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3.CRYSTAL STRUCTURE OF METALSA crystal structure is composed of a pattern, a setof atoms arranged in a particular way. Patterns are located upon the points of a lattice,which is an array of points repeating periodically in three dimensions. The points can bethought of as forming identical tiny boxes, called unit cells, that fill the space of the lattice.The lengths of the edges of a unit cell and the angles between them are called the lattice

parameters.UNIT CELLThe crystal structure of a material (the arrangement of atoms within a giventype of crystal) can be described in terms of its unit cell. The unit cell is a small boxcontaining one or more atoms, a spatial arrangement of atoms. The unit cells stacked inthree-dimensional space describe the bulk arrangement of atoms of the crystal. The crystalstructure has a three-dimensional shape. The unit cell is given by its lattice parameters,which are the length of the cell edges and the angles between them, while the positions ofthe atoms inside the unit cell are described by the set of atomic positions (xi , yi , zi)measured from a lattice point. Body-Centered Cubic (BCC); Face-Centered Cubic (FCC);Hexagonal Close-Packed (HCP).4.GRAIN BOUNDARIES Grain boundaries are interfaces where crystals of different

orientations meet. A grain boundary is a single-phase interface, with crystals on each side ofthe boundary being identical except in orientation. Grain boundary areas contain thoseatoms that have been perturbed from their original lattice sites, dislocations, and impuritiesthat have migrated to the lower energy grain boundary. Since grain boundaries are defectsin the crystal structure they tend to decrease the electrical and thermal conductivity of thematerial. The high interfacial energy and relatively weak bonding in most grain boundariesoften makes them preferred sites for the onset of corrosion and for the precipitation of newphases from the solid.7.POLYMORPHISM refers to the ability of a solid to exist in more than one crystalline formor structure. These unique crystalline phases will be dependent on intensive variables suchas pressure and temperature. Polymorphs have different stabilities and may spontaneouslyconvert from a metastable form (or thermodynamically unstable form) to the stable form at aparticular temperature. They also exhibit different melting points, solubilities, and X-raydiffraction patterns.5.Metallographic SamplesThe surface of a metallographic specimen is prepared byvarious methods of grinding, polishing, and etching. After preparation, it is often analyzedusing optical or electron microscopy. Successively finer abrasive particles are used toremove material from the sample surface until the desired surface quality is achieved.Thespecimen is wet ground to reveal the surface of the metal. The specimen is successivelyground with finer and finer abrasive media. Typically it is polished with a slurry of alumina,silica, or diamond on a napless cloth to produce a scratch-free mirror finish, free fromsmear, drag, or pull-outs and with minimal deformation remaining from the preparationprocess. After polishing, certain microstructural constituents can be seen with themicroscope.6.Nonmetallic solidshave 4 main classes of crystals:

Metallic, Individual metal atoms of metallic crystals sit on lattice sites. This leaves the outer

electrons of these atoms free to float around the lattice. Metallic crystals tend to be very

dense and have high melting points.

Ionic, the atoms of ionic crystals are held together by electrostatic forces (ionic bonds). Ionic

crystals are hard and have relatively high melting points. This structure is based on the FCC

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lattice, but there are two atoms per lattice point, one centered on the point and another at

distance away along the edge of the FCC unit cell.

Covalent, in which the atoms are bound together with chemical covalent bonds; diamond;

covalent crystals has true covalent bonds between all of the atoms in the crystal.

Molecular, which includes those crystals made up of chemically saturated molecules

held together by Van der Waals forces. Molecular crystals tend to be soft with relatively low

melting points.

10.Strength-The ability of a material to stand up to forces being applied without it bending,breaking, shattering or deforming in any way. Elasticity-The ability of a material to absorbforce and flex in different directions, returning to its original position. Toughness- Acharacteristic of a material that does not break or shatter when receiving a blow or under asudden shock. Hardness- The ability of a material to resist scratching, wear and tear andindentation.