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Metals Engineering materials are typically categorized into four groups metals polymers ceramics and composites Understanding their properties and how to use them effectively is essential in engineering In this video sponsored by CuriosityStream we will focus on metals their microstructures and techniques like alloying and heat treatment used to enhance their properties Types of Metals Two thirds of the elements in the periodic table are metals but for engineering purposes we are interested in only a few of them Iron is the most important as it is used to create steel a high strength material with a wide range of applications Aluminum is also commonly used because its alloys have high strength to weight ratios a low melting temperature and are relatively inexpensive Titanium is a popular choice for aerospace applications due to its excellent strength to weight properties and high melting point but it is more expensive than aluminum Other important metals include magnesium copper and nickel Crystal Lattice and Defects The atoms of a pure metal are packed closely together and arranged in a regular grid called the crystal lattice Metal is a crystalline material but not all materials have a regular structure like this There are several different ways the atoms of a metal can pack together leading to different types of unit cells Copper atoms for example pack together in a face centered cubic structure while iron atoms prefer to pack together in a body centered cubic structure and titanium atoms prefer the hexagonal close packed structure The close packing of atoms is one reason why metals have higher densities than most other materials Lattices contain numerous defects like vacancy interstitial and substitution al defects They also contain linear defects called dislocations where atoms are offset from their usual position in the lattice Dislocations move through the lattice by breaking and reforming atomic bonds and the process is irreversible This is the underlying mechanism behind plastic deformation in metals Polycrystalline Materials As metal cools down atoms group together and form multiple lattices that are oriented in different directions creating grains within the metal s structure Materials made up of a collection of these grains are said to be polycrystalline The smaller the grain size the stronger the material will be and impurities called inoculants can intentionally be added to the molten metal to lead to smaller grain sizes Grain boundary strengthening is just one of many techniques used to intentionally strengthen metals Metals and their alloys are divided into ferrous and nonferrous categories depending on whether or not the base metal of the alloy is iron Brass for example is a nonferrous alloy of copper and zinc Aluminum alloys are important in engineering and are often used for their good strength properties at a lightweight and reasonable cost Steel is probably the most important engineering alloy of all Types of Steel Low carbon or mild steel contains up to 0 25 carbon It does not have particularly high strength but is ductile and relatively low cost Medium carbon steel contains between 0 25 and 0 6 carbon High carbon steel contains between 0 6 and 2 carbon Because these steels contain a larger amount of carbon they are stronger and can be more easily strengthened using different heat treatment methods like quenching and tempering Cast iron contains between 2 and 4 carbon It has good fluidity and a lower melting point making it good for casting but it tends to be brittle Alloys are created by melting the base metal and various alloying elements together They can either be substitutional or interstitial depending on the relative size of the atoms The presence of alloying elements distorts the crystal lattice which tends to impede the motion of dislocations and so has a strengthening effect Phase Diagram for Iron Carbon Alloy Because of the nature of the bcc structure ferrite can only hold a small amount of interstitial carbon When the solubility of ferrite is exceeded a new phase called cementite forms alongside the ferrite Cementite is a hard brittle compound made up of one carbon atom for every three iron atoms The presence of a cementite phase can have a significant strengthening effect which is part of the reason steel is much stronger than pure iron

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UA CHEM 380 - Metals

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