AEM 338 Engineered Materials TestingIntroduction to Materials Technology

Sergio SgroEastern Kentucky University

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Lecture ObjectivesState reasons for studying materialsList and describe common terms related to the study of materialsDescribe and define terms and conditions associated with atomic structure and atomic theoryRecognize and describe how the periodic table of elements is used and the structure of the tableList and describe how the various bonding forces act to hold atoms togetherDefine the various structures of materials in crystal lattice formationDescribe the solidification processList and recognize various organizations and their purposes.

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1.1 IntroductionMaterials TechnologySpecifications, properties, selection, and testing of engineering materials

Engineering MaterialsPrimarily used in construction of various structures, machines, etc.

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1.2 Atomic theoryWhy study engineering materialsTo understand the characteristics of materials and their structuresCategories (ways to classify)Chemical compositionMaterials natural stateManufacturing/refining to bring to useful stateAtomic structure

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1.2 Atomic theoryClassification into one of the following:Elements

Compounds

mixtures

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1.2 Atomic theoryBy studying elements, scientists discoveredRepetitive patternsAllowed them to predict nature and properties of elements not discovered until much later

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1.2 Atomic Theory

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1.2 Atomic theoryAn atom consists of

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1.2 Atomic theoryThe number of PROTONS in the nucleus determines what element the atom represents

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1.2 Atomic theoryAtoms are held together in molecules by various types of bondsPrimary bonds - generally associated with formation of moleculesSecondary bonds - generally associated with attraction between moleculesPrimary bonds are much stronger than secondary bonds

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1.2 Atomic theoryPRIMARY BONDSCharacterized by strong atomtoatom attractions that involve exchange of valence electrons Following forms: IonicCovalentMetallic

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1.2 Atomic theory: Ionic BondingAtoms of one element give up their outer electron(s), which are in turn attracted to atoms of some other element to increase electron count in the outermost shell to eight

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1.2 Atomic theory: Covalent BondingElectrons are shared (as opposed to transferred) between atoms in their outermost shells to achieve a stable set of eight

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1.2 Atomic theory: Metallic BondingSharing of outer shell electrons by all atoms to form a general electron cloud that permeates the entire block

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1.2 Atomic theory: Secondary BondsWhereas primary bonds involve atomtoatom attractive forces, secondary bonds involve attraction forces between molecules No transfer or sharing of electrons in secondary bondingBonds are weaker than primary bondsThree forms: Dipole forcesLondon forcesHydrogen bonding

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1.2 Atomic theory: Dipole ForcesArise in a molecule comprised of two atoms with equal and opposite electrical chargesEach molecule therefore forms a dipole that attracts other molecules

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1.2 Atomic theory: London ForcesAttractive force between nonpolar molecules, i.e., atoms in molecule do not form dipolesHowever, due to rapid motion of electrons in orbit, temporary dipoles form when more electrons are on one side

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1.2 Atomic theory: Hydrogen BondingOccurs in molecules containing hydrogen atoms covalently bonded to another atom (e.g., H2O)Since electrons to complete shell of hydrogen atom are aligned on one side of nucleus, opposite side has a net positive charge that attracts electrons in other molecules

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1.2 Atomic theory: Macroscopic Structures of MatterAtoms and molecules are the building blocks of more macroscopic structure of matterWhen materials solidify from the molten state, they tend to close ranks and pack tightly, arranging themselves into one of two structures: CrystallineNoncrystalline

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1.4 Crystalline structuresCrystalline structuresSimple, body-centered cubic (bcc), face-centered cubic (fcc), close-packed hexagonal

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1.2 Atomic theory: Crystalline StructureStructure in which atoms are located at regular and recurring positions in three dimensions Unit cell - basic geometric grouping of atoms that is repeated The pattern may be replicated millions of times within a given crystal Characteristic structure of virtually all metals, as well as many ceramics and some polymers

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Progression of crystal structuresIllustration of the BCC

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Metal latticesSome metals undergo a change in structure at different temperaturesIRONBCC at room temperatureChanges to FCC 1674 F (912 C)Back to BCC above 2550 F (1400 C)

Referred to as ALLOTROPIC or polymorphs

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Supercooling processMetals in liquid state (VERY HOT)As heat decreasesPrimary and secondary forces develop in distinct patterns characteristic for that material (lattices) this is called the FREEZING POINTLattice formation creates heat, grows until stopped by another lattice or containerWhere lattice structures collide, it is a grain boundary GRAIN BOUNDARY: Bounding surfaces between crystals

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1.5 Specification of materialWhat and why?Clear and accurate descriptions of technical requirements of materials, products, or services. They may state requirements for quality, use of material and methods to produce a desired product, system, application, or finish

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1.6 standardsWhat are they?A consortium or group of people defining common definitions and proceduresOnly work when people adopt and use them properlyASTM standardsAmerican Society for Testing Materials (ASTM)

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REVIEWQuestions?

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