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Aerospace Materials > Important Definitions > Flashcards

Important Definitions Flashcards

1
Q

PAULI’S EXCLUSION PRINCIPLE

A

Each electron state can hold no more than two electrons, which must have opposite spins.

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2
Q

Ground state

A

When all the electrons occupy the lowest possible energies.

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3
Q

Valence electron

A

Those that occupy the outermost shell.

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4
Q

Ionic Bonding (primary)

A

Metallic + Nonmetalic elements. All atoms acquire stable configuration and electrical charge.

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5
Q

Coulombic Force

A

Attractive bonding force between positive and negative ions.

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6
Q

Covalent Bonding (primary)

A

Shared pair of electrons form stable configurations.

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7
Q

Metallic bonding (primary)

A

Found in metals and their alloys. One, two or three valence electrons ‘free’ to drift through the metal.

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8
Q

Secondary, van der Waals, or physical bonds

A

Weak compared to primary/chemical bonds. Exists between virtually all atoms or molecules, but obscured if any primary bond is present.

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9
Q

Dipole (secondary bonds arise from dipoles)

A

Exist whenever there is some separation of positive and negative portions of an atom or molecule.

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10
Q

Hydrogen Bonding (secondary)

A

Exists between some molecules that have hydrogen as one of the constituents.

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11
Q

CRYSTALLINE

A

A material in which the atoms are situated in a repeating or periodic array over large atomic distances.

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12
Q

Crystal structure

A

The manner in which atoms, ions, or molecules are spatially arranged in a crystalline material.

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13
Q

LATTICE

A

in the context of crystal structures A three-dimensional array of points coinciding with atom positions (or sphere centers).

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14
Q

Unit cells

A

Small repeat entities arising from subdivisions of a crystal structure.

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15
Q

Face-centered cubic (FCC)

A

A unit cell of cubic geometry, with atoms located at each of the corners and the centers of all the cube faces.

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16
Q

COORDINATION NUMBER

A

Number of nearest-neighbor/touching atoms.

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17
Q

Atomic packing factor (APF)

A

The sum of the sphere volumes of all atoms within a unit cell divided by the unit cell volume.

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18
Q

Body-centered cubic (BCC)

A

A unit cell of cubic geometry with atoms located at all eight corners and a single atom at the cube center.

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19
Q

Hexagonal close-packed (HCP)

A

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.

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20
Q

POLYMORPHISM

A

Metal or nonmetal having more than one crystal structure.

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21
Q

Allotropy

A

Polymorphism condition found in elemental solids.

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22
Q

Lattice parameters

A

The three interaxial angles alpha, beta, and gamma.

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23
Q

Crystal system

A

Groups of crystal structures defined according to unit cell configuration and/or atomic arrangements.

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24
Q

Miller indices

A

Crystallographic planes are specified by three Miller indices as (hkl).

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25
Q

Single crystal

A

When the periodic and repeated arrangement of atoms is perfect or extends throughout the entirety of the specimen without interruption.

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26
Q

POLYCRYSTALLINE

A

A crystalline solid that is composed of a collection of may small crystals or grains.

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27
Q

Grain boundary

A

An atomic mismatch within the region where two grains meet.

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28
Q

ANISOTROPIC

A

Substances in which measured properties are dependent on the direction of measurement.

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29
Q

ISOTROPIC

A

Substances in which measured properties are independent of the direction of measurement.

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30
Q

NONCRYSTALLINE/AMORPHOUS

A

Solids which lack a systematic and regular arrangement of atoms over relatively large atomic distances.

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31
Q

Point defect

A

Defects associated with one or two atomic positions.

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32
Q

Vacancy

A

The simplest of the point defects.

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33
Q

Self-interstitial

A

An atom from the crystal that is crowded into an interstitial site, a small void space that under ordinary circumstances is not occupied.

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34
Q

Alloy

A

A solid in which impurity atoms have been added intentionally to impart specific characteristics to the material.

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35
Q

Solid solution

A

Formed as a result of adding impurity atoms to a metal.

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36
Q

Solvent

A

The element or compound that is present in the greatest amount.

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37
Q

Solute

A

An element or compound present in a minor concentration.

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38
Q

SUBSTITUTIONAL POINT DEFECT

A

Solute or impurity atoms replace or substitute for the host atoms.

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39
Q

INTERSTITIAL POINT DEFECT

A

Impurity atoms fill the voids or interstices among the host atoms.

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40
Q

Weight percent

A

The weight of a particular element relative to the total alloy weight.

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41
Q

Atom percent

A

The number of moles of an element in relation to the total moles of the elements in the alloy.

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42
Q

DISLOCATION

A

A linear or one-dimensional defect around which some of the atoms are misaligned.

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43
Q

Edge dislocation

A

A linear defect that centers on the line that is defined along the end of the extra half-plane atoms.

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44
Q

Dislocation line

A

For the edge dislocation the dislocation line is perpendicular to the page.

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45
Q

Screw dislocation

A

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.

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46
Q

BURGERS VECTOR

A

The magnitude and direction of the lattice distortion associated with a dislocation.

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47
Q

Engineering stress

A

(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.

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48
Q

Engineering strain

A

(∆l/l) where ∆l is the change in length and l is the original length.

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49
Q

MODULUS OF ELASTICITY or Young’s modulus

A

The constant of proportionality in the relationship between stress and strain.

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50
Q

ELASTIC DEFORMATION

A

Deformation in which stress and strain are proportional.

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51
Q

Anelasticity

A

Time-dependent elastic strain behavior once a load is released. For metals this component is normally small and is often neglected.

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52
Q

Viscoelastic behavior

A

When the magnitude of the time-dependent elastic behavior is significant it is termed viscoelastic behaior.

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53
Q

Poisson’s ratio

A

The ratio of the lateral and axial strains.

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54
Q

PLASTIC DEFORMATION

A

Permanent, nonrecoverable deformation where stress is no longer proportional to strain.

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55
Q

Yielding

A

The stress level at which plastic deformation begins.

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56
Q

YIELD STRENGTH

A

The stress corresponding to the intersection of the strain offset line and the stress-strain curve as it bends over in the plastic region.

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57
Q

TENSILE STRENGTH

A

Corresponds to the maximum stress that can be sustained by a structure in tension. If applied and maintained, fracture will result.

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58
Q

DUCTILITY

A

The measure of the degree of plastic deformation that has been sustained at fracture.

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59
Q

Resilience

A

The capacity of a material to absorb energy when it is deformed elastically and then, upon unloading, to have this energy recovered.

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60
Q

TOUGHNESS (fracture toughness)

A

A material’s resistance to fracture when a crack is present.

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61
Q

True stress

A

Stress calculated using the instantaneous cross-sectional area.

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62
Q

True strain

A

The natural logarithm of instantaneous length divided by original length.

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63
Q

HARDNESS

A

A measure of a material’s resistance to localized plastic deformation (e.g., a small dent or a scratch)

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64
Q

Design stress

A

The calculated stress level (on basis of estimated max load) multiplied by a design factor, N’.

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65
Q

Safe stress or working stress

A

The yield strength divided by a factor of safety, N.

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66
Q

SLIP

A

The process by which plastic deformation is produced by dislocation motion.

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67
Q

DISLOCATION

A

The total dislocation length per unit volume.

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68
Q

SLIP SYSTEM

A

The combination of the slip plane and the slip direction.

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69
Q

SOLID-SOLUTION STRENGTHENING

A

Alloying with impurity atoms that go into either substitutional or interstitial solid solution.

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70
Q

STRAIN HARDENING OR COLD WORKING

A

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.

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71
Q

GRAIN GROWTH

A

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.

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72
Q

CLEAVAGE

A

The successive and repeated breaking of atomic bonds along specific crystallographic planes.

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73
Q

Transgranular fracture

A

Fracture that occurs as a result of cleavage, due to the fracture cracks passing through the grains.

74
Q

Intergranular fracture

A

Crack propagation along grain boundaries.

75
Q

Stress raisers

A

Microscopic flaws or cracks that always exist under normal conditions at the surface and within the interior of a body of material.

76
Q

Impact energy

A

The energy absorption, computed from the differences in height of the pendulum used in an impact test.

77
Q

FATIGUE

A

A form of failure that occurs in structures subjected to dynamic and fluctuating stresses.

78
Q

Fatigue limit or endurance limit

A

Below this stress level fatigue failure will not occur.

79
Q

Fatigue strength

A

The stress level at which failure will occur for some specified number of cycles.

80
Q

Fatigue life

A

The number of cycles to cause failure at a specified stress level, as taken from the S-N plot.

81
Q

CASE HARDENING

A

A technique by which both surface hardness and fatigue life are enhanced for steel alloys. Accomplished by a carburizing or nitriding process.

82
Q

Thermal fatigue

A

Normally induced at elevated temperatures by fluctuating thermal stresses.

83
Q

Corrosion fatigue

A

Failure that occurs by the simultaneous action of a cyclic stress and chemical attack.

84
Q

CREEP

A

Deformation that occurs when a material is placed at elevated temperatures and exposed to static mechanical stresses.

85
Q

Solubility limit

A

The maximum concentration of solute atoms that may dissolve in the solvent to form a solid solution (at a specific temperature).

86
Q

Phase

A

A homogeneous portion of a system that has uniform physical and chemical characteristics.

87
Q

FREE ENERGY

A

A function of the internal energy of a system, and also the randomness or disorder of the atoms or molecules (or entropy).

88
Q

Equilibrium

A

A system is at equilibrium if its free energy is at a minimum under some specified combination of temperature, pressure, and composition.

89
Q

Phase equilibrium

A

Phase equilibrium is reflected by a constancy with time in the phase characteristics of a system.

90
Q

Nonequillibrium or Metastable

A

A state in which equilibrium is never completely achieved because the rate of approach to equilibrium is extremely slow.

91
Q

ISOMORPHUS

A

A system in which there is complete liquid and solid solubility of the two components in the system. e.g. Copper - Nickel

92
Q

Tie line

A

A horizontal line drawn from a given temperature.

93
Q

LEVER RULE

A
  1. Tie line constructed across two-phase region at given temp.
  2. Overall alloy composition located on tie line.
  3. 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.
94
Q

Invariant point

A

The point at which the liquidus lines, as well as the isotherm line, meet.

95
Q

Eutectic reaction

A

Where two solid phases, upon heating, turn into a liquid phase at temperature T(E).

96
Q

Eutectic structure

A

The microstructure of the solid that results from the cooling of a eutectic alloy.

97
Q

Microconstituent

A

An element of the microstructure having an identifiable and characteristic structure.

98
Q

EUTECTOID REACTION

A

The reaction that occurs upon heating two solid phases allowing a transformation into a one solid delta phase.

99
Q

Peritectic reaction

A

Involving three phases at equilibrium, this reaction, upon heating, one solid phase transforms into a liquid phase and another solid phase.

100
Q

Congruent transformations

A

Those transformations for which there are no compositional alterations.

101
Q

GIBBS PHASE RULE

A

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).

102
Q

Ferrite

A

The BCC stable form of pure iron.

103
Q

Austenite

A

The polymorphic transformation of ferrite upon heating to an FCC austenite. Up to 1394˚C

104
Q

Cementite

A

The intermediate compound iron carbide.

105
Q

Pearlite

A

The microstructure for eutectoid steel that is slowly cooled through the eutectoid temperature.

106
Q

Hypoeutectoid (less than eutectoid) alloys

A

Microstructures containing less than 0.76 wt% C

107
Q

Proeutectoid (before eutectoid)

A

Constituents that were formed before the eutectoid constituents in a microstructure.

108
Q

Hypereutectoid alloys

A

Microstructures containing between 0.76 and 2.14 wt% C.

109
Q

NUCLEATION

A

The appearance of very small particles, or nuclei of the new phase.

110
Q

GROWTH

A

The process by which these nuclei increase in size, which results in the disappearance of some (or all) of the parent phase.

111
Q

Kinetics of transformation

A

The time dependence of rate of transformation.

112
Q

SUPERCOOLING/SUPERHEATING

A

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.

113
Q

Bainite

A

A microstructure consisting of ferrite and cementite phases.

114
Q

Spheroidite

A

A microstructure where the Fe3C phase appears as spherelike particles embedded in a continuous alpha-phase matrix.

115
Q

Martensite

A

Formed when austenitized iron-carbon alloys are rapidly cooled (or quenched) to a relatively low temperature.

116
Q

Ferrous alloys

A

Alloys of which iron is the prime constituent.

117
Q

Plain carbon steels

A

Contain only residual concentrations of impuities other than carbon.

118
Q

Stainless steels

A

Highly resistant to corrosion. Their predominant alloying element is chromium; a concentration of at least 11 wt% Cr is required.

119
Q

Cast irons

A

Class of ferrous alloys with carbon contents abover 2.14 wt%.

120
Q

Bronzes

A

Alloys of copper and several other elements, including tin, aluminium, silicon, and nickel.

121
Q

Hot working

A

Deformation achieved at a temperature above that at which recrystallization occurs.

122
Q

Forging

A

mechanically working or deforming a single pice of a normally hot metal.

123
Q

Rolling

A

Most widely used deformation process, consists of passing a piece of metal between two rolls.

124
Q

Extrusion

A

A bar of metal is forced through a die orifice by a compressive force that is applied to a ram.

125
Q

Drawing

A

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.

126
Q

Powder metallurgy

A

The compaction of powdered metal, followed by a heat treatment to produce a more dense piece.

127
Q

Welding

A

Two or more metal parts are joined to form a single piece when one-part fabrication is expensive or inconvenient.

128
Q

ANNEALING

A

A heat treatment in which material is exposed to an elevated temperature for an extended time period and then slowly cooled.

129
Q

Process annealing

A

A heat treatment that is used to negate the effects of cold work.

130
Q

Normalizing

A

An annealing heat treatment used to refine the grains and produce a more uniform and desirable size distribution.

131
Q

Hardenability (not hardness)

A

The ability of an alloy to be hardened by the formation of martensite as a result of a given heat treatment.

132
Q

Jominy end-quench test

A

A procedure to determine hardenability.

133
Q

PRECIPITATION HARDENING

A

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)

134
Q

Solution heat treatment

A

Solute atoms are dissolved to form a single-phase solid solution.

135
Q

Overaging

A

The reduction in strength and hardness that occurs after long time periods.

136
Q

Unsaturated (molecules)

A

Molecules that have double and triple covalent bonds.

137
Q

Saturated

A

All bonds are single ones, and no new atoms may be joined without the removal of others that are already bonded.

138
Q

ISOMERISM

A

Hydrocarbon compounds with the same composition but different atomic arrangements.

139
Q

Macromolecules

A

Molecules in polymers in comparison to hydrocarbon molecules.

140
Q

MONOMER

A

The small molecule from which a polymer is synthesized.

141
Q

Homopolymer

A

When all of the repeating units along a chain are of the same type.

142
Q

Copolymers

A

Chains composed of two or more different repeat units.

143
Q

Bifunctional

A

A monomer with an active bond that may react to form two covalent bonds with other monomers forming a two-dimensional chainlike molecular structure.

144
Q

Functionality

A

The number of bonds that a given monomer can form.

145
Q

Degree of polymerization

A

The average number of repeat units in a chain.

146
Q

LINEAR POLYMERS

A

Those in which the repeat units are joined together end to end in single chains.

147
Q

BRANCHED POLYMERS

A

Polymers synthesized in which side-branch chains are connected to the main ones.

148
Q

CROSSLINKED POLYMERS

A

Adjacent linear chains are joined one to another at various positions by covalent bonds.

149
Q

NETWORK POLYMERS

A

Multifunctional monomers forming three or more active covalent bonds make three-dimensional networks.

150
Q

Stereoisomerism

A

Denotes the situation in which atoms are linked together in the same order (head-to-tail) but differ in their spatial arrangement.

151
Q

Isotactic configuration

A

Configuration in which the R groups are situated on the same side of the chain.

152
Q

Syndiotactic configuration

A

Configuration in which the R groups alternate sides of the chain.

153
Q

Atactic configuration

A

Configuration in which the R groups are randomly positioned.

154
Q

cis (structure)

A

A structure in which the CH3 group and H atom are positioned on the same side of the double bond (isoprene example).

155
Q

trans (structure)

A

The CH3 and H reside on opposite sides of the double bond.

156
Q

THERMOPLASTICS

A

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

157
Q

THERMOSET

A

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

158
Q

Polymer crystallinity

A

The packing of molecular chains to produce an ordered atomic array.

159
Q

Crystallite

A

Small crystalline region.

160
Q

Chain-folded model

A

Platelets forming a multilayered structure. The molecular chains within each platelet fold back and forth on themselves, with folds occurring at the faces.

161
Q

Spherulite

A

A semicrystalline bulk polymer that is crystallized from a melt.

162
Q

ELASTOMER

A

A class of polymers with rubberlike elasticity (large recoverable strains produced at low stress levels).

163
Q

VISCOELASTIC

A

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.

164
Q

RELAXATION MODULUS

A

A time-dependent elastic modulus for viscoelastic polymers. Defined as the measured time-dependent stress and the strain level, which is maintained constant.

165
Q

Viscoelastic Creep

A

Time-dependent deformation when stress level is maintained constant.

166
Q

Drawing (mechanism of plastic deformation)

A

The appreciable tensile deformation of semicrystalline polymers producing a highly oriented structure. Drawing is the process of this orientation.

167
Q

Vulcanization

A

The crosslinking process in elastomers.

168
Q

Glass transition temperature

A

The temperature at which the polymer experiences the transition from rubbery to rigid states. Reverse order for rigid glass below Tg being heated.

169
Q

Plastics

A

Materials that have some structural rigidity under load and are used in general-purpose applications.

170
Q

Fiber polymers

A

Capable of being drawn into long filaments having at least a 100:1 length-to-diameter ratio.

171
Q

Foams

A

Plastic materials that contain a relatively high volume percentage of small pores and trapped gas bubbles.

172
Q

Ultra-High-Molecular-Weight Polyethylene (UHMWPE)

A

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.

173
Q

Liquid crystal polymers (LCPs)

A

A group of chemically complex and structurally distinct materials that have unique properties and are used in diverse applications.

174
Q

Addition polymerization

A

A process by which monomer units are attached one at a time in chainlike fashion to form a linear macromolecule.

175
Q

Condensation polymerization

A

The formation of polymers by stepwise intermolecular chemical reactions that may involve more than one monomer speicies.

176
Q

Filler

A

Materials most often added to polymers to improve tensile and compressive strengths, abrasion resistance, toughness, dimensional and thermal stability, and other properties.

177
Q

Plasticizers

A

Additives used to improve the flexibility, ductility, and toughness of polymers. Also produces reductions in hardness and stiffness.

178
Q

Stabilizer

A

Additives that counteract deteriorative processes.

179
Q

Colorants

A

Impart a specific color to a polymer.

180
Q

Molding

A 
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
181
Q

Spinning

A

The process by which fibers are formed from bulk polymer material.

Aerospace Materials (2 decks)

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