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02 - NATURE OF MATERIALS


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Elements
Elements are building blocks of materials and can be grouped into 3 categories: Metals, metaloids (transition zone), and nonmetals.
Atomic structure
Accounts for many similarities between elements. Number of atoms in the uttermost shell of the atom account for most of the chemical affinity for other atoms ( valence electrons) - the more electrons there are in the outer shell, the more stable the atom is.
Bonding between atoms and molecules
Atoms are held together in molecules by various types of bonds that depend on the valence electrons. By comparison, molecules are attracted to each other by weaker bonds, which generally result from the electron configuration in the individual molecules. Thus, we have two types of bonding: (1) primary bonds, generally associated with the formation of molecules; and (2) secondary bonds, generally associated with attraction between molecules. Primary bonds are much stronger than secondary bonds.
Primary bonds
Strong connections between atoms via the exchange of valence electrons. Has 3 types: [1] IONIC BOND - transfer of electrons between atoms such as in salt, results in poor ductility and low conductivity. [2] COVALENT BOND - electrons are shared between atoms which usually results in high hardness and low electrical conductivity (diamond). [3] METALLIC BONDING - all the atoms share their outer shell electrons and create an "electron cloud" which usually grants the material good conductivity and ductility
Secondary bonds
Bond that stem from attraction between molecules and are much weaker than primary bonds. [1] Dipole forces - molecules attracted to each other due to difference in charge. [2] Lindon forces - molecules attracted to each other by temporary dipoles. [3] Hydrogen bonding - some form of covalent bonding, just like wanter, where hydrogen mixes with oxygen to produce the substance that drowns people so well.
Crystalline structure
The atoms in a crystalline structure are located at regular and repeating lattice positions in three dimensions; thus, the crystal structure possesses a long-range order which allows a high packing density - GEOMETRICAL REPETITION OF AN ATOMIC PATTER WITHIN A CERTAIN AREA (CRYSTAL GRAIN).
Types of crystal structures
Metals usually have three basic cristaline structures: [1] BCC: Body centered cubic. [2] FCC - Face centered cubic. [3] HCP - Hexagonal close packed, each of those structures gives a metal different properties and some metals undergo structural changes at certain temperatures >>> BCC, FCC, HPC CHANGE WHEN ROASTED INTENSELY.
Imperfections in crystals
Due to physical limits, such as grain boundaries or the inability to propagate indefinitely, imperfections in crystal structures arise. Common types of imperfections: [1] Point defects. [2] Line defects. [3] Surface defects.
Point defects
IMPERFECTIONS IN THE CRYSTAL STRUCTURE INVOLVING A SINGLE ATOM OR A PAIR FEW ATOMS. The common point defects are (1) vacancy - a missing atom in the lattice structure; (2) ion-pair vacancy (Schottky defect) - a missing pair of ions of opposite charge in a compound; (3) interstitialcy - a distortion in the lattice caused by an extra atom present; and (4) Frenkel defect - an ion is removed from a regular position in the lattice and inserted into an interstitial position not normally occupied by such an ion.
Line defects
A CONNECTED GROUP OF POINT DEFECTS THAT FORM A LINE IN THE LATTICE STRUCTURE. Most important line defect is a dislocation which may take two forms: [1] Edge dislocation - an edge of an extra plane of atoms that exist in the lattice. [2] Screw dislocation - a spiral within the lattice structure wrapped around an imperfection, like a screw is wrapped around an axis.
Deformation in metallic crystals
Elastic deformation is a temporary distortion of the lattice structure; plastic deformation distorts the lattice structure in a permanent manner via the mechanisms of "slip" and "twinning". Cristal size matters - the smaller the crystal is the harder it is to plastically deform the material, since the deformations stop at the crystal boundary >>> SMALL GRAINS TRAP AND STOP TWINS AND SLIPS.
Slip deformation
Material deforms due to a relative movement of atoms on opposite sides of a plane in a lattice due to shear stress. Number of slip direction depends on a lattice type (BCC - most slip directions, FCC, HCP - least slip directions)
Twinning deformation
Mechanism of plastic deformation in which atoms on one side of the twinning plane are shifted to form a mirror image. Mechanism is important for HCP metals: magnesium, zinc, since those metals don't slip easily. Rate of deformation also plays a factor: twinning can occur instantaneously, yet slipping requires time, thus if you the rate of deformation is very high, metals can twin instead of slipping, such as low carbon steels. >>> FAST RATE AND HCP PRODUCE A MIRROR IMAGE
Grain boundaries stop dislocations
Grain boundaries block the continued movement of dislocations in the metal during straining. As more dislocations become blocked, the metal becomes more difficult to deform; in effect it becomes stronger. Faster cooling usually means smaller grain size. >>> FAST COOLED SMALL GRAINS BLOCK DISLOCATIONS.
Noncrystalline structures
Crystalline structures undergo an abrupt volumetric change as they transform from liquid to solid state and vice versa. This is accompanied by an amount of energy called the heat of fusion that must be added to the material during melting or released during solidification. Noncrystalline materials melt and solidify without the abrupt volumetric change and heat of fusion and they lack the long-range order in their molecular structure >>> AMORPHOUS GLASS MELTS SMOOTHLY AND LACKS ORDERED STRUCTURE.