Materials Science Flashcards
Ionic bond
where one or more electrons completely transfers from an atom of one element to the atom of another. The force of attraction due to the opposite polarity of the charge holds the element together.
Covalent bond
the bond formed by shared electrons when an atom needs electrons to complete its outer shell it shares those electrons with its neighboring atom. The electrons become a part of both atoms, filling both atoms’ electron shells.
Metallic bond
the atoms do not share or exchange electrons to bond together. Many electrons, roughly one for each atom, are more or less free to move throughout the metal; each electron can interact with many of the fixed atoms.
Molecular bond
a temporary weak charge exists when electrons of neutral atoms spend more time in one region of their orbit than in another region. The molecule weakly attracts other molecules. This molecular bond also called a van der Waals bond
Hydrogen bond
similar to the molecular bond, a hydrogen bond occurs because of the ease with which hydrogen atoms are willing to give up an electron to atoms of oxygen, fluorine, or nitrogen.
Describe the following types and features of solids:
Amorphous
Amorphous materials have an irregular arrangement of atoms or molecules; they exhibit properties of solids. Amorphous solids do not have a repeating crystalline structure. These materials have definite shape and volume and diffuse slowly; however, they lack sharply defined melting points. As solids, they resemble liquids that flow slowly at room temperature. Glass and paraffin are examples of amorphous materials. Other examples of amorphous materials include thin gels and thin films.
Describe the following types and features of solids:
Crystalline solids
Arrays of atoms in regular patterns create crystal structures in metals and other solids. Crystalline structures have repeating units of atoms, ions, and molecules. A crystal structure has atoms arranged in a pattern that repeats periodically in a three-dimensional geometric lattice. Forces associated with chemical bonding result in this repetition and produce properties such as strength, ductility, density, conductivity, and shape. Ductility is the metal’s ability to bend
Describe the following types and features of solids:
Grain structures
Examining a thin section of a common metal under a microscope illustrates the molecular structure similar to that shown below in the figure. Each of the light areas is a grain, or crystal, which is the region of space occupied by a continuous crystal lattice. Grain boundaries are the dark lines surrounding the grains. The term grain structure refers to the arrangement of the grains in a metal. Each grain has a particular crystal structure determined by the type of metal and its composition.
Body-Centered Cubic (BCC):
The unit cell consists of eight atoms at the corners of a cube and one atom at the body center of the cube in a body-centered cubic (BCC) arrangement of atoms.
These BCC metals have two properties in common, high-strength and low-ductility
Face-Centered Cubic (FCC):
In a face-centered cubic (FCC) arrangement of atoms, the unit cell consists of eight atoms at the corners of a cube and one atom at the center of each of the faces of the cube.
These FCC metals generally have lower strength and higher ductility than BCC metals.
Hexagonal Close-Packed (HCP):
The unit cell consists of three layers of atoms in a hexagonal close-packed (HCP) arrangement of atoms. The top and bottom layers each contain six atoms at the corners of a hexagon and one atom at the center of each hexagon. The middle layer contains three atoms nestled between the atoms of the top and bottom layers, therefore, the name close-packed.
HCP metals are not as ductile as FCC metals.
Point Imperfections… List 3 types
Vacancy Defects
Substitutional Defects
Interstitial Defects
Describe Vacancy Defects
Vacancy defects, the simplest defect, result from a missing atom in a lattice position. This defect results from imperfect packing during the crystallization process, or may be due to increased thermal vibrations of the atoms from elevated temperatures.
Describe Substitutional Defects
Substitutional defects result from an impurity present at a lattice position. An alloying material added to the metal, such as carbon (carbon steel) creates an impurity at a lattice position.
Describe Interstitial Defects
Interstitial refers to locations between atoms in a lattice structure. They result from an impurity located at an interstitial site or one of the lattice atoms being in an interstitial position instead of its lattice position. Interstitial impurities called network modifiers act as point defects in amorphous solids.
Describe Edge Dislocations
Edge dislocations consist of an extra row or plane of atoms in the crystal structure, shown below in the figure. The imperfection may extend in a straight line all the way through the crystal, or it may follow an irregular path. The edge dislocation may be short, extending only a small distance into the crystal causing a slip of one atomic distance along the glide plane (direction the edge imperfection is moving).