Please enable JavaScript.
Coggle requires JavaScript to display documents.
Metals (Crystalline structure) (Imperfection in the atomic and…
Metals
(Crystalline structure)
Crystalline structure
when
molten
metals
cool & solidify
, they generally form a crystalline structure called a
space lattice
that consists of orderly rows of atoms
the
imaginary lines
connecting the
centers
of the atoms together is called a
lattice structure
the
orderly
rows of atoms in a
space lattice
can also be described as having
repeating
unit building blocks
called
unit cells
Unit Cell
metals solidify into
14
known crystal structures, but most commercially important is
three
types of lattice structures
BCC (BODY-CENTERED CUBIC)
FCC (FACE-CENTERED CUBIC)
HCP (HEXAGONAL CLOSE PACKED
Space lattice
atomic arrangements is crystalline solids can be described with respect to a
network of lines
in three dimensions
the
intersection
of the lines are called
lattice sites
,
(site has the same environment in the same direction)
a particular arrangement of atoms in a crystal structure can be described by specifying the atom positions in a repeating
unit cell
Crystal Structures for metals
BCC
has atoms at each of the
eight corners
of a cube
(plus one atom in the center of the cube)
each of the corner atoms is the
corner of another cube
so the corner atoms are shared among eight unit cells
a coordination number of 8
a
unit cell
consists of a
net total of two atoms
; one in the center & eight eights from corners atoms
the volume of atoms in a cell per the total volume of a cell is called the
packing factor
the bcc unit cell has a packing factor of
0.68
FCC
has atoms located at each of the corners & the centers of all the cubic
faces
each of the corner atoms is the corner of another cube, so the corner atoms are shared among eight unit cells
each of its six face centered atoms is shared with a adjacent atom
since 12 of its atoms are shared , it it said to have a coordination number of 12
the fcc unit cell consists of a
net total of four atoms
; eight eights from corners atoms and six halves of the face atoms
the packing factor is
0.74
HCP
the hcp structure has
three layers of atoms
in each the top & bottom layer, there are
six atoms
that arrange themselves in the
shape of a hexagon
& a seventh atom that sits in the middle of the hexagon
the middle layer has
three atoms
nestle in the triangular "groves" of the top and bottom plane
each of the
12 atoms in the corners
of the top & bottom layers contribute
1/6 atom
to the
unit cell
, the
two
atoms in the center of the hexagon of both the top & bottom layers each contribute
1/2 atom
& each of the
three
atom in the middle layer contribute
1 atom
coordination number is
12
there are six nearest neighbors in the same close packed layer,three in the layer above and three in the layer below
packing factor is
0.74
Imperfection in the atomic and arrangement
these defect often have profound effect on properties of materials
the material is not considered defective from an application viewpoint
the arrangement of atoms or ions in engineering materials contain imperfections or defect
for example, defect known as dislocations are useful in increasing the strength of metals and alloy
three basic types of imperfections
Point defects
localized disruptions
in otherwise perfect atomic or ionic arrangements in crystal structure
the disruption affects a region involving several atoms or ions
may be introduced by
movement
of the atoms or ions when gain energy by
heating
, during
processing of the materials
, by introduction of
impurities
and
doping
cause by
vacancy
interstitial atom
small substitutional atom
large substitutional atom
frenkel defect
Schottky defect
Line defect
Dislocation
line imperfections in otherwise perfect crystal
was introduced typically into the crystal during solidification of material or when the material is deformed permanently
types of dislocations
Screw dislocation
illustrated by cutting partway through a perfect crystal, then screwing the crystal one atom spacing
if we follow a crystallographic plane one revolution around the axis on which the crystal was skewed, starting at point x and travelling equal atom spacings in each direction, we finish one atom spacing below our starting point (point y)
the
vector
required to complete the
loop
and return to starting point is
Burgers vectors
the
axis or line
around which we
trace out
this path, is screw dislocation
Edge dislocation
illustrated by
slicing part away
through a perfect crystal,
spreading the crystal part
, and
partly filling the cut
with
extra
plane of atoms
the
bottom edge
of this
inserted plane
represents the edge dislocation
Mixed dislocation
have both
edge and screw components
, with a
transition region
between them
the
burgers vector
however
remains the same
for
all portions
of the mixed dislocations
Surface defect
are the
boundaries or plane
that
separate
a material into
regions
, each region having the
same
crystal structure but
different
orientations
Material surface
the
exterior dimensions
of the material represent surfaces at which the crystal abruptly ends
each atom at the surface
no longer has
the proper
coordination number
and
atomic bonding
is
discrupted
Grain boundaries
the microstructure of many engineered ceramic and metallic materials is nearly consist of many grain
Grain is a
portion of the material
within the
arrangement
of the atoms is nearly
identical
, however the
orientation
of the atom arrangement or crystal structure is
different
for each
adjoining grain
Grain boundary
, the
surface
that
separate
the individual grains , is narrow zone in which atom are
not properly spaced