Page 159 - Materials Science and Engineering An Introduction

Summary • 131

(b) The value of G is determined by substitution of this value for / into Equation 4.19a;
therefore,
G = -6.6457 log / - 3.298
= (-6.6457) log(0.0377) - 3.298
= 6.16

SUMMARY
Vacancies and • Point defects are those associated with one or two atomic positions; these include
Self-Interstitials vacancies (or vacant lattice sites) and self-interstitials (host atoms that occupy inter-
stitial sites).
• The equilibrium number of vacancies depends on temperature according to Equation 4.1.

Impurities in Solids • An alloy is a metallic substance that is composed of two or more elements.
• A solid solution may form when impurity atoms are added to a solid, in which case
the original crystal structure is retained and no new phases are formed.
• For substitutional solid solutions, impurity atoms substitute for host atoms.
• Interstitial solid solutions form for relatively small impurity atoms that occupy inter-
stitial sites among the host atoms.
• For substitutional solid solutions, appreciable solubility is possible only when atomic
diameters and electronegativities for both atom types are similar, when both elements
have the same crystal structure, and when the impurity atoms have a valence that is
the same as or greater than the host material.

Specification of • Composition of an alloy may be specified in weight percent (on the basis of mass frac-
Composition tion, Equations 4.3a and 4.3b) or atom percent (on the basis of mole or atom fraction,
Equations 4.5a and 4.5b).
• Expressions were provided that allow conversion of weight percent to atom percent
(Equation 4.6a) and vice versa (Equation 4.7a).
• Computation of average density and average atomic weight for a two-phase alloy is
possible using other equations cited in this chapter (Equations 4.10a, 4.10b, 4.11a,
and 4.11b).

Dislocations—Linear • Dislocations are one-dimensional crystalline defects of which there are two pure
Defects types: edge and screw.
An edge may be thought of in terms of the lattice distortion along the end of an
extra half-plane of atoms.
A screw is as a helical planar ramp.
For mixed dislocations, components of both pure edge and screw are found.
• The magnitude and direction of lattice distortion associated with a dislocation are
specified by its Burgers vector.
• The relative orientations of Burgers vector and dislocation line are (1) perpendicular
for edge, (2) parallel for screw, and (3) neither perpendicular nor parallel for mixed.

Interfacial Defects • Within the vicinity of a grain boundary (which is several atomic distances wide), there
is some atomic mismatch between two adjacent grains that have different crystallo-
graphic orientations.