Chapter 1 Manufacturing and Engineering Technology *All matter is made up of atoms containing a nucleus of protons and neutrons and surrounding clouds, or orbits, of electrons. *Atoms can transfer or share electrons; in doing so, multiple atoms combine to form molecules. Molecules are held together by attractive forces called bonds * * * 1. Ionic bonds-one or more electrons from an outer orbit are transferred from one material to another (example Na+ and Clform salt) 2. Covalent bonds- electrons in outer orbits are shared by atoms to form molecules (H20 water). Typically low conductivity and high hardness 3. Metallic bonds-available electrons are shared by all atoms in contact. The resultant electron cloud provides attractive forces to hold the atoms together and results in generally high thermal and electrical conductivity. 4. Van Der Waals forces are weak attractions occurring between molecules. *The crystal structure of metals- when metals solidify from a molten state, the atoms arrange themselves into various orderly configurations called CRYSTALS. 1.Body-centered cubic (BCC) least dense 2.Face-centered cubic (FCC) more dense 3.Hexagonal close-packet (HCP) most dense * * * * * minimize At different temperatures the same metal may form different structures * - the appearance of more than one type of crystal structure *Elastic deformation- a single crystal is subject to an external force, but returns to its original shape when the force is removed *Plastic deformation-a permanent deformation when the crystal does not return to its original shape * * Slipping of one plane of atoms over another adjacent plane (slip plane) under shear stress * Twinning- the second and less common mechanism of plastic deformation where a portion of the crystal forms a mirror image of itself across the plane of twinning * Definition: Anisotropy-a single crystal exhibits different properties when tested in different directions (ex. Woven cloth, plywood) * * *Point defects-vacancy, missing atoms, interstitial atom extra atom in the lattice or impurity foreign atom that has replaced the atom of pure metal *Linear defections called dislocations *Planar imperfections such as grain boundaries and phase boundaries *Volume or bulk imperfections-voids, inclusions, other phases, cracks * * Dislocations-defects in the orderly arrangement of a metal’s atomic structure. Because a slip plane containing a dislocation requires less shear stress to allow slip than does a plane in a perfect lattice, dislocations are the most significant defects that explain the discrepancy between the actual and theoretical strengths of metals. * * *Dislocations can become entangled and interfere with each other and be impeded by barriers such as grain boundaries, impurities, and inclusions in the material. The increased shear stress required to overcome entanglements and impediments results in an increase in overall strength and hardness of the metal and is known as work hardening or strain hardening. (Ex. Cold rolling, forging, drawing) * *When molten metal solidifies, crystals begin for form independently of each other. They have random and unrelated orientations. Each of these crystals grows into a crystalline structure or GRAIN. *The number and size of the grains developed in a unit volume of the metal depends on the rate at which NUCLEATION (the initial stage of crystal formation) takes place Is this what I mean by grain? * * • Rapid cooling – smaller grains • Slow cooling – larger grains • Grain boundaries – the surfaces that separate individual grains • Grain size- at room temperature a large grain size is generally associated with low strength, low hardness, and low ductility (ductility is a solid material's ability to deform under tensile stress) • Grain size is measured by counting the number of grains in a given area or by counting the number of grains that intersect a length of line randomly drawn on an enlarged photograph of the grains * * Cold working – a polycrystalline metal with uniform equiaxed grains is subject to plastic deformation at room temperature. * The grains become deformed and elongated. * The deformed metal exhibits higher strength because of the entanglement of dislocations with grain boundaries and with each other. * The higher the deformation, the stronger the metal becomes. * Strength is higher for metals with small grains because they have larger grain-boundary surface area per unit volume of metal hence more entanglements of dislocations * * *Metal properties are different in the vertical direction from those in the horizontal direction *It influences both mechanical and physical properties of metals * * *Annealing – heating metal to a specific temperature range for a given period of time * New equiaxed and strain-free grains are formed replacing the older grains. Between .3Tm and .5Tm where Tm is melting point of the metal on the absolute scale. Recrystallization temperature is defined as the temperature at which complete recrystallization occurs in approximately one hour. o Decrease density of dislocations o Lowers strength o Raises ductility * *temperature of metal increases further, the grain size grows and the size may exceed the original grain size We grow lots of grain in Indiana, but this is not what is meant by grain growth * * * *Cold working- plastic deformation at room temperature *Hot working – deformation occurs above the recrystallization temperature *Warm working is carried out at intermediate temperatures, thus it is a compromise between cold working and hot working *