DT Materials and Processes in Design UNIT 1 - PROPERTIES OF MATERIALS Sub Atomic Particles CONSIST OF: PROTONS NEUTRONS ELECTRONS Protons Located in the Nucleus of and Atom Has a positive charge Number of Protons is the Atomic Number Neutrons Located in the nucleus of an Atom Has NO electrical charge Number of Neutrons is usually equal to the number or protons. Electrons Located outside the nucleus of the atom Housed in Electrical Clouds, Shells, Orbits Has a negative charge Moves around the nucleus in an orbit Atoms have a maximum of 8 Valence Electrons and are considered to be stable Electron Arrangement The electrons in the outermost shell (if it is not completely full) are available for bonding These electrons are called Valence Electrons Bonding is the chemical combining of 2 or more atoms. Valence The Capacity of an atom to combine with other atoms to form a molecule. Valence considers positive and negative charge of atoms, as determined by the gain or lose of electrons. Bonding The process when 2 or more atoms combine chemically is known as Bonding The 2 we are learning today are Ionic Bonding Covalent Bonding Ionic Bonding Electrons are either gained or lost by atom One atom is positive +, One is negative - Occurs between Metals and Nonmetals Metals lose electrons Nonmetals gain electrons Very strong bond usually solids - with high melting temperatures Covalent Bonding Electrons are shared between atoms (co) Neither atom has an electrical charge Forces between atoms are weak have low boiling or melting points Usually are liquids or gasses If a solid is usually a crystal Metallic Bond The metallic bond is actually a special case of an ionic bond. Think of metallic bonding as a more or less static arrangement of positive ions within a moving array of mobile electrons. This unique arrangement of cations and electrons give metals their characteristic properties. Metallic properties include: Heat conductivity (mobile electrons can carry the kinetic energy of heat), Heat conductivity Shiny appearance (the rapidly moving electrons emit energy in the form of light), Electricity conductors (electricity is the flow of electrons), and Electricity conductors Malleability (ability to be easily shaped into flat sheets or drawn into wires). Van der Waals Bond A weak attractive force between atoms or nonpolar molecules caused by a temporary change in dipole moment arising from a brief shift of orbital electrons to one side of one atom or molecule, creating a similar shift in adjacent atoms or molecules. Allotropy The ability of a material to exist in several crystalline forms. Metals ARE MATERIALS THAT OCCUPY THE LEFT SIDE OF THE PERIODIC TABLE AND ARE CHARACTERIZED BY HAVING ONE, TWO, OR THREE VALENCE ELECTRONS, AND BOND WITH THE METALLIC BOND. MOST OF THE ELEMENTS ON THE PERIODIC TABLE ARE METALS, INCLUDING GOLD, SILVER, PLATINUM, MERCURY, URANIUM, ALUMINUM, SODIUM AND CALCIUM. ALLOYS, SUCH AS BRASS AND BRONZE, ALSO ARE METALS. Properties of Metals Metals are shiny solids are room temperature (except mercury, which is a shiny liquid element), with characteristically high melting points and densities.liquid element Many of the properties of metals, including large atomic radius, low ionization energy, and low electronegativity, that are due to the fact that the electrons in the valence shell of a metal atoms can be removed easily. One characteristic of metals is their ability to be deformed without breaking. Malleability is the ability of a metal to be hammered into shapes. Ductility is the ability of a metal to be drawn into wire. Because the valence electrons can move freely, metals are good heat conductors and electrical conductors. Solidification of Metal To form the strongest metallic bonds, metals are packed together as closely as possible. Several packing arrangements are possible. Instead of atoms, imagine marbles that need to be packed in a box. As atoms of melted metal begin to pack together to form a crystal lattice at the freezing point, groups of these atoms form tiny crystals. These tiny crystals increase in size by the progressive addition of atoms. The resulting solid is not one crystal but actually many smaller crystals, called grains.grains These grains grow until they impinge upon adjacent growing crystals. The interface formed between them is called a grain boundary. Grains are sometimes large enough to be visible under an ordinary light microscope or even to the unaided eye. The spangles that are seen on newly galvanized metals are grains. (See A Particle Model of Metals Activity) Figure 5 shows a typical view of a metal surface with many grains, or crystals.grain boundary Grain In metals, a structure containing atoms of one crystalline orientation. Grains form during the solidification (or crystallization) of metal; they may be re-formed during recrystallization (re- heating) The Grain of Steel under a Microscope Processing (Changing the Properties of metals) It has long been known that the properties of some metals could be changed by heat treating. Grains in metals tend to grow larger as the metal is heated. A grain can grow larger by atoms migrating from another grain that may eventually disappear. Dislocations cannot cross grain boundaries easily, so the size of grains determines how easily the dislocations can move. As expected, metals with small grains are stronger but they are less ductile.heat treating Processing (Changing the Properties of metals) There are many ways in which metals can be heat treated. Annealing is a softening process in which metals are heated and then allowed to cool slowly. Most steels may be hardened by heating and quenching (cooling rapidly). This process was used quite early in the history of processing steel. In fact, it was believed that biological fluids made the best quenching liquids and urine was sometimes used. In some ancient civilizations, the red hot sword blades were sometimes plunged into the bodies of hapless prisoners! Today metals are quenched in water or oil.Annealingquenching Quenching results in a metal that is very hard but also brittle. Gently heating a hardened metal and allowing it to cool slowly will produce a metal that is still hard but also less brittle. This process is known as tempering. (See Processing Metals Activity). It results in many small Fe 3 C precipitates in the steel, which block dislocation motion which thereby provide the strengthening. Processing (Changing the Properties of metals) Because plastic deformation results from the movement of dislocations, metals can be strengthened by preventing this motion. When a metal is bent or shaped, dislocations are generated and move. As the number of dislocations in the crystal increases, they will get tangled or pinned and will not be able to move. This will strengthen the metal, making it harder to deform. This process is known as cold working. At higher temperatures the dislocations can rearrange, so little strengthening occurs.pinnedcold working You can try this with a paper clip. Unbend the paper clip and bend one of the straight sections back and forth several times. Imagine what is occurring on the atomic level. Notice that it is more difficult to bend the metal at the same place. Dislocations have formed and become tangled, increasing the strength. The paper clip will eventually break at the bend. Cold working obviously only works to a certain extent! Too much deformation results in a tangle of dislocations that are unable to move, so the metal breaks instead. Heating removes the effects of cold-working. When cold worked metals are heated, recrystallization occurs. New grains form and grow to consume the cold worked portion. The new grains have fewer dislocations and the original properties are restored. Plastics A PLASTIC MATERIAL IS ANY OF A WIDE RANGE OF SYNTHETIC OR SEMI-SYNTHETIC ORGANIC SOLIDS THAT ARE MOLDABLE. PLASTICS ARE TYPICALLY ORGANIC POLYMERS OF HIGH MOLECULAR MASS, BUT THEY OFTEN CONTAIN OTHER SUBSTANCES. THEY ARE USUALLY SYNTHETIC, MOST COMMONLY DERIVED FROM PETROCHEMICALS, BUT MANY ARE PARTIALLY NATURAL.SYNTHETICORGANICMOLDABLEORGANIC POLYMERSMOLECULAR MASSPETROCHEMICALS Polymer All plastics are polymers, but not all polymers are plastics. The simplified diagram below shows the relationship between monomers and polymers. The monomers that are found in many plastics include organic compounds like ethylene, propylene, styrene, phenol, formaldehyde, ethylene glycol, vinyl chloride and acetonitrile. Because there are so many different monomers that can combine in many different ways, we can make many kinds of plastics. Thermosetting Thermoset, or thermosetting, plastics are synthetic materials that strengthen during being heated, but cannot be successfully remolded or reheated after their initial heat- forming. This is in contrast to thermoplastics, which soften when heated and harden and strengthen after cooling. Thermoplastics can be heated, shaped and cooled as often as necessary without causing a chemical change, while thermosetting plastics will burn when heated after the initial molding. Additionally, thermoplastics tend to be easier to mold than thermosetting plastics, which also take a longer time to produce (due to the time it takes to cure the heated material). Thermosetting plastics, however, have a number of advantages. Unlike thermoplastics, they retain their strength and shape even when heated. This makes thermosetting plastics well-suited to the production of permanent components and large, solid shapes. Additionally, these components have excellent strength attributes (although they are brittle), and will not become weaker when the temperature increases. Thermoplastic Is a material that is capable of softening of fusing when heated and hardens when cooled again. A thermoplastic (sometimes written as thermo plastic) is a type of plastic made from polymer resins that becomes a homogenized liquid when heated and hard when cooled. When frozen, however, a thermoplastic becomes glass-like and subject to fracture. These characteristics, which lend the material its name, are reversible. That is, it can be reheated, reshaped, and frozen repeatedly. This quality also makes thermoplastics recyclable. There are dozens of kinds of thermoplastics, with each type varying in crystalline organization and density. Some types that are commonly produced today are polyurethane, polypropylene, polycarbonate, and acrylic. Ceramics (or ceramic materials) A FAMILY OF MATERIALS THAT ARE COMPOUNDS, TRADITIONALLY CONSISTING OF A METAL AND AN OXIDE, BUT THEY MAY ALSO BE CARBIDES, SULFIDES, NITRIDES AND INTERMETALLIC COMPOUNDS THEY GENERALLY HAVE AN IONIC BOND, ARE VERY HARD AND BRITTLE, AND CAN WITHSTAND HIGH TEMPERATURES.