Important Definitions Flashcards
PAULI’S EXCLUSION PRINCIPLE
Each electron state can hold no more than two electrons, which must have opposite spins.
Ground state
When all the electrons occupy the lowest possible energies.
Valence electron
Those that occupy the outermost shell.
Ionic Bonding (primary)
Metallic + Nonmetalic elements. All atoms acquire stable configuration and electrical charge.
Coulombic Force
Attractive bonding force between positive and negative ions.
Covalent Bonding (primary)
Shared pair of electrons form stable configurations.
Metallic bonding (primary)
Found in metals and their alloys. One, two or three valence electrons ‘free’ to drift through the metal.
Secondary, van der Waals, or physical bonds
Weak compared to primary/chemical bonds. Exists between virtually all atoms or molecules, but obscured if any primary bond is present.
Dipole (secondary bonds arise from dipoles)
Exist whenever there is some separation of positive and negative portions of an atom or molecule.
Hydrogen Bonding (secondary)
Exists between some molecules that have hydrogen as one of the constituents.
CRYSTALLINE
A material in which the atoms are situated in a repeating or periodic array over large atomic distances.
Crystal structure
The manner in which atoms, ions, or molecules are spatially arranged in a crystalline material.
LATTICE
in the context of crystal structures A three-dimensional array of points coinciding with atom positions (or sphere centers).
Unit cells
Small repeat entities arising from subdivisions of a crystal structure.
Face-centered cubic (FCC)
A unit cell of cubic geometry, with atoms located at each of the corners and the centers of all the cube faces.
COORDINATION NUMBER
Number of nearest-neighbor/touching atoms.
Atomic packing factor (APF)
The sum of the sphere volumes of all atoms within a unit cell divided by the unit cell volume.
Body-centered cubic (BCC)
A unit cell of cubic geometry with atoms located at all eight corners and a single atom at the cube center.
Hexagonal close-packed (HCP)
Unit cell is hexagonal. The top and bottom faces consist of six atoms that form regular hexagons and surround a single atom in the center. Another plan that provides three additional atoms is situated between the top and bottom planes.
POLYMORPHISM
Metal or nonmetal having more than one crystal structure.
Allotropy
Polymorphism condition found in elemental solids.
Lattice parameters
The three interaxial angles alpha, beta, and gamma.
Crystal system
Groups of crystal structures defined according to unit cell configuration and/or atomic arrangements.
Miller indices
Crystallographic planes are specified by three Miller indices as (hkl).
Single crystal
When the periodic and repeated arrangement of atoms is perfect or extends throughout the entirety of the specimen without interruption.
POLYCRYSTALLINE
A crystalline solid that is composed of a collection of may small crystals or grains.
Grain boundary
An atomic mismatch within the region where two grains meet.
ANISOTROPIC
Substances in which measured properties are dependent on the direction of measurement.
ISOTROPIC
Substances in which measured properties are independent of the direction of measurement.
NONCRYSTALLINE/AMORPHOUS
Solids which lack a systematic and regular arrangement of atoms over relatively large atomic distances.
Point defect
Defects associated with one or two atomic positions.
Vacancy
The simplest of the point defects.
Self-interstitial
An atom from the crystal that is crowded into an interstitial site, a small void space that under ordinary circumstances is not occupied.
Alloy
A solid in which impurity atoms have been added intentionally to impart specific characteristics to the material.
Solid solution
Formed as a result of adding impurity atoms to a metal.
Solvent
The element or compound that is present in the greatest amount.
Solute
An element or compound present in a minor concentration.
SUBSTITUTIONAL POINT DEFECT
Solute or impurity atoms replace or substitute for the host atoms.
INTERSTITIAL POINT DEFECT
Impurity atoms fill the voids or interstices among the host atoms.
Weight percent
The weight of a particular element relative to the total alloy weight.
Atom percent
The number of moles of an element in relation to the total moles of the elements in the alloy.
DISLOCATION
A linear or one-dimensional defect around which some of the atoms are misaligned.
Edge dislocation
A linear defect that centers on the line that is defined along the end of the extra half-plane atoms.
Dislocation line
For the edge dislocation the dislocation line is perpendicular to the page.
Screw dislocation
Thought of as being formed by a shear stress. The upper front region of the crystal is shifted one atomic distance to the right relative to the bottom portion. Also linear and along a dislocation line.
BURGERS VECTOR
The magnitude and direction of the lattice distortion associated with a dislocation.
Engineering stress
(F/A) where F is the instantaneous load applied perpendicular to the specimen cross section, and A is the original cross-sectional area before any load is applied.
Engineering strain
(∆l/l) where ∆l is the change in length and l is the original length.
MODULUS OF ELASTICITY or Young’s modulus
The constant of proportionality in the relationship between stress and strain.
ELASTIC DEFORMATION
Deformation in which stress and strain are proportional.
Anelasticity
Time-dependent elastic strain behavior once a load is released. For metals this component is normally small and is often neglected.
Viscoelastic behavior
When the magnitude of the time-dependent elastic behavior is significant it is termed viscoelastic behaior.
Poisson’s ratio
The ratio of the lateral and axial strains.
PLASTIC DEFORMATION
Permanent, nonrecoverable deformation where stress is no longer proportional to strain.
Yielding
The stress level at which plastic deformation begins.
YIELD STRENGTH
The stress corresponding to the intersection of the strain offset line and the stress-strain curve as it bends over in the plastic region.
TENSILE STRENGTH
Corresponds to the maximum stress that can be sustained by a structure in tension. If applied and maintained, fracture will result.
DUCTILITY
The measure of the degree of plastic deformation that has been sustained at fracture.
Resilience
The capacity of a material to absorb energy when it is deformed elastically and then, upon unloading, to have this energy recovered.
TOUGHNESS (fracture toughness)
A material’s resistance to fracture when a crack is present.
True stress
Stress calculated using the instantaneous cross-sectional area.
True strain
The natural logarithm of instantaneous length divided by original length.
HARDNESS
A measure of a material’s resistance to localized plastic deformation (e.g., a small dent or a scratch)
Design stress
The calculated stress level (on basis of estimated max load) multiplied by a design factor, N’.
Safe stress or working stress
The yield strength divided by a factor of safety, N.
SLIP
The process by which plastic deformation is produced by dislocation motion.
DISLOCATION
The total dislocation length per unit volume.
SLIP SYSTEM
The combination of the slip plane and the slip direction.
SOLID-SOLUTION STRENGTHENING
Alloying with impurity atoms that go into either substitutional or interstitial solid solution.
STRAIN HARDENING OR COLD WORKING
The phenomenon whereby a ductile metal becomes harder and stronger as it is plastically deformed. Termed cold working as the temperature at which deformation takes place is “cold” relative to the absolute melting temperature of the metal.
GRAIN GROWTH
The process by which the strain-free grains, after complete recrystallization, will continue to grow if the metal specimen is left at the elevated temperature.
CLEAVAGE
The successive and repeated breaking of atomic bonds along specific crystallographic planes.
Transgranular fracture
Fracture that occurs as a result of cleavage, due to the fracture cracks passing through the grains.
Intergranular fracture
Crack propagation along grain boundaries.
Stress raisers
Microscopic flaws or cracks that always exist under normal conditions at the surface and within the interior of a body of material.
Impact energy
The energy absorption, computed from the differences in height of the pendulum used in an impact test.
FATIGUE
A form of failure that occurs in structures subjected to dynamic and fluctuating stresses.
Fatigue limit or endurance limit
Below this stress level fatigue failure will not occur.
Fatigue strength
The stress level at which failure will occur for some specified number of cycles.
Fatigue life
The number of cycles to cause failure at a specified stress level, as taken from the S-N plot.
CASE HARDENING
A technique by which both surface hardness and fatigue life are enhanced for steel alloys. Accomplished by a carburizing or nitriding process.
Thermal fatigue
Normally induced at elevated temperatures by fluctuating thermal stresses.
Corrosion fatigue
Failure that occurs by the simultaneous action of a cyclic stress and chemical attack.
CREEP
Deformation that occurs when a material is placed at elevated temperatures and exposed to static mechanical stresses.
Solubility limit
The maximum concentration of solute atoms that may dissolve in the solvent to form a solid solution (at a specific temperature).
Phase
A homogeneous portion of a system that has uniform physical and chemical characteristics.
FREE ENERGY
A function of the internal energy of a system, and also the randomness or disorder of the atoms or molecules (or entropy).
Equilibrium
A system is at equilibrium if its free energy is at a minimum under some specified combination of temperature, pressure, and composition.
Phase equilibrium
Phase equilibrium is reflected by a constancy with time in the phase characteristics of a system.
Nonequillibrium or Metastable
A state in which equilibrium is never completely achieved because the rate of approach to equilibrium is extremely slow.
ISOMORPHUS
A system in which there is complete liquid and solid solubility of the two components in the system. e.g. Copper - Nickel
Tie line
A horizontal line drawn from a given temperature.
LEVER RULE
- Tie line constructed across two-phase region at given temp.
- Overall alloy composition located on tie line.
- Fraction of one phase computed by taking length from alloy composition point to phase boundary of other phase, and dividing by total tie line length.
Invariant point
The point at which the liquidus lines, as well as the isotherm line, meet.
Eutectic reaction
Where two solid phases, upon heating, turn into a liquid phase at temperature T(E).
Eutectic structure
The microstructure of the solid that results from the cooling of a eutectic alloy.
Microconstituent
An element of the microstructure having an identifiable and characteristic structure.
EUTECTOID REACTION
The reaction that occurs upon heating two solid phases allowing a transformation into a one solid delta phase.
Peritectic reaction
Involving three phases at equilibrium, this reaction, upon heating, one solid phase transforms into a liquid phase and another solid phase.
Congruent transformations
Those transformations for which there are no compositional alterations.
GIBBS PHASE RULE
Represents a criterion for the number of phases that will coexist within a system at equilibrium; P + F = C + N
P = number of phases present
F = number of degrees of freedom
C = number of components in system
N = number of noncompositional variables (e.g. temp and pressure).
Ferrite
The BCC stable form of pure iron.
Austenite
The polymorphic transformation of ferrite upon heating to an FCC austenite. Up to 1394˚C
Cementite
The intermediate compound iron carbide.
Pearlite
The microstructure for eutectoid steel that is slowly cooled through the eutectoid temperature.
Hypoeutectoid (less than eutectoid) alloys
Microstructures containing less than 0.76 wt% C
Proeutectoid (before eutectoid)
Constituents that were formed before the eutectoid constituents in a microstructure.
Hypereutectoid alloys
Microstructures containing between 0.76 and 2.14 wt% C.
NUCLEATION
The appearance of very small particles, or nuclei of the new phase.
GROWTH
The process by which these nuclei increase in size, which results in the disappearance of some (or all) of the parent phase.
Kinetics of transformation
The time dependence of rate of transformation.
SUPERCOOLING/SUPERHEATING
For other than equilibrium cooling or heating, transformations are shifted to lower or higher temperatures (respectively) than indicated by the phase diagram. The degree of each depends on the rate of temperature change.
Bainite
A microstructure consisting of ferrite and cementite phases.
Spheroidite
A microstructure where the Fe3C phase appears as spherelike particles embedded in a continuous alpha-phase matrix.
Martensite
Formed when austenitized iron-carbon alloys are rapidly cooled (or quenched) to a relatively low temperature.
Ferrous alloys
Alloys of which iron is the prime constituent.
Plain carbon steels
Contain only residual concentrations of impuities other than carbon.
Stainless steels
Highly resistant to corrosion. Their predominant alloying element is chromium; a concentration of at least 11 wt% Cr is required.
Cast irons
Class of ferrous alloys with carbon contents abover 2.14 wt%.
Bronzes
Alloys of copper and several other elements, including tin, aluminium, silicon, and nickel.
Hot working
Deformation achieved at a temperature above that at which recrystallization occurs.
Forging
mechanically working or deforming a single pice of a normally hot metal.
Rolling
Most widely used deformation process, consists of passing a piece of metal between two rolls.
Extrusion
A bar of metal is forced through a die orifice by a compressive force that is applied to a ram.
Drawing
The pulling of a metal piece through a die having a tapered bore by means of a tensile force that is applied on the exit side.
Powder metallurgy
The compaction of powdered metal, followed by a heat treatment to produce a more dense piece.
Welding
Two or more metal parts are joined to form a single piece when one-part fabrication is expensive or inconvenient.
ANNEALING
A heat treatment in which material is exposed to an elevated temperature for an extended time period and then slowly cooled.
Process annealing
A heat treatment that is used to negate the effects of cold work.
Normalizing
An annealing heat treatment used to refine the grains and produce a more uniform and desirable size distribution.
Hardenability (not hardness)
The ability of an alloy to be hardened by the formation of martensite as a result of a given heat treatment.
Jominy end-quench test
A procedure to determine hardenability.
PRECIPITATION HARDENING
A heat treatment that induces phase transformations that result in extremely small uniformly dispersed particles of a second phase within the original phase matrix. (these small particles are termed precipitates)
Solution heat treatment
Solute atoms are dissolved to form a single-phase solid solution.
Overaging
The reduction in strength and hardness that occurs after long time periods.
Unsaturated (molecules)
Molecules that have double and triple covalent bonds.
Saturated
All bonds are single ones, and no new atoms may be joined without the removal of others that are already bonded.
ISOMERISM
Hydrocarbon compounds with the same composition but different atomic arrangements.
Macromolecules
Molecules in polymers in comparison to hydrocarbon molecules.
MONOMER
The small molecule from which a polymer is synthesized.
Homopolymer
When all of the repeating units along a chain are of the same type.
Copolymers
Chains composed of two or more different repeat units.
Bifunctional
A monomer with an active bond that may react to form two covalent bonds with other monomers forming a two-dimensional chainlike molecular structure.
Functionality
The number of bonds that a given monomer can form.
Degree of polymerization
The average number of repeat units in a chain.
LINEAR POLYMERS
Those in which the repeat units are joined together end to end in single chains.
BRANCHED POLYMERS
Polymers synthesized in which side-branch chains are connected to the main ones.
CROSSLINKED POLYMERS
Adjacent linear chains are joined one to another at various positions by covalent bonds.
NETWORK POLYMERS
Multifunctional monomers forming three or more active covalent bonds make three-dimensional networks.
Stereoisomerism
Denotes the situation in which atoms are linked together in the same order (head-to-tail) but differ in their spatial arrangement.
Isotactic configuration
Configuration in which the R groups are situated on the same side of the chain.
Syndiotactic configuration
Configuration in which the R groups alternate sides of the chain.
Atactic configuration
Configuration in which the R groups are randomly positioned.
cis (structure)
A structure in which the CH3 group and H atom are positioned on the same side of the double bond (isoprene example).
trans (structure)
The CH3 and H reside on opposite sides of the double bond.
THERMOPLASTICS
Soften when heated (and eventually liquefy) and harden when cooled–processes that are totally reversible and may be repeated. Most linear polymers and those with some branched structures with flexible chains
THERMOSET
Become permanently hard during their formation and do not soften upon heating. Generally harder and stronger than thermoplastics and have better dimensional stability. Most crosslinked and network polymers
Polymer crystallinity
The packing of molecular chains to produce an ordered atomic array.
Crystallite
Small crystalline region.
Chain-folded model
Platelets forming a multilayered structure. The molecular chains within each platelet fold back and forth on themselves, with folds occurring at the faces.
Spherulite
A semicrystalline bulk polymer that is crystallized from a melt.
ELASTOMER
A class of polymers with rubberlike elasticity (large recoverable strains produced at low stress levels).
VISCOELASTIC
The condition by which at intermediate temperatures the polymer is a rubbery solid that exhibits the combined mechanical characteristics of the two extremes; elastic and viscous behaviour.
RELAXATION MODULUS
A time-dependent elastic modulus for viscoelastic polymers. Defined as the measured time-dependent stress and the strain level, which is maintained constant.
Viscoelastic Creep
Time-dependent deformation when stress level is maintained constant.
Drawing (mechanism of plastic deformation)
The appreciable tensile deformation of semicrystalline polymers producing a highly oriented structure. Drawing is the process of this orientation.
Vulcanization
The crosslinking process in elastomers.
Glass transition temperature
The temperature at which the polymer experiences the transition from rubbery to rigid states. Reverse order for rigid glass below Tg being heated.
Plastics
Materials that have some structural rigidity under load and are used in general-purpose applications.
Fiber polymers
Capable of being drawn into long filaments having at least a 100:1 length-to-diameter ratio.
Foams
Plastic materials that contain a relatively high volume percentage of small pores and trapped gas bubbles.
Ultra-High-Molecular-Weight Polyethylene (UHMWPE)
A linear polyethylene that has an extremely high molecular weight.
Characteristics include:
1. An extremely high impact resistance
2. Outstanding resistance to wear and abrasion
3. A very low coefficient of friction
4. A self-lubricating and nonstick surface
5. Very good chemical resistance to normally encountered solvents.
6. Excellent low-temperature properties
7. Outstanding sound damping and energy absorption
8. Electrically insulating and excellent dielectric properties.
Liquid crystal polymers (LCPs)
A group of chemically complex and structurally distinct materials that have unique properties and are used in diverse applications.
Addition polymerization
A process by which monomer units are attached one at a time in chainlike fashion to form a linear macromolecule.
Condensation polymerization
The formation of polymers by stepwise intermolecular chemical reactions that may involve more than one monomer speicies.
Filler
Materials most often added to polymers to improve tensile and compressive strengths, abrasion resistance, toughness, dimensional and thermal stability, and other properties.
Plasticizers
Additives used to improve the flexibility, ductility, and toughness of polymers. Also produces reductions in hardness and stiffness.
Stabilizer
Additives that counteract deteriorative processes.
Colorants
Impart a specific color to a polymer.
Molding
The most common method for forming plastic polymers. Several techniques include: 1. Compression and transfer molding 2. Injection molding 3. Extrusion 4. Blow molding 5. Casting
Spinning
The process by which fibers are formed from bulk polymer material.
