Overview. Metals. Metals. Metallic Bonding. Metallic Structure. Metallic Bonding. Atomic structure - metallic bond, lattice, unit cell, allotropy

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1 Overview Metals Dr. Kimberly Kurtis School of Civil Engineering Georgia Institute of Technology Atlanta, Georgia Atomic structure - metallic bond, lattice, unit cell, allotropy Microstructure - grains, grain boundaries, phases Mechanical Behavior - strength, E, ductility, toughness, effects of high T, strengthening mechanism, fracture, fatigue Durability - corrosion, fire Metals Metals - an element characterized by the tendency to give up electrons and by good thermal and electrical conductivity Metallic Bonding The difference in chemical properties between metals and non-metals lies mainly in the fact that atoms of non-metals will readily fill their valence shells by sharing electrons with or transferring electrons to other atoms. In metals, the outer shell is usually less than half full. Each atom will donate one or more electrons into a cloud, which is shared by all metal atoms. Mg Metallic Bonding In a pure metal, there are no molecules (i.e., each atom is independent of its neighbor). Valence electrons move randomly among other atoms present in the lattice, forming an electron cloud. Thus, we can think of the atoms in a metal lattice as positively charged ions. Metallic Structure The atomic structure results from the equilibrium created through (1) net repulsion of positively charged ion cores from one another (2) the attraction of these positively charged ion cores to the electron cloud The resulting structure is very stable, and if disturbed, will attempt to return to this stable configuration. This stretching of bonds occurs during elastic deformation under loading, for example. The sea of cloud of electrons is what gives metals their high electrical and thermal conductivity.

2 Metallic Structure At equilibrium, the mean interatomic distance must be the same for all atoms (in a pure metal). This results in the formation of a regular threedimensional pattern in the structure or a lattice structure, such as the one seen here. The characteristic arrangement in a crystal allows for its identification by the use of x-ray diffraction, using Bragg s law: nλ=2dsinθ Where n= integer λ = wavelength of incident x-ray d = distance between atomic layers in a crystal θ = diffraction angle Metallic Structure X-ray Diffraction Within the lattice structure, small groups of atoms form repeating units or unit cells. Smallest configuration of atoms that preserves the arrangement of the lattice Building blocks of the lattice structure About 90% of all elemental metals crystallize upon solidification into three densely packed unit cell or crystalline structures: Body-centered cubic (BCC) Face-centered cubic (FCC) Hexagonal close-packed (HCP) grain grain boundary : BCC Total of 2 atoms/unit cell Each atom has 8 nearest neighbors Ex. Chromium, iron, tungsten, molybdenum a =4R/ 3

3 : FCC Total of 4 atoms/unit cell (1/2 of each face and 1/8 of each corner atom) Each atom has 12 nearest neighbors Ex. Copper, aluminum, silver, gold, lead : HCP Total of 6 atoms/unit cell (1/6 of each of the 12 top and bottom face corner atoms, 1/2 of each of the 2 center face atoms, and all 3 midplane interior atoms) Each atom has 12 nearest neighbors Ex. Cobalt, titanium, zinc, magnesium, beryllium a =2R 2 and Interatomic Spacing : Atomic Packing Factor APF = (vol of atoms in a unit cell)/(total unit cell vol) For BCC, APF = (2x4/3πR 3 )/(4R/ 3x4R/ 3x4R/ 3)= 0.68 For FCC, APF = (4x4/3πR 3 )/(2R 2x 2R 2x 2R 2)= 0.74 For HCP APF = = 0.74 Most efficient use of space possible Thus, metal lattices based upon FCC and HCP unit cells can be considered to be more closely-packed or more densely packed than those based on BCC. This will have some implications on strength and ductility, as we will see Allotropy Allotropy or polymorphism - different crystalline configurations may be possible for a substance (usually an elemental solid) Alloys Alloying - addition of another element to a pure metal to form a solid solution; may result in changes to microstructure and material properties Interstitial system Solute radius must be <41% solvent radius Substitutional system Atomic radii must be +15%

4 Alloys: Substitutional Brass is a solid solution of copper (Cu) and zinc (Zn) Steel is a solid solution of iron (Fe) and carbon (C) Alloys: Interstitial Phase Diagram These changes in structure with temperature and alloying, lead us to our next topic. Phase Diagrams Because the mechanical properties (strength, E, hardness, toughness, ductility) and durability (corrosion resistance, weldability) of engineering materials depend strongly upon microstructure, it is important that engineers possess a basic understanding of how the microstructure is formed, and how this structure influences the engineering properties. Microstructure-Property Relationships: Steel Phase diagrams can be thought of as a map, showing the phase(s) existing at equilibrium for an alloy of a given composition at a given temperature

5 Phase Diagram Component - distinct chemical substance (e.g., Fe, C, Sn, Pb, etc.) from which a phase is formed Phase - a chemically homogeneous portion of the microstructure; may be polycrystalline, but each crystal varies only in orientation, not composition (e.g., solid solution, liquid, pure solid)