38 terms

Chapter 4 Material

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Terms in this set (...)

imperfections
all
contain large numbers of various defects
Crystalline defect
lattice irregularity having one or more of its
dimensions on the order of an atomic diameter
point defects
(those associated with one or two atomic positions), linear (or one-dimensional)
defects, and interfacial defects, or boundaries, which are two-dimensional
The simplest of the point defects is a vacancy
vacant lattice site, one normally
occupied from which an atom is missing; All crystalline solids contain vacancies
vacancies is explained using principles of thermodynamics
the presence of vacancies increases the entropy (i.e.,
the randomness) of the crystal.
dislocation
is a linear or one-dimensional defect around which some of the atoms are misaligned
edge dislocation
it is a linear defect that centers on the line that
is defined along the end of the extra half-plane of atoms;This is sometimes termed the dislocation line, which, for the edge dislocation, is perpendicular to the plane of the page
The magnitude of this distortion decreases with distance away from the dislocation
line; at positions far removed, the crystal lattice is virtually perfect
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screw dislocation
being formed by a shear stress that is applied to produce the distortion;the upper front region of the crystal is shifted one atomic distance to the right relative to the bottom portion.
mixed dislocations
pure edge and pure screw
Burgers vector
denoted by a b. Burgers vectors are
indicated for edge and screw dislocations; For an edge, they are perpendicular ; whereas for a screw, they are parallel; they are neither perpendicular nor parallel for a mixed dislocation; Burgers vector will be the same at all points along its line
the permanent deformation of most crystalline materials
is by the motion of dislocations.
Burgers vector is an element
of the theory that has been developed to explain this type of deformation.
Virtually all crystalline materials contain some dislocations
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Dislocations are involved in the plastic
deformation of crystalline materials, both metals and ceramics
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The equilibrium number of vacancies for a given quantity of material depends on and increases with temperature according
the number of vacancies increases exponentially with temperature
Boltzmann's constant
k is the gas
For most metals, the fraction of vacancies Nv/N
just below the melting temperature is on the order of 10^-4; that is, one lattice site out of 10,000 will be empty.
self-interstitial
is an atom from the crystal that is crowded into an interstitial
site, a small void space that under ordinary circumstances is not occupied; In metals, a self-interstitial introduces
relatively large distortions in the surrounding lattice because the atom is substantially larger than the interstitial position in which it is situated; Consequently,
the formation of this defect is not highly probable, and it exists in very small concentrations,
which are significantly lower than for vacancies.
IMPURITIES IN SOLIDS .
it is difficult to refine metals to a purity in excess of 99.9999%;Most familiar metals are not highly pure; rather, they are alloys.
alloys,
in which impurity atoms have been added
intentionally to impart specific characteristics to the material; alloying is used in metals to improve mechanical strength and corrosion resistance
For example, sterling silver is a 92.5% silver/7.5% copper alloy
In normal ambient environments,
pure silver is highly corrosion resistant, but also very soft. Alloying with copper significantly enhances the mechanical strength without depreciating the corrosion
resistance appreciably.
solid solution and/or a new second phase
depending on the kinds of impurity, their concentrations,
and the temperature of the alloy
With regard to alloys, solute and solvent are terms that are commonly employed
Solvent represents the element or compound that is present in the greatest amount; on occasion,
solvent atoms are also called host atoms. Solute is used to denote an element or compound present in a minor concentration.
A solid solution forms when, as the solute atoms are added to the host material,
the crystal structure is maintained and no new structures are formed
A solid solution
is also compositionally homogeneous; the impurity atoms are randomly and uniformly
dispersed within the solid.
Impurity point defects are found in solid solutions, of which there are two types: substitutional and interstitial
For the substitutional type, solute or impurity atoms
replace or substitute for the host atoms
Type of solute and solvent
Atomic size factor (only when the difference in atomic radii between the two atom types is less than about Otherwise the solute atoms will create substantial lattice distortions and a new phase will form +-15%), Crystal structure (For appreciable solid solubility the crystal structures for metals of both atom types must be the same.),
type of solute and solvent
Electronegativity- The more electropositive one element and the more electronegative the other, the greater the likelihood that they will form an intermetallic compound instead of a substitutional solid solution
type of solute and solvent
Valences- Other factors being equal, a metal will have more of a tendency to dissolve another metal of higher valency than one of a lower valency.
An example of a substitutional solid solution
found for copper and nickel
For interstitial solid solutions
For metallic materials that have relatively high
atomic packing factors, these interstitial positions are relatively small; Normally, the maximum allowable concentration of interstitial
impurity atoms is low (less than 10%).
Even very small impurity atoms are
ordinarily larger than the interstitial sites, and as a consequence they introduce some
lattice strains on the adjacent host atoms
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composition (or concentration)
The two most common ways to specify composition
are weight (or mass) percent and atom percent
weight percent(wt%)
is the weight of a particular element relative to the total alloy weight
atom percent (at%)
is the number of moles of an
element in relation to the total moles of the elements in the alloy
Composition Conversions
from weight percent to atom percent
Interfacial defects
are boundaries that have two dimensions and normally separate regions of the materials that have different crystal structures and/or crystallographic orientations.These imperfections include external surfaces, grain boundaries, phase boundaries, twin boundaries, and stacking faults
External Surfaces
Surface atoms are not bonded to the maximum number of nearest neighbors, and are therefore in a higher energy state than the atoms at interior positions. The bonds of these surface atoms that are not satisfied give rise to a surface
energy, expressed in units of energy per unit area. To reduce this energy, materials tend to minimize, if at all possible, the total surface area
Grain Boundaries (Another interfacial defect)
boundary separating two small grains or crystals having different crystallographic orientations in polycrystalline materials; small- (or low-) angle grain boundary; tilt boundary-