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
HCP
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
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
Line defect Dislocation
Surface defect
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 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
Edge dislocation
Mixed 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
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
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
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