
-the lowest temperature at which two solids are at equilibrium w/ liquid phase
peritectic
a three phase reaction where a solid phase and liquid phase transform to a single solid phase upon cooling
Austenite
One of the allotropes of iron, also known as gamma iron. It is formed when iron is between 912 ° and 1,394 °C and has a face-centred cubic structure. Also found in carbon steel.
Allotropy
The ability of a substance to exist in more than one phase in the solid (or indeed, liquid and gaseous) state.
Annealing
A heat treatment designed to soften a metal or alloy. Microstructurally, annealing is associated with recovery, recrystallization and/or grain growth.
Bainite
A non-equilibrium phase, usually in steel, which is formed by quenching from the austenite phase. The rate of quenching required is slower than that necessary to form martensite but faster than that which produce the equilibrium phase of pearlite.
BCC
Body-centered cubic packing describes a way in which atoms (considered as hard spheres) pack together to fill space. It comprises a cube of 8 atoms, with another atom at the center.
FCC
Face-centered cubic describes a way in which atoms pack together to fill space. The first layer (A) consists of an hexagonal array of atoms. The next layer (B) sits in the hollows of the first layer. The third layer (C) does not duplicate either A or B layer, giving an ABCABC... structure.
Fatigue
The failure of a structure subjected to repeated loading at stress levels below those required to cause general yielding. The process of fatigue may involve the initiation and growth of cracks from stress concentrations. However, the initiation stage is often by-passed since micro-cracks are usually introduced into a structure during processing.
S-N curve
A graph plotting stress amplitude, S versus number of cycles to failure, N for a fatigue test.
Some materials, notably low carbon steels exhibit a fatigue limit, below which stress, failure never occurs. More commonly, no such limit exists and an endurance limit must be defined as the stress required to cause failure after 108 cycles.
Ferrite
Pure iron up to 912 ºC has a bcc structure and is known as alpha ferrite. Between 1394 ºC and the melting point of iron the bcc structure is now known as delta ferrite. Also found in carbon steel.
Fick's first law of diffusion
States that the rate of diffusion, or flux, J of a species is proportional to the concentration gradient.
allotropy
the possibility of the existence of two or more different crystal structures for a substance (generally an elemental solid)
amorphous
(adj.) shapeless, without definite form; of no particular type or character; without organization, unity, or cohesion
anion
A negatively charged ion
anisotropy
having a different value when measured in different directions
atomic packing factor (APF)
the volume of atoms in a selected unit cell divided by the volume of the unit cell
Bragg's law
nλ=2dsinΘ
cation
A positively charged ion
coordination number
The number of immediately adjacent (i.e. touching) atoms to a given atom is called the:
crystalline
A solid that is made up of crystals in which particles are arranged in a regular, repeating pattern
crystal structure
The arrangement of the atoms in a material into a regular repeatable lattice.
crystal system
any of the seven groups (cubic, hexagonal, rhombohedral, tetragonal, orthorhombic, monoclinic, and triclinic) of crystals
grain boundary
The interface separating two adjoining grains having different crystallographic orientations.
hexagonal close-packed (HCP)
most common non-cubic bravais lattice, 6 atoms form a hexagon on both the top and bottom and a single atom positioned in the center between the two hexagonal rings
isotropic
Having the same property in all directions.
lattice parameters
the edge lengths and angles of a unit cell
Miller indices
A shorthand notation to describe certain crystallographic directions and planes in a material. Denoted by [ ] brackets. A negative number is represented by a bar over the number.
octahedral position
the void space among close-packed, hard-sphere atoms or ions for which there are six nearest neighbors; an octahedron (double pyramid) is circumscribed by lines constructed from centers of adjacent spheres
tetrahedral position
interstitial position where the atoms surrounding the interstitial atom forms a tetrahedron
unit cell
A simple arrangement of atoms or molecules which is repeated to develop a solid crystal structure is called unit cell.
alternating copolymer
Alternating sequence of monomers
atactic configuration
a type of polymer chain configuration (stereoisomer) wherein side groups are randomly positioned on one side of the chain or the other
block copolymer
identical repeat units are clustered in blocks along the chain
branched polymer
a polymer having a molecular structure of secondary chains that extend from the primary main chains
copolymer
a polymer consisting of two or more different monomers
crosslinked polymer
Permanently bonded, tangled polymers. A crosslinked polymer can never melt. Most flexible and is the most dense polymer
degree of polymerization
number of repeating units within the chain
isotactic configuration
a type of polymer chain configuration (stereoisomer) wherein all side groups are positioned on the same side of the chain molecule
linear polymer
polymer in which the molecules form long chains without branches or cross-linked structures
molecular weight
The molecular weight of a compound is the sum of the atomic weights of all atoms in the molecule.
monomer
A simple molecule that can combine with other like or unlike molecules to make a polymer
network polymer
A polymer in which monomers are connected in a three-dimensional cross-linked network.
polymer
A compound consisting of repeated linked monomers.
random copolymer
A copolymer in which different monomers are linked in no particular order.
syndiotactic
In polymers, an orientation where the side groups alternate in a regular pattern from side to side along the backbone of the polymer.
thermoplastic polymer
A polymer that can be melted and solidified multiple times. Conversely, heat-set elastomers and thermosets undergo a crosslinking reaction upon heating to form a 3-D covalently bonded network and cannot be remelted. Thermoplastics include PE, PP, PVC, PS, PMMA, PVDC, PET, PC, Nylon, and Kevlar.
A semi crystalline polymeric material that softens when heated and hardens when cooled. While in the softened state, articles may be formed by molding or extrusion
thermosetting polymer
polymers that undergo a chemical reaction to produce a rigid cross-linked network
alloy
A COMBINATION; A MIXTURE OF TWO OR MORE METALS
atom percent
(at%) the number of moles of an element in relation to the total moles of the elements in the alloy
Burgers vector
The direction and magnitude of the slip caused by a single dislocation
dislocation line
the line extending along the extra partial plane of atoms in an edge dislocation
edge dislocation
a line defect; a slip of the part of crystal over an atomic plane relative to another part is perpedicular to this plane.
Frenkel defect
point defect found in ceramic materials that occurs when a cation diffuses into an interstitial site on the lattice
interstitial solid solution
a solid solution wherein relatively small solute atoms occupy interstitial positions between the solvent or host atoms.
microstructure
the structural features of an alloy that can be seen under a microscope (ex: grain and phase structure)
point defect
a flaw in the structure of a material that occurs at a single site in the lattice, such as vacancies, substitutions, and interstitial defects
scanning electron microscope (SEM)
A microscope that is especially useful for the detailed study of the surface of a specimen; electron beams scan the surface of the sample, which is usually coated with a thin film of gold, allowing electrons on the surface to be deleted and translated into an image; result is a 3D topographic image
Schottky defect
small cation and anion vacancy clusters that are formed in ionic solids. The cation to anion ration in these clusters is adjusted to maintain electroneutrality.
screw dislocation
a line defect in which the Burgers vector is parallel to the dislocation line
self-interstitial
Atom crowded into an interstitial site, that under normal circumstances normally isn't occupied.
solid solution
Is a metallic substance that is composed of two or more metallic elements.
solute
A substance that is dissolved in a solution.
solvent
The dissolving agent of a solution
substitutional solid solution
A solution where a solute atom replaces a host atom on a lattice site in a crystal
vacancy
Type of point defect where there is a vacant lattice site
weight percent
mass of one component divided by the total mass of the mixture, multiplied by 100%
activation energy
Energy needed to get a reaction started
carburizing
produces a high carbon layer on the surface of a metal by exposing the metal to a carbon-containing atmosphere at elevated temperatures, such that carbon atoms diffuse into the surface of the metal.
concentration gradient
A difference in the concentration of a substance across a distance
concentration profile
curve of the concentration v. position graph
diffusion
Movement of molecules from an area of higher concentration to an area of lower concentration.
diffusion coefficient
a factor that determines the rate of net movement of a substance by diffusion, it is a property that depends on the particle size of the substance and the nature of the medium in which diffusion is occurring.
diffusion flux
how fast the foreign mass can transfer across and area J=M/At
driving force
A force that tends to encourage change in a particular direction.
Fick's first law
use for steady state diffusion; net flux (#atoms/area-time) of molecules of material A diffusing in the x-direction (dx) because of a concentration gradient (dCa/dx)--> A can diffuse through material B.
Fick's second law
describes the diffusion of atoms when the concentration is changed with time
interstitial diffusion
atoms migrate from an interstitial position to a neighboring one that is empty , this occurs more rapidly than vacancy diffusion because interstitial atoms are smaller and more mobile
self-diffusion
the mechanism by which a species diffuses in itself
steady-state diffusion
diffusion flux does not change with time
vacancy diffusion
one mechanism involves the interchange of an atom from a normal lattice position to an adjacent vacant lattice site or vacancy
ductility
Ability of a material to stretch or deform under a load without breaking:
elastic deformation
Occurs when an object changes shape because of a stress is being applied, but snaps back into shape when the stress is removed.
elastomer
a material that can undergo a substantial change in shape via stretching, bending, or compression and return to its original shape upon release of the distorting force
engineering strain
the change in length of sample divided by the original length of the sample
engineering stress
Average force divided by the original cross-sectional area.
flexural strength
the amount of flexural stress a material can withstand before breaking. measured through the bend test.
hardness
-Refers to various properties of matter in the solid phase that give it a high resistance to its shape changing when force is applied
modulus of elasticity
a coefficient of elasticity of a material, expressing the ratio between a unit stress and the corresponding unit strain caused by the stress, as derived from Hooke's law and represented by the slope of the straight-line portion of the stress-strain diagram (coefficient of elasticity)
plastic deformation
Occurs when a material deforms, or changes shape, as a stress is applied and remains in the new shape when the stress is released.
proportional limit
Point at which the deformation is no longer directly proportional to the applied force. Hooke's Law no longer applies
safe stress
A stress used for design purposes; for ductile metals, it is the yield strength divided by a factor of safety.
Ultimate tensile strength
Maximum stress which can be applied to a material before failure
toughness
Mechanical property of a material that indicates the ability of the material to handle overloading before it fractures.
true strain
strain measure in which increments in strain are based on current dimensions of the structure
true stress
a ratio of the force applied to a sample and the instantaneous cross-sectional area of the sample
viscoelasticity
property deforms slowly with load and returns originial shape slowly with removal of load
yield strength
point a which, when exceeded, a material will no longer completely return to its original shape after removing the applied load
cold working
permanent deformation of metals and alloys below the temperature at which a strain-free microstructure is produced continuously
critical resolved shear stress
Stress required to cause slip in pure metal single crystal. Depends on crystal structure, atomic bonding characteristics, temp, orientation of slip plane
grain growth
the third step in the formation of crystallites, which is dependent on temperature and can be described using the arrhenius equation
recrystallization
Is the formation of a new set of strain-free and equiaxed grains (i.e. having approximately equal dimensions in all directions) that have low dislocation densities and characterization of period-worked condition.
recrystallization temperature
The temperature at which atomic mobility is sufficient to affect mechanical properties as a result of recrystallization.
resolved shear stress
A material property that depends on angles fie and lambda, where lambda is the angle between the slip direction and force axis and fie is the angle between the normal direction of the slip plane.
slip system
composed of both the slip plane and the slip directions
solid-solution strengthening
alloying metals with impurity atoms that go into either substitutional or intersitital solid solution in order to strengthen or harden
strain hardening
the increase in hardness and strength of a ductile metal as it is plastically deformed below its recrystallization temperature.
viscosity
A liquid's resistance to flowing
vulcanization
process of treating rubber or rubberlike materials with sulphur at great heat to improve elasticity and strength or to harden them
brittle fracture
fracture that occurs by rapid crack propagation and without appreciable macroscopic deformation
Charpy test
impact test in which the standard specimen contains a notch where an impact occurs to complete a three-point bending
creep
THE GRADUAL, PERMANENT DEFORMATION OF A BODY PRODUCED BY CONTINUED APPLICATION OF A LOAD
ductile fracture
a mode of fracture that is attended by extensive gross plastic deformation
ductile-to-brittle transition
the transition from ductile to brittle behavior with a decrease in temperature exhibited by some low-strength steel (BCC) alloys; the temperature range over which the transition occurs is determined by Charpy and Izod impact test
fatigue life
the number of stress cycles that will cause a fatigue failure at some specified stress amplitude
fatigue limit
stress below which there is no fatigue failure whatever the number of cycles
fatigue strength
The stress to which a metal can be subjected for a specified number of cyclic changes of stress.
fracture mechanics
study of crack growth leading to material failure
fracture toughness
Critical Value of stress-intensity factor that causes failure of the plane KIC =Yσ*√ (π*crack length)
impact energy
a measure of the energy absorbed during the fracture of a specimen of standard dimensions and geometry when subjected to very rapid (impact) loading; Charpy and Izod impact tests are used to measure this parameter, which is important in assessing the ductile-to-brittle transition behavior of a material
intergranular fracture
a fracture that follows the grains of the material. If the material has multiple lattice organizations, when one lattice ends and another begins, the fracture changes direction to follow the new grain. This results in a fairly jagged looking fracture with bumpy edges.
stress raiser
A small flaw (internal or surface) or a structural discontinuity at which an applied tensile stress will be amplified and from which cracks may propagate
transgranular fracture
a fracture that follows the edges of lattices in a granular material, ignoring the grains in the individual lattices. This results in a fairly smooth looking fracture with less sharp edges than one that follows the changing grains.
solidus line
alloys are entirely solid for temperatures and compositions below this curve
solubility limit
maximum amount of solute that may be added to dissolve in a solution without forming a new phase.
terminal solid solution
A solid solution that exists over a composition range extending to either composition extremity of a binary phase diagram.
austenite
at 912 the ferrite turns into austenite and has FCC structure, more carbon is soluble
cementite
the intermetallic compound Fe3C, a hard and brittle substance
eutectic reaction
upon cooling one liquid phase is transformed into two solid phases
eutectic structure
A two-phase micro-structure resulting from the solidification of a liquid having the eutectic composition; the phases exist as lamellae that alternate with one another
eutectoid reaction
Solid phase transforms to two different solid phases.
ferrite
an interstitial solid solution of carbon in BCC iron
free energy
Energy that is available to do work
hypereutectoid alloy
An alloy with a greater concentration of solute than an alloy system that displays a eutectoid (to right in diagram)
hypoeutectoid alloy
An alloy with a smaller concentration of solute than an alloy system that displays a eutectoid (to left in diagram)
intermediate solid solution
a solid solution or phase having a composition range that does not extend to either of the pure components of the system
intermetallic compound
compound resulting from when atoms of the alloying element interact with the atoms of the base metal in definite proportions and in definite geometric relationships
invariant point
any spot on a phase diagram where three phase are in equilibrium
isomorphous
Having the same structure. When applied to a phase diagram, indicating that the solid phase has the same structure and hence complete solubility at every composition.
lever rule
mechanical analog for the mass balance with which one can calculate the amount of each phase present in a two phase microstructure
liquidus line
line on a phase diagram above which only liquid exists at equilibrium
pearlite
a two phase microstructure of alternate ferrite and cementite lamellae occurring in some steels- forms by the decompositions of austenite
peritectic reaction
A reaction in which a solid and a liquid upon cooling at the peritectic temperature and composition, transform into a different solid. Ex: L+ S1 -> S2
phase
Any part of a sample with uniform composition and properties
phase diagram
A graphic representation of the equilibria among the solid, and liquid phases of a substance as a function of temperature and weight percent.
phase equilibrium
equilibrium as it applies to a system where there are more than one phase.
solidus line
The line across the phase diagram below which all material is solid.
solubility limit
maximum amount of solute that will dissolve in a given amount of solvent
solvus line
lines defining border between the pure solid phase and a blend of two solid phases on a phase diagram
terminal solid solution
A solid solution that exists over a composition range extending to either composition extremity of a binary phase diagram.
tie line
A horizontal line that extends across a two phase region at a particular temperature and intersects with the solubility limits of each phase in the two phase region.
alloy steel
a form of steel containing carbon plus other metals such as chromium, cobalt, manganese, and molybdenum
coarse pearlite
thick layers of alpha-ferrite and Fe3C phases; at high temperatures high diffusion rates --> thick lamelle (layers); at low temperatures low diffusion rates --> thin lamelle
fine pearlite
thin alternating layers of cementite and pearlite and forms at low temperatures; diffusion rate is slower; harder and stronger than coarse pearlite, less ductile
glass transition temperature
temp at which a polymer changes from a rigid to rubbery structure; a feature of non-crystalline materials
isothermal transformation diagram
figure used to summarize the time needed to complete a specific phase transformation as a function of temperature for a given material
martensite
hard and brittle steel structure formed after severe quenching.
nucleation
Nucleation is the onset of a phase transition in a small region such as with the formation of a bubble or of a crystal from a liquid.
overaging
during precipitation hardening, aging beyond the point at which strength and hardness are at their maxima
phase transformation
A change in the number and/or character of the phases that constitute the microstructure of an alloy.
precipitation hardening
strength and hardness increased by the formation of extremely small uniformly dispersed particles of a second phase within the original matrix, accomplished by heat treatments.
precipitation heat treatment
A heat treatment used to precipitate a new phase from a supersaturated solid solution. For precipitation hardening it is termed artificial aging
solution heat treatment
first step in age hardening that involves heating until one phase has completely dissolved the other
spheroidite
product of austenitic transfomation; formed when steel alloy with either pearlitic or bainitc microstructures is heated to and left at temperature below eutectoid for 18-24 hours; sphere-like particle
supercooling
When you cool something below its normal freezing point
atomic mass unit (amu)
a unit used to measure the mass of particles in atoms
atomic number
Number of protons in the nucleus
Bohr atomic model
Atoms described as electrons orbiting the nucleus in well defined paths.
bonding energy
Net energy of attraction (or repulsion) as a function of separation distance between two atoms or ions
covalent bond
A chemical bond that involves sharing a pair of electrons between atoms in a molecule
electron configuration
the arrangement of electrons in an atom
electronegative
the ability of an atom of an element to attract electrons when the atoms is in a compound
electropositive
When something is not at all electronegative. In fact, it tends to lose electrons rather than to gain them. Elements that are electropositive are generally to the left and bottom of the periodic table.
hydrogen bond
A type of weak chemical bond formed when the slightly positive hydrogen atom of a polar covalent bond in one molecule is attracted to the slightly negative atom of a polar covalent bond in another molecule.
ionic bond
A chemical bond resulting from the attraction between oppositely charged ions.
metallic bond
A bond formed by the attraction between positively charged metal ions and the electrons around them.
periodic table
A table that shows the elements, their atomic number, symbol, and average atomic mass; elements with similar chemical properties are grouped together.
primary bond
ionic bonds, covalent bonds, metallic bonds
secondary bond
These are uneven distribution of electrons around an atom or molecule the charge can be permanent or temporary weak or strong
valence electron
The electrons in the outermost shell (main energy level) of an atom; these are the electrons involved in forming bonds.
van der Waals bond
formed when the fluctuating electrostatic charge in adjacent atoms of different molecules produces a weak electrostatic force between the molecules. Also present as a secondary bond between the long-chain molecules of polymers. Ice is an example.

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