As shown on the periodic table of elements, the majority of the chemical elements in pure form are classified as metals. Physical properties show that metals are good electrical conductors and heat conductors, and exhibit good ductility and strength. Shown in chemical properties, metals usually have 1-3 electrons in their outer shell, and loose their valence electrons easily.
Metals are composed of atoms held together by strong, delocalized bonds called metallic bonding: arrangement of positive ions surrounded by a cloud of delocalized electrons. Above their melting point, metals are liquids, and their atoms are randomly arranged and relatively free to move. However, when cooled below their melting point (solidification), metals rearrange to form ordered, crystalline structures. The smallest repeating array of atoms in a crystal is called a unit cell. In a unit cell, atoms are packed together as closely as possible to form the strongest metallic bonds. Typical packing or stacking arrangements are: face-centered cubic (FCC), body-centered cubic (BCC), and hexagonal close packing (HCP). As atoms of a 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. These grains grow until they impinge upon adjacent growing crystals. The interface formed between them is called a grain boundary. Metallic crystals are not perfect. Sometimes there are empty spaces called vacancies, where an atom is missing. Another common defect in metals are dislocations, which are lines of defective bonding. These and other imperfections, as well as the existence of grains and grain boundaries, determine many of the mechanical properties of metals. When a stress is applied to a metal, dislocations are generated and move, allowing the metal to deform.
When loads (stresses) are applied to metals they deform. If the load is small, metals experience elastic deformation, which involves temporary stretching or bending of bonds between atoms. When higher stresses are applied, permanent (plastic) deformation occurs. This plastic deformation involves the breaking of bonds, often by the motion of dislocations. If placed under too large of a stress, metals will mechanically fail, or fracture. The most common reason for metal failure is fatigue, i.e., a fracture process resulting from the application and release of small stresses and re-application of the load (as many as millions of times).
In industry, molten metal is cooled to form the solid (casting). The solid metal is then thermomechanically shaped to form a particular product. Processes such as extrusion and sheet forming are used for this purpose. During this shaping process, the application of heat and plastic deformation can strongly affect the mechanical properties of a metal. Heat treating induces microstructure changes, such as grain growth, that modify the properties of some metals. 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). Quenching produces a metal that is very hard but also brittle. Because plastic deformation results from the movement of dislocations, metals can be strengthened by preventing this motion. When a metal is 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. This process is known as cold working. At higher temperatures the dislocations can rearrange, so little strengthening occurs. Heating removes the effects of cold-working. When cold worked metals are heated, recrystallization occurs, a process where new grains form and grow to consume the cold worked portion. The new grains have fewer dislocations and the original properties are restored.
At CAVS at Mississippi State University, we perform research and application work for metals in two branches of materials - lightweight materials of magnesium and aluminum, and steel materials. The material research around these two branches is broad enough to attract various funding sources, from federal agencies to local manufaturers. We form interdisciplinary teams to support the material research. The team includes physicists, chemists, material scientists, mechancial/aerospace/civil engineers to develop multiscale material length scale models for use that are validated using a wide range of experimental equipment.
Aluminum alloys have been a focus in lightweight designs. Understanding the energy absorption, mechanical behavior and strength, creep resistance, and corrosion resistance are key research opportunities.
Magnesium alloys have been a focus in lightweight designs. Understanding the energy absorption, mechanical behavior and anisotropy, creep resistance, and corrosion resistance are key research opportunities.
Nickel has been in use since 3500BCE, is one of the few room temperature ferromagnetic elements, and today is utilized in alloys, superalloys and catalysis.
Here we can discuss applications to iron with links to projects.
- ↑ 1.0 1.1 Tschopp, M. A., & McDowell, D.L. (2007). Structures and energies of Sigma3 asymmetric tilt grain boundaries in Cu and Al. Philosophical Magazine, 87, 3147-3173 (http://dx.doi.org/10.1080/14786430701455321).
- ↑ 2.0 2.1 Tschopp, M. A., & McDowell, D.L. (2007). Asymmetric tilt grain boundary structure and energy in copper and aluminum. Philosophical Magazine, 87, 3871-3892 (http://dx.doi.org/10.1016/j.commatsci.2010.02.003).
- ↑ Spearot, D.E., Tschopp, M.A., Jacob, K.I., McDowell, D.L., "Tensile strength of <100> and <110> tilt bicrystal copper interfaces," Acta Materialia 55 (2007) p. 705-714 (http://dx.doi.org/10.1016/j.actamat.2006.08.060).
- ↑ Tschopp, M.A., Spearot, D.E., McDowell, D.L., "Atomistic simulations of homogeneous dislocation nucleation in single crystal copper," Modelling and Simulation in Materials Science and Engineering 15 (2007) 693-709 (http://dx.doi.org/10.1088/0965-0393/15/7/001).
- ↑ 5.0 5.1 5.2 Tschopp, M.A., McDowell, D.L., "Influence of single crystal orientation on homogeneous dislocation nucleation under uniaxial loading," Journal of Mechanics and Physics of Solids 56 (2008) 1806-1830. (http://dx.doi.org/10.1016/j.jmps.2007.11.012).
- ↑ Tang, T., Kim, S., & Horstemeyer, M. (2010). Fatigue Crack Growth in Magnesium Single Crystals under Cyclic Loading: Molecular Dynamics Simulation. Computational Materials Science, 48, 426., 48, 426-439 (http://dx.doi.org/10.1080/14786430701255895).
- ↑ Barrett, C.D., El Kadiri, H., Tschopp, M.A. (2011). Breakdown of the Schmid Law in Homogenous and Heterogenous Nucleation Events of Slip and Twinning in Magnesium. Journal of Mechanics and Physics of Solids, in review.
- ↑ B. Jelinek, J. Houze, Sungho Kim, M. F. Horstemeyer, M. I. Baskes, and Seong-Gon Kim, "Modified embedded-atom method interatomic potentials for the Mg-Al alloy system" Phys. Rev. B 75, 054106 (2007)