MODULE 2 Flashcards
is one in which the atoms are situated in a repeating or periodic array over large atomic distances—that is, long-range order exists, such that upon solidification, the atoms
will position themselves in a repetitive three-dimensional pattern, in which each atom is bonded
to its nearest neighbor atoms.
Crystalline material
-do not crystallize and the long-range atomic order is absent.
Non-crystalline/Amorphous Material
the manner in which atoms, ions, or molecules are spatially arranged.
Crystal structure
where spheres representing nearest-neighbor atoms touch one
another.
Atomic hard-sphere model
means a three-dimensional array of points coinciding with atom positions (or
sphere centers).
Lattice
small groups of atoms form a repetitive pattern.
UNIT CELLS
are parallelepipeds or prisms having three sets of parallel faces; one is drawn within the
aggregate of spheres which in this case happens to be a cube.
UNIT CELLS
is the basic structural unit or building block of the crystal structure and defines the crystal
structure by virtue of its geometry and the atom positions within.
UNIT CELLS
What are three of the simple crystal strcutures found in metals?
- face-centered cubic,
- body centered cubic, and
- hexagonal close-packed.
The crystal structure found for many metals has a unit cell of cubic geometry, with atoms
located at each of the corners and the centers of all the cube faces.
face-centered cubic,
the cube edge length a and the atomic radius R are related through of a fCC is
a = 2R√2
Numbers of atoms per unit cell in FCC can be calculated using
N = Ni +Nf/2+Nc/8
each atom has the same number of nearest-neighbor or touching atoms
coordination number
What is the coordination number of FCC?
12
is the sum of the sphere volumes of all atoms within a unit cell (assuming the atomic hard-sphere model) divided by the unit cell volume—that is,
Atomic packing factor (APF)
Equation of APF
volume of atoms in a unit cell/Total unit cell volume
Another common metallic crystal structure also has a cubic unit cell with atoms located at
all eight corners and a single atom at the cube center.
body-centered cubic
(BCC) crystal structure.
Center and corner atoms touch one another along cube diagonals, and unit cell length a
and atomic radius R are related through
a =4R√3
Number of atoms in FCC?
4
NUMBER OF ATOMS IN BCC?
2
coordination number for bcc?
8
BCC atom packing factor
0.68
FCC atom packing factor
0.74
It is also possible to have a unit cell that consists of atoms situated only at the corners of a cube.
simple cubic (SC) crystal structure;
The only simple-cubic element is
polonium,
The top and bottom faces of the unit cell consist of six atoms that form regular hexagons and
surround a single atom in the center.
The Hexagonal Close-Packed Crystal Structure
In order to compute the number of atoms per unit cell for the HCP crystal structure,
N = Ni +Nf/2+Nc/6
number of atoms of hexagonal is?
6
FCC Unit Cell Volume
16R3√2
THEORETICAL DENSITY COMPUTATION
ρ =nA/(Vc xNa)
when the periodic and repeated arrangement of atoms is perfect or extends throughout the entirety of the specimen without interruption,
single crystal.
Most crystalline solids are composed of a collection of many small crystals or grains;
polycrystalline.
Also, there exists some atomic mismatch within the region
where two grains meet; this area,
grain boundary.
It lack a systematic and regular
arrangement of atoms over relatively large atomic distances.
noncrystalline solids
meaning of amorphous?
“without form”
The simplest of the point defects is
vacancy,
The equilibrium number of vacancies Ny for a given quantity of material (usually per meter cubed) depends on and increases with temperature according to
Nv = N exp(−Qv/kt)
The
value of k is
1.38 × 10−23 J/atom∙K, or 8.62 × 10−5
eV/atom∙K
is an atom from the crystal that is crowded into an interstitial
site
self-interstitial
small void space that under ordinary circumstances is not occupied.
interstitial site
is the element or compound that is present in the greatest amount; on occasion,
Solvent
used to denote an element or compound present in a minor concentration.
Solute
is a linear or one-dimensional defect around which some of the atoms are misaligned.
dislocation
it is a linear defect that centers on the line that is defined along the end of the extra
half-plane of atoms.
edge dislocation;
being formed by a shear stress that is applied to produce the distortion shown in Figure, the upper front region of the crystal is shifted one atomic distance to the right relative
to the bottom portion.
screw dislocation,
are boundaries that have two dimensions and normally separate
regions of the materials that have different crystal structures and/or crystallographic orientations.
Interfacial defects
Within the boundary region, which is probably just several atom distances wide, there is some
atomic mismatch in a transition from the crystalline orientation of one grain to that of an adjacent
one.
grain boundary
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.
External Surfaces
exist in multiphase materials, in which a different phase exists on each
side of the boundary; furthermore, each of the constituent phases has its own distinctive physical
and/or chemical characteristics.
Phase boundaries
is a special type of grain boundary across which there is a specific mirror
lattice symmetry; that is, atoms on one side of the boundary are located in mirror image positions
to those of the atoms on the other side
twin boundary
are defects in 3-dimensions. These include pores,
cracks, foreign inclusions and other phases.
BULK OR VOLUME DEFECTS
Most dislocations found in crystalline materials are probably neither pure edge nor pure
screw but exhibit components of both types; these are termed
mixed dislocations
The magnitude and direction of the lattice distortion associated with a dislocation are
expressed in terms
a Burgers vector
