Course CHEM111E (Chemistry for Engineers) Title of the Module Unit III. Chemistry of Engineering Materials – Metals Learning Objectives At the end of this module, the student shall be able:
To distinguish different types of fuels.
To calculate the amount of energy that can be derived from fuels. Content 1. Structure of Metals
Properties of Metals
Classification of Metals
Introduction Metals, from the Greek word ‘’metallon” (mine or quarry), are used in various aspects of our daily lives. Roughly 25% of the earth’s crust is comprised of metals and due to this abundance and metals’ varied properties, they can be used in construction, home appliances, tools, decorative items and jewelry, coinage and more. The history of refined metals is very long. The smelting and use of copper has been show to date back to the Bronze age, at around 3500 B.C. Gold, silver, meteoric iron, and lead have been in use earlier than that. The smelting of iron dates back to around 1500 B.C. and subsequent developments of early forms of steel followed at around 1200 B.C. Further discoveries of new types of metals and the development of modern alloys from early simple steel alloys to more complicated and specialized ones have happened over the years, to cater to various needs and applications.
Figure 1. metal atoms are stacked similarly to how we see fruit being stacked or how any large or heavy spheres are stored/arranged
This arrangement is the most efficient as it minimizes the empty space between the spheres. Metal crystals have unit cells – the smallest section of a crystal lattice that still retails the overall structure and symmetry of the lattice – wherein the atoms are arranged in such close-packed structures. Most pure metals and metallic alloys naturally adopt one of these three arrangements which provide the closest packing structures:
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Figure 2. The common structures of the unit cells of most metals
These different structures affect certain metallic behaviors particularly strength and ductility. Because of the close-packed nature of the atoms, the attraction between the atomic nuclei and the surrounding electrons may occur in all directions and we can say that the bonding in metals in nondirectional; this means that upon the application of pressure, a metal will deform rather than break. 2. Properties of Metals Metals are recognized as a type of matter that exhibit the following physical properties: 1. Metallic luster. Metals appear shiny and lustrous (i.e. they reflect light) when they are fractured, cut, or polished.
Figure 3. Metallic vs non-metallic luster 2. High thermal and electrical conductivity. Metals have electron clouds where the valence electrons can be removed easily. This means that a metal atom’s electrons are highly mobile and are able to pass on heat-induced vibrational energy easily. These valence electrons are also responsible for allowing metals to conduct electricity more easily than other materials. When exposed to an electric field, these electrons move across the lattice of the metal structure like billiard balls hitting each other one after another. Conductive metals like Cu, Au, and Ag have atoms with only one valence electron which moves more freely and is able to pass on more energy. Metalloids or semiconductor metals on the other hand have four or more valence electrons which means that their electrons cannot move as freely in the metal’s lattice. This also means that even though they can still conduct electricity, they are less-efficient at it compared to conductor metals. 3. Malleability. Metals can be hammered or rolled into thin sheets. As we have mentioned, the bonding of metal atoms is nondirectional. When compression forces are applied to a metal, this causes the atoms of the metal to just roll over each other into new positions while maintaining their metallic bonds with each other. 4. Ductility. Metals can be stretched and drawn into thin wires without damage or breakage. This property also results from the nondirectional bonding of metal atoms. While metals are malleable and ductile in general, some metals are more so than others. Those with more close-packed structures (FCC and HCP structures) are generally more malleable and ductile than those with the BCC structure.
Figure 4. How metallic crystal structure affects ductility
III. Classification of Metals
Ferrous vs. Nonferrous Metals
This is the most general/broad classification of metals. Ferrous metals, as the name implies, contain iron (“ferrum” being the Latin name for iron). Iron (Fe) is a very abundant element and although difficult to find pure in nature, it is present in many minerals. It is also relatively cheap due to the fact that it is relatively easy to refine. When combined with other metals in alloys, or even by itself, it displays different properties which results in its versatility as an alloying component. Ferrous metals are also magnetic and strong but are prone to corrosion. Around 90% of all manufactured metals contain iron. Non- ferrous metals, like brass, on the other hand do not have iron in them. They do not have any magnetic properties. The most common non-ferrous metals used in the industry are Cu, Al, Zn, Sn, Pb, Co, and Ni. The non-ferrous metals and the alloys based on them have names that are based on their element’s names (e.g. copper alloy, aluminum alloy, etc.)
FERROUS METALS
Properties
Uses
Pig Iron/Crude Iron
(92%iron, up to 3.5% C and other impurities)
First product in the smelting of iron
Can be hardened but not tempered
Brittle (not malleable)
Mixed with other metals or elements in making steel Cast Iron (Iron with up to 2-6.7% C)
Does not corrode easily unless exposed to saline water
Cannot be used in forging because it is a bit brittle
Making gates, lamp posts, railings, etc.
Making machinery parts and agricultural tools Wrought Iron (iron with less than 0.08% C)
Ductile, malleable, tough
Not affected by saline water and can resist corrosion
Can be welded but fuses with difficulty
Decorative/ornamental iron works like gates, outdoor stairs, railings
Steel (iron, small amounts of C + other elements in various %s) At present, over 3500 grades of steel are available in the market.
NON-FERROUS METALS Even though all metals share some similar mechanical properties, individually they have properties that may or may not be advantageous for certain applications. These individual properties can be exploited by mixing various metals in different proportions when creating alloys. Some of the most commonly used non-ferrous metals in the industry are:
Soft and has low melting point
Corrosion resistant and highly machinable
Is toxic and may have byproducts that cause serious health complications USE: car batteries, radiation protection, cable sheathing and ammunition; solder (Zn-Pb alloy) is used to join electrical components and metallic items
Summary
Hexagonal close-packed (hcp) structure. Atoms occupying all corners of a hexagonal prism + one atom at the center of each of the two hexagonal faces.
Face-centered cubic (fcc) structure. One atom at the center of each of the six faces of the cube + eight atoms at each corner.
Body-centered cubic (bcc) structure.
Eight metal atoms occupying all corners of a cube + 1 atom at the very center of the cube.
Properties of Metals
Metallic luster
High thermal and electrical conductivity
Malleability
Ductility
Classification of Metals
Main classification of metals FERROUS NON-FERROUS * Pig iron
Aluminum
Zinc
Tin
Lead
Cobalt
Nickel, etc. And their alloys
References: Askeland, D. et. al. (2010). The Science and Engineering of Materials (6th Ed). Cengage Learning, Inc. Mortimer, Charles E. (1975). Chemistry: A Conceptual Approach (3rd ed.). New York: D. Van Nostrad Company. Russell, A. M; Lee, K. L. (2005). Structure–Property Relations in Nonferrous Metals. Structure-Property Relations in Nonferrous Metals. Hoboken, NJ: John Wiley & Sons.