Topic 2: Nature of Materials PDF

Topic 2: Nature of Materials ME 380  Nature of Materials Major topics: 1.Atomic structure and the arrangement of the elements? what type of 2.Types of interatomic bonding material corresports what type...

Topic 2: Nature of Materials ME 380  Nature of Materials Major topics: 1.Atomic structure and the arrangement of the elements? what type of 2.Types of interatomic bonding material corresports what type of 3.Ordering of atoms and ions Material ? to Nature of Materials Prof. Abiade 2  Nature of Materials Manufacturing is a process that transforms materials The ability to predict the behavior of a material when force, heat, etc. are applied determines success of operation Material properties related to: Electronic configuration Atomic/molecular bonding Atomic/molecular structure (or lack thereof) Nature of Materials Prof. Abiade 3  Atomic models-FYI I willasked not on quit be Many phenomena involving electrons in solids could not be explained in terms of classical mechanics. specific energy each From quantum mechanics we know electronic energies are quantized and energy states are discrete. the electron G assumes M Early attempt to describe atoms in terms of both L position (electron orbital) and energy (quantized energy levels). K Assumed electron revolve around the atomic nucleus in discrete orbitals. Position of any particular electron is more or less well defined in terms of its orbital. Bohr atomic model ↓ Not really accurate, but a good it war of depicting Nature of Materials Prof. Abiade 4  Electron Energy States-FYI Electrons... have discrete energy states tend to occupy lowest available energy state. 4d 4p N-shell n = 4 · 3d 4s Energy 3p M-shell n = 3 3s Adapted from Fig. 2.4, Callister & Rethwisch 3e. 50 2p can 2s accomodate L-shell n = 2 3 selections 3 electrons Nature of Materials 1s - can accommodate 2 elections Prof. Abiade K-shell n = 1 3-2 5  SURVEY OF ELEMENTS-FYI Electron configuration represents manner in which electron levels are filled. Element Atomic # Hydrogen 1 Electron configuration 1s 1 shell completed y Helium 2 1s 2 (stable) Lithium 3 1s 2 2s 1 Beryllium 4 1s 2 2s 2 Adapted from Table 2.2, Boron 5 1s 2 2s 2 2p 1 Callister & Rethwisch 3e. Carbon 6 1s 2 2s 2 2p 2...... Neon 10 1s 2 2s 2 2p 6 (stable) Sodium 11 1s 2 2s 2 2p 6 3s 1 Magnesium 12 1s 2 2s 2 2p 6 3s 2 Aluminum 13 1s 2 2s 2 2p 6 3s 2 3p 1...... Argon 18 1s 2 2s 2 2p 6 3s 2 3p 6 (stable) ↓ if interested......... Look up Krypton 36 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 (stable) completely a Electors in the outer shell Valence - that is not Nature of Materials Prof. Abiade 6  Valence electrons – those in unfilled shells Filled shells more stable Valence electrons are most available for bonding and tend to control the chemical properties example: C (atomic number = 6) 1s2 2s2 2p2 Carbon has ↓ 4 Valence electrons Nature of Materials Prof. Abiade 7  Valence electrons – those in unfilled shells Filled shells more stable Valence electrons are most available for bonding and tend to control the chemical properties example: C (atomic number = 6) 1s2 2s2 2p2 valence electrons Nature of Materials Prof. Abiade 7  Atomic Structure Valence electrons determine all of the following properties 1) Chemical 2) Electrical 3) Thermal 4) Optical 5) Mechanical Nature of Materials Prof. Abiade 8  Periodic Table Nature of Materials Prof. Abiade 9  The Periodic Table-FYI Iceramics clements Form from from these two Elements arranged in increasing atomic number in 7 horizontal rows (periods). All elements in given column (group) have similar valencies and chem. & phys. properties. Generally electronegativity increases from left to right and bottom left to top right Group IA - alkali metals 1 excess electron Group IIA – alkaline earth metals 2 excess electrons Group IIIB – IIB transition metals, incomplete d shells Group IIIA – VA intermediate by virtue of electronic structures (some are semiconductors, Si) Group VI A – 2 electrons deficient from stable configuration Group VII – 1 electron deficient Group 0 – inert gases Nature of Materials Prof. Abiade 10  Atomic Bonding in Solids Nature of Materials Prof. Abiade 11  Primary Bonding Nature of Materials Prof. Abiade 12  Ionic bond – metal + nonmetal - Generally ceramics donates accepts electrons electrons is often Although there some sort of as Valence bonding well in ceramics Dissimilar electronegativities & Tendency of an an atom electron ↳> to give up ex: MgO Mg 1s2 2s2 2p6 3s2 O 1s2 2s2 2p4 [Ne] 3s2 Nature of Materials Prof. Abiade 13  Ionic bond – metal + nonmetal donates accepts electrons electrons Dissimilar electronegativities ex: MgO Mg 1s2 2s2 2p6 3s2 O 1s2 2s2 2p4 [Ne] 3s2 Nature of Materials Prof. Abiade 13  Ionic bond – metal + nonmetal donates accepts electrons electrons Dissimilar electronegativities ex: MgO Mg 1s2 2s2 2p6 3s2 O 1s2 2s2 2p4 [Ne] 3s2 Mg2+ 1s2 2s2 2p6 O2- 1s2 2s2 2p6 Nature of Materials Prof. Abiade 13  Ionic bond – metal + nonmetal donates accepts electrons electrons Dissimilar electronegativities ex: MgO Mg 1s2 2s2 2p6 3s2 O 1s2 2s2 2p4 [Ne] 3s2 -Mg2+ 1s2 2s2 2p6 [Ne] O2- 1s2 2s2 2p6 [Ne] Nature of Materials Prof. Abiade 13  Ionic Bonding Occurs between + and - ions. d Powered by Requires electron transfer. Attraction - Columbing Large difference in electronegativity required. of Example: NaCl In the process electron transfer both elements Na (metal) Cl (nonmetal) become charged unstable unstable electron Na (cation) stable + - Cl (anion) Coulombic stable Attraction I Dissimilar Nature of Materials Prof. Abiade charges 14  Covalent Bonding similar electronegativity ∴ share electrons ↓ sharing bonds determined by valence – s & p orbitals dominate bonding electrons Example: CH4 ↓ Generally of the type shared electrons for H bunding from carbon atom C: has 4 valence e -, CH 4 Polymers. needs 4 more ↓ But metals 3) ceramics H: has 1 valence e-, H C H can also have needs 1 more covalent shared electrons banding Electronegativities H from hydrogen are comparable. atoms Nature of Materials Prof. Abiade 15  Metallic Bonding Metallic Bond: I Generally Delocalized as electron cloud used Results in good electrical and thermal conductivity by metals Nature of Materials Prof. Abiade 16  SECONDARY BONDING - Primary Bunding is Arises from interaction between dipoles much stronger Fluctuating dipoles (London forces) ex: liquid H2 - Secondary Bonding Atums asymmetric electron H 2 H 2 occurs over Shorter distances clouds ↓ + - + - H H H H secondary secondary weaken bonding bonding r Y ( Molecules Permanent dipoles-molecule induced + - secondary + - -general case: S bonding Adapted from Fig. 2.14, Callister & Rethwisch 3e. H Cl secondary H Cl -ex: liquid HCl bonding Stronger ? - > covalent bunding secon Within the chan -ex: polymer dary secondarysecondary bonding bond ing Nature of Materials bonding Prof. Abiade across chains 17  Hydrogen Bonding Occurs in molecules containing hydrogen atoms covalently bonded to another atom (e.g., H2O) - occurrs across molecules Since electrons to complete shell of hydrogen atom - occurs at shorter are aligned on one side of nucleus, opposite side range has a net positive charge that attracts electrons in other molecules Nature of Materials Prof. Abiade 18  Ordering of Atoms and molecules Yons have change Atoms and molecules are the building - a ~ Atoms are neutral blocks of matter When materials solidify from the molten state, they tend to aggregate tightly, arranging themselves into one of two forms: Crystalline Noncrystalline Nature of Materials Prof. Abiade 19  Crystalline Structure ↳ las long range or den Manner 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 - Pretty much All metals are crystalline out so are many ceramics - Polymers are almost never 100 % crystalline Nature of Materials Prof. Abiade 20  Three Crystal Structures in Metals Three types of crystal structure: (a) body- centered cubic, (b) face-centered cubic, and (c) hexagonal close-packed Magnesium Zinc Aluminum Juctile more Iron Copper - then BCC Matinum materials atoms al each & cunerner ) & in the recken -2abousell atoms on corner and in thecenter of every Pace in the one aton & likely to more center and "slip" than BC Nature of Materials Prof. Abiade materials 21 Crystal Structures for Common Metals Room temperature crystal structures for some of the common metals: Body‑centered cubic (BCC) Z Chromium, Iron, Molybdenum, Tungsten Face‑centered cubic (FCC) Aluminum, Copper, Gold, Lead, Silver, Nickel Hexagonal close‑packed (HCP) Magnesium, Titanium, Zinc Nature of Materials Prof. Abiade 22  Imperfections (Defects) in Crystals Nearly all engineering materials possess defects Defects are often introduced during solidification Imperfections can also be introduced purposely; e.g., addition of alloying ingredient in metal defects, (3)C & Types of defects: (1) point defects, Flora (2) line surface (interfacial) defects few atonic defects E FCC can materials undergo Juctile are more more B & plastic deformation da lineen-up Nature of Materials Prof. Abiade 23  Point Defects - anions are generally Imperfections in crystal structure involving either a cations then larger single atom or a small number of atoms cation vacancy + interstitial defect Basic Interstitial O Vacancy - > schoftly Defect Ratios of Interstitial) - - anions v ↓ defect than is smaller host atoms & missing cations - atom o 00 - Point Defects in O- occur ceramics Mostly appear in ceramics in pairs (in order electrically Point defects: (a) vacancy, (b) ion-pair vacancy (Schottky to remain neutral) Defect), (c) interstitial, (d) displaced ion (Frenkel Defect). Nature of Materials Prof. Abiade 24  Line Defects - ID defects Connected group of point defects that forms a line in the lattice structure Most important line defect is a dislocation, which can take two forms: of A connected line Edge dislocation - defina Be able point defects to S - Screw dislocation 3) discuss what if An extra half plane of has to to with - atums Strengthening of metal requires metals - strengthening inhibiting dislocations ↓ Quiz Nature of Materials Prof. Abiade clue 25  Edge Dislocation Edge of an extra plane of atoms that exists in the lattice Nature of Materials Prof. Abiade 26  Screw Dislocation puto imperfection - stain Spiral within the lattice - Defects something structure wrapped interrupts around an that order the regular imperfection line, like of the atoms a screw is wrapped around its axis Nature of Materials Prof. Abiade 27  Interfacial or Surface Defects Imperfections that extend in two directions to form a boundary Examples: External: the surface of a crystalline object is an interruption in the lattice structure Internal: grain boundaries are internal surface - interruptions ↓ change in atomic from one another order to Nature of Materials Prof. Abiade 28  Polycrystalline Nature of Metals A block of metal may contain millions of individual crystals, called grains Such a structure is called polycrystalline Each grain has its own unique lattice orientation But collectively, the grains are randomly oriented in the block crystalline & -- polycrystilline single ↓ one crystalline ↓ Nature of Materials Prof. Abiade structure 29 Grains and Grain Boundaries in Materials molten metal ↓ ↓ temp decreasing Eachgrahanbea have O Temp ↓ g O recreasing Most engineering materials are polycrystalline. Each "grain" is a single crystal. If grains are randomly oriented, overall component properties are not directional. Grain sizes typ. range from 1 nm to 2 cm (i.e., from a few to millions of atomic layers). Nature of Materials Prof. Abiade 30  Single vs. Polycrystals & Single Crystals E (diagonal) = 273 GPa ↓ Data from Table 3.7, -Properties vary with Callister & Rethwisch 3e direction: anisotropic. -Example: the modulus Differ of elasticity (E) in BCC iron: For single crystals E (edge) = 125 GPa Nature of Materials Prof. Abiade 31  Single vs. Polycrystals Single Crystals E (diagonal) = 273 GPa Data from Table 3.7, -Properties vary with Callister & Rethwisch 3e direction: anisotropic. -Example: the modulus of elasticity (E) in BCC iron: E (edge) = 125 GPa Polycrystals 200 µm random orientation-Properties may/may not Adapted from Fig. 5.19(b), Callister & leads to isotropy vary with direction. Rethwisch 3e. (Fig. 5.19). -If grains are randomly - structure of Y oriented: isotropic. materials determine (Epoly iron = 210 GPa) properties -If grains are textured, anisotropic. Nature of Materials Prof. Abiade 31  20 nm Electron microscopy image showing grain boundaries 5 nm Nature of Materials Prof. Abiade 32  Polymorphism - changing ↓ structure change properties Two or more distinct crystal structures for the same material (allotropy/polymorphism) iron system titanium liquid α, β-Ti 1538ºC BCC δ-Fe carbon 1394ºC diamond, graphite FCC γ-Fe 912ºC BCC α-Fe Nature of Materials Prof. Abiade 33  Noncrystalline (Amorphous) Structures ↓ No long range structure/order/shape but be amorphous metals are mostly polymorphous , can when it is Liquid Amorphous materials do not have long-range atomic/ molecular order A metal loses its crystalline structure when melted Some engineering materials are amorphous in their solid states refer to either 5:82 or Glass can Glass non-crystalline amorphous , Many plastics Rubber Nature of Materials Prof. Abiade 34  Features of Amorphous Materials Two features differentiate noncrystalline (amorphous) from crystalline materials: 1. Absence of long‑range order in molecular structure 2. Differences in melting and thermal expansion characteristics Nature of Materials Prof. Abiade 35  Crystalline vs. Amorphous materials Difference in structure between: (a) crystalline and (b) noncrystalline materials Crystal structure is regular, repeating; amorphous materials are less tightly packed and randomly oriented crystalline in Thermal Differences -. Expansion & A certain point where the material becomes plasticy ↓ - (a) (b) - Glass > transition Temp Nature of Materials Prof. Abiade 36