Metals and Alloys

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Engineering Materials Four types: Metals, Ceramics, Polymers, Composites Metals: Properties satisfy a wide variety of design requirements. Manufacturing processes are well-developed. Engineers understand metals. Why Metals Are Important High stiffness and strength; can be alloyed for rigidity, strength, and hardness. Toughness (capacity to absorb energy) better than other materials. Good electrical and thermal conductivity. Cost-competitive (e.g., steel). Starting Forms of Metals in Manufacturing Cast metal: starting form is a casting. Wrought metal: metal has been worked or can be worked after casting. Powdered metal: very small powders converted into parts using powder metallurgy. Classification of Metals Ferrous: based on iron (Steels, Cast irons). Nonferrous: all other metals (e.g., Aluminum, Magnesium, Copper, Nickel, Titanium, Zinc, Lead, Tin, Molybdenum, Tungsten, Gold, Silver, Platinum). Superalloys Metals and Alloys Some metals are important as pure elements (e.g., gold, silver, copper). Most engineering applications require enhanced properties obtained by alloying. Alloying increases strength, hardness, and other properties compared to pure metals. Alloys Definition: A mixture or compound of two or more elements, at least one of which is metallic. Two main categories: Solid solutions Intermediate phases Solid Solutions An alloy where one element is dissolved in another to form a single-phase structure. Phase: Any homogeneous mass of material with the same crystal lattice structure. Solvent (base element) is metallic; dissolved element can be metallic or nonmetal. Two Forms: Substitutional: Atoms of solvent element replaced by dissolved element in unit cell. Interstitial: Atoms of dissolving element fit into vacant spaces between base metal atoms. Both forms generally result in stronger and harder alloys than component elements. Intermediate Phases Limits to solubility of one element in another. When dissolving element exceeds solid solubility limit, a second phase forms. Chemical composition is intermediate between the two pure elements. Crystalline structure differs from pure metals. Types: Metallic compounds: Metal and nonmetal (e.g., Fe$_3$C). Intermetallic compounds: Two metals form a compound (e.g., Mg$_2$Pb). Some two-phase alloys can be heat treated for much higher strength than solid solutions. Phase Diagrams Graphical representation of phases in a metal alloy system as a function of composition and temperature. Binary Phase Diagram: For an alloy system with two elements at atmospheric pressure. Composition on horizontal axis, temperature on vertical axis. Any point indicates overall composition and phases present at given temperature under equilibrium conditions. Copper-Nickel (Cu-Ni) Alloy System Solid solution alloy throughout entire range of compositions below the solidus. No intermediate solid phases. Mixture of phases (solid + liquid) in the region bounded by the solidus and liquidus. Chemical Compositions of Phases Overall alloy composition given by horizontal axis position. Compositions of liquid and solid phases differ. Draw a horizontal line at the temperature of interest. Intersections with solidus and liquidus indicate compositions of solid and liquid phases. Inverse Lever Rule Determines amounts of each phase present at a given temperature. Measure distances between aggregate composition and intersection points with liquidus ($L$) and solidus ($S$), identified as $C_L$ and $C_S$. Liquid phase proportion: $\frac{C_S}{C_S + C_L}$ Solid phase proportion: $\frac{C_L}{C_S + C_L}$ Applicable wherever two phases are present. Does not apply when only one phase is present (composition is aggregate composition). Tin-Lead (Sn-Pb) Alloy System Widely used in soldering. Includes two solid phases: alpha ($\alpha$) and beta ($\beta$). $\alpha$-phase: solid solution of tin in lead (left side of diagram). $\beta$-phase: solid solution of lead in tin (right side of diagram, around $200^\circ C$). Mixture of $\alpha + \beta$ lies between these solid solutions. Melting: Pure tin melts at $232^\circ C (449^\circ F)$. Pure lead melts at $327^\circ C (621^\circ F)$. Tin-lead alloys melt at lower temperatures. Two liquidus lines meet at $61.9\%$ Sn, which is the eutectic composition . Eutectic Alloy A composition where solidus and liquidus are at the same temperature. Eutectic temperature: Melting point of the eutectic composition, always the lowest melting point for an alloy system. Derived from Greek "eutektos" (easily melted). Ferrous Metals Based on iron, important engineering alloys of iron and carbon. Divide into two major groups: Steel and Cast Iron. Constitute $\sim 85\%$ of metal tonnage in the US. Iron-Carbon Phase Diagram Up to about $6\%$ carbon. Phases of Iron: Room temperature: alpha ($\alpha$) called ferrite (BCC). At $912^\circ C (1674^\circ F)$: ferrite transforms to gamma ($\gamma$) called austenite (FCC). At $1394^\circ C (2541^\circ F)$: transforms to delta ($\delta$) (BCC). Pure iron melts at $1539^\circ C (2802^\circ F)$. Iron as a Commercial Product Electrolytic iron: $\sim 99.99\%$ pure, for research. Ingot iron: $\sim 0.1\%$ impurities (including $0.01\%$ carbon), for high ductility or corrosion resistance. Wrought iron: $\sim 3\%$ slag, very little carbon, easily shaped in hot forming. Solubility Limits of Carbon in Iron Ferrite can dissolve $\sim 0.022\%$ carbon at $723^\circ C (1333^\circ F)$. Austenite can dissolve up to $\sim 2.1\%$ carbon at $1130^\circ C (2066^\circ F)$. Solubility difference provides opportunities for strengthening by heat treatment. Steel and Cast Iron Defined Steel: Iron-carbon alloy with $0.02\%$ to $2.1\%$ carbon. Cast iron: Iron-carbon alloy with $2.1\%$ to $\sim 4\%$ or $5\%$ carbon. Both can contain other alloying elements. Cementite in Iron-Carbon System At room temperature, iron-carbon alloys with carbon levels slightly above zero form a two-phase system. Second phase is Fe$_3$C, known as cementite . Cementite: Intermediate phase, metallic compound of iron and carbon, hard and brittle. Eutectic and Eutectoid Compositions Eutectic composition (Fe-C): $4.3\%$ C. Phase changes from solid ($\gamma$ + Fe$_3$C) to liquid at $1130^\circ C (2066^\circ F)$. Eutectoid composition (Fe-C): $0.77\%$ C. Phase changes from $\alpha$ to $\gamma$ above $723^\circ C (1333^\circ F)$. Below $0.77\%$ C: hypoeutectoid steels . From $0.77\%$ to $2.1\%$ C: hypereutectoid steels . Iron and Steel Production Iron making: Iron reduced from its ores. Steel making: Iron refined to obtain desired purity and composition (alloying). Iron Ores in Iron-making Principal ore: hematite (Fe$_2$O$_3$). Other ores: magnetite (Fe$_3$O$_4$), siderite (FeCO$_3$), limonite (Fe$_2$O$_3 \cdot$xH$_2$O). Iron ores contain $50\%$ to $70\%$ iron (hematite $\sim 70\%$). Scrap iron and steel widely used as raw materials. Other Raw Materials in Iron-making Coke (C): Supplies heat and produces carbon monoxide (CO) to reduce iron ore. Limestone (CaCO$_3$): Acts as a flux to remove impurities as slag. Hot gases (CO, H$_2$, CO$_2$, H$_2$O, N$_2$, O$_2$, fuels): Used to burn coke. Iron-making in a Blast Furnace Refractory-lined chamber (diameter $\sim 9-11$ m, height $\sim 40$ m). Charge of ore, coke, and limestone dropped into top. Hot gases forced into lower part for combustion and reduction of iron. Chemical Reactions in Iron-Making Using hematite (Fe$_2$O$_3$) as starting ore: Fe$_2$O$_3$ + CO $\rightarrow$ 2FeO + CO$_2$. CO$_2$ reacts with coke to form more CO: CO$_2$ + C (coke) $\rightarrow$ 2CO. Final reduction of FeO to iron: FeO + CO $\rightarrow$ Fe + CO$_2$. Proportions of Raw Materials in Iron-Making Approx. 7 tons of raw materials to produce 1 ton of iron: 2.0 tons of iron ore 1.0 ton of coke 0.5 ton of limestone 3.5 tons of gases Significant proportion of byproducts are recycled. Iron from the Blast Furnace Pig iron: Iron tapped from blast furnace, contains $>4\%$ C, plus impurities (Si, Mn, P, S). Further refinement required for cast iron and steel. Cupola furnace converts pig iron into gray cast iron. For steel, compositions are tightly controlled, impurities much lower. Steel-making Processes developed since mid-1800s to refine pig iron into steel. Two most important processes today: Basic oxygen furnace (BOF) Electric furnace Both produce carbon and alloy steels. Basic Oxygen Furnace (BOF) Accounts for $\sim 70\%$ of US steel production. Adaptation of Bessemer converter (used air to burn off impurities). BOF uses pure oxygen. Typical vessel: $\sim 5$ m diameter, processes $150-200$ tons per heat. Cycle time (tap-to-tap) $\sim 45$ min. Electric Arc Furnace Accounts for $\sim 30\%$ of US steel production. Scrap iron and steel are primary raw materials. Capacities: $25-100$ tons per heat. Complete melting: $\sim 2$ hr; tap-to-tap time: $4$ hr. Associated with alloy steels, tool steels, stainless steels. Better quality steel but higher cost per ton vs. BOF. Casting Processes in Steel-making Steels solidified for subsequent processing as ingots or by continuous casting. Casting of Ingots: Discrete castings, from $ Molds of high carbon iron, tapered for removal. Mold on a stool ; lifted after solidification, leaving casting. Continuous Casting: Widely applied in aluminum and copper production. Most noteworthy in steel-making. Dramatic productivity increases over ingot casting. Reduces solidification time by an order of magnitude. Steel Alloy of iron with $0.02\%$ to $2.11\%$ carbon by weight. Often includes other alloying elements: nickel, manganese, chromium, molybdenum. Grouped into four categories: Plain carbon steels Low alloy steels Stainless steels Tool steels Plain Carbon Steels Carbon is principal alloying element; small amounts of other elements ($\sim 0.5\%$ manganese normal). Strength increases with carbon content, but ductility reduces. High carbon steels can be heat treated to form martensite, making steel very hard and strong. Types: Low carbon steels ($ Automobile sheet metal, plate steel, railroad rails. Medium carbon steels ($0.20\%$ to $0.50\%$ C): Machinery components, engine parts (crankshafts, connecting rods). High carbon steels ($>0.50\%$ C): Springs, cutting tools, blades, wear-resistant parts. AISI-SAE Designation Scheme (Plain Carbon Steel) 4-digit number system: 10XX. 10 indicates plain carbon steel. XX indicates carbon $\%$ in hundredths of percentage points (e.g., 1020 steel has $0.20\%$ C). Low Alloy Steels Iron-carbon alloys with additional alloying elements totaling $ Superior mechanical properties to plain carbon steels for given applications. Higher strength, hardness, hot hardness, wear resistance, toughness, and desirable combinations. Heat treatment often required for improved properties. AISI-SAE Designation Scheme (Alloy Steel) 4-digit number system: YYXX. YY indicates alloying elements, XX indicates carbon $\%$ in hundredths of $\%$ points. Examples: 13XX - Manganese steel 20XX - Nickel steel 31XX - Nickel-chrome steel 40XX - Molybdenum steel 41XX - Chrome-molybdenum steel Stainless Steel (SS) Highly alloyed steels designed for corrosion resistance. Principal alloying element is chromium ($>15\%$). Cr forms a thin impervious oxide film protecting the surface. Nickel (Ni) increases corrosion protection in some SS. Carbon strengthens and hardens SS, but high C reduces corrosion protection (chromium carbide forms). Properties: good corrosion resistance, strength, ductility. Often difficult to work in manufacturing and more expensive than plain carbon or low alloy steels. Types (classified by predominant phase at ambient temperature): Austenitic stainless: Typical $18\%$ Cr and $8\%$ Ni. Ferritic stainless: $\sim 15-20\%$ Cr, low C, no Ni. Martensitic stainless: Up to $18\%$ Cr, no Ni, higher C than ferritic. Precipitation hardening stainless: Typical $17\%$ Cr and $7\%$ Ni, with Al, Cu, Ti, Mo. Duplex stainless: Mixture of austenite and ferrite in roughly equal amounts. Designation Scheme for Stainless Steels (AISI) Three-digit AISI numbering scheme. First digit indicates general type, last two digits specific grade. Examples: Type 302 - Austenitic SS ($18\%$ Cr, $8\%$ Ni, $2\%$ Mn, $0.15\%$ C) Type 430 - Ferritic SS ($17\%$ Cr, $0\%$ Ni, $1\%$ Mn, $0.12\%$ C) Type 440 - Martensitic SS ($17\%$ Cr, $0\%$ Ni, $1\%$ Mn, $0.65\%$ C) Tool Steels Highly alloyed steels for industrial cutting tools, dies, molds. Must possess high strength, hardness, hot hardness, wear resistance, toughness under impact. Heat treated. AISI Classification: T, M: High-speed tool steels (machining). H: Hot-working tool steels (forging, extrusion, die-casting). D: Cold-work tool steels (sheet metal pressworking, cold extrusion). W: Water-hardening tool steels (high carbon, little else). S: Shock-resistant tool steels (high toughness for punching, bending). P: Mold steels (for molding plastics and rubber). Cast Irons Iron alloys with $2.1\%$ to $\sim 4\%$ C and $1\%$ to $3\%$ Si. Composition makes them highly suitable as casting metals. Tonnage of cast iron castings exceeds all other cast metal parts (excluding steel ingots). Overall tonnage second only to steel. Types: Gray cast iron (most important). Ductile iron, white cast iron, malleable iron, alloy cast irons. Ductile and malleable irons have similar chemistries to gray and white, but special processing. Nonferrous Metals Metal elements and alloys not based on iron. Most important: Al, Cu, Mg, Ni, Ti, Zn. Not as strong as steels, but have corrosion resistance and/or strength-to-weight ratios competitive with steels in moderate to high stress applications. Many have properties (other than mechanical) making them ideal for applications unsuitable for steel. Aluminum and Magnesium Light metals, often specified for this feature. Abundant on Earth (Al on land, Mg in sea), but not easily extracted. Aluminum Production Principal ore: bauxite (hydrated aluminum oxide, Al$_2$O$_3$-H$_2$O). Extraction: Washing and crushing ore into fine powders. Bayer process: Conversion of bauxite into pure alumina (Al$_2$O$_3$). Electrolysis: Separation of alumina into aluminum and oxygen (O$_2$). Properties of Aluminum High electrical and thermal conductivity. Excellent corrosion resistance due to hard, thin oxide surface film. Very ductile, noted for formability. Pure Al has low strength, but can be alloyed and heat treated to compete with some steels (especially considering weight). Designation Scheme for Aluminum Four-digit code number to identify composition. Two designations for wrought vs. cast aluminums. Difference: decimal point after third digit for cast, no decimal for wrought. Properties influenced by work hardening and heat treatment; temper designated by hyphen. Temper treatments for strain hardening don't apply to cast alloys. Temper Designations: F: As fabricated (no special treatment). H: Strain hardened (wrought aluminums). O: Annealed to relieve strain hardening and improve ductility. T: Thermal treatment for stable tempers other than F, H, O. Magnesium and Its Alloys Lightest structural metals. Available in wrought and cast forms. Relatively easy to machine. Small particles (e.g., cutting chips) oxidize rapidly; care needed to avoid fire hazards. Magnesium Production Sea water contains $\sim 0.13\%$ MgCl$_2$ (source of commercial Mg). Extraction: Sea water mixed with milk of lime (Ca(OH)$_2$). Reaction precipitates magnesium hydroxide (Mg(OH)$_2$), which settles as a slurry. Slurry filtered to increase Mg(OH)$_2$ content. Slurry mixed with HCl to form concentrated MgCl$_2$. Electrolysis decomposes salt into Mg and chlorine gas (Cl$_2$). Mg cast into ingots for processing. Chlorine recycled to form more MgCl$_2$. Properties of Magnesium Pure Mg is relatively soft and lacks strength for most engineering applications. Can be alloyed and heat treated for strengths comparable to aluminum alloys. High strength-to-weight ratio is advantageous in aircraft and missile components. Designation Scheme for Magnesium Three-to-five character alphanumeric code. First two characters: letters identifying principal alloying elements (up to two). Followed by two-digit number indicating amounts of two alloying ingredients to nearest percent. Example: AZ63A ($6\%$ Aluminum, $3\%$ Zinc, $93\%$ Magnesium). Last symbol: letter for composition variation or chronological order of availability. Requires temper specification (same basic scheme as aluminum). Copper One of the oldest metals known to mankind. Low electrical resistivity; pure copper widely used as electrical conductor. Excellent thermal conductor. Noble metal (like gold, silver), so corrosion resistant. Copper Production Ancient times: available as free element. Today: extracted from ores like chalcopyrite (CuFeS$_2$). Process: Ore crushed, concentrated by flotation. Smelted (melted/fused with chemical reaction) to separate metal. Resulting copper is $98\%$ to $99\%$ pure. Electrolysis used for higher purity levels. Copper Alloys Pure copper has relatively low strength and hardness; often alloyed to improve. Bronze: Alloy of copper and tin (typical $\sim 90\%$ Cu, $10\%$ Sn), used since Bronze Age. Brass: Alloy of copper and zinc (typical $\sim 65\%$ Cu, $35\%$ Zn). Highest strength alloy: beryllium-copper ($\sim 2\%$ Be), heat treated for high strengths, used for springs. Designation Scheme for Copper (UNS) Unified Numbering System (UNS), five-digit number preceded by 'C'. Includes both wrought and cast copper and alloys. Examples: C10100 – $99.99\%$ pure copper C17000 – $98\%$ Cu, $1.7\%$ Be (beryllium-copper) C24000 – $80\%$ Cu, $20\%$ Zn (brass) C52100 – $92\%$ Cu, $8\%$ Sn (bronze) Nickel and Its Alloys Similar to iron: magnetic, modulus of elasticity $\approx E$ for iron and steel. Differences: Much more corrosion resistant (alloying element in SS, plating metal). High temperature properties of Ni alloys are superior. Nickel Production Ore: pentlandite ((Ni,Fe)$_9$S$_8$). Process: Ore crushed and ground with water. Flotation separates sulfides. Nickel sulfide heated to burn off sulfur, then smelted to remove iron and silicon. Further refinement for high-concentration nickel sulfide (NiS). Electrolysis recovers high-purity nickel from NiS. Nickel Alloys Commercially important for corrosion resistance and high temperature performance. Number of superalloys based on nickel. Applications: SS alloying ingredient, plating metal, high temperature/corrosion resistance. Titanium and Its Alloys Abundant in nature ($\sim 1\%$ of Earth's crust, Al is $\sim 8\%$). Density between aluminum and iron. Importance grown due to aerospace applications for lightweight and good strength-to-weight ratio. Titanium Production Principal ores: rutile ($98-99\%$ TiO$_2$) and ilmenite (FeO and TiO$_2$). Process (Kroll process): TiO$_2$ converted to titanium tetrachloride (TiCl$_4$) by reacting with chlorine gas. TiCl$_4$ reduced to metallic titanium by reaction with magnesium. Resulting metal used to cast ingots of titanium and its alloys. Properties of Titanium Relatively low coefficient of thermal expansion. Stiffer and stronger than Al. Retains good strength at elevated temperatures. Pure Ti is reactive, presenting processing problems, especially in molten state. At room temperature, forms thin adherent oxide coating (TiO$_2$) for excellent corrosion resistance. Applications of Titanium Commercially pure state: corrosion-resistant components (marine, prosthetic implants). Titanium alloys: high strength components at temperatures up to $550^\circ C (1000^\circ F)$, exploiting strength-to-weight ratio (aircraft, missile components). Alloying elements: aluminum, manganese, tin, vanadium. Zinc and Its Alloys Low melting point, attractive for casting (especially die casting). Provides corrosion protection when coated onto steel or iron. Galvanized steel refers to steel coated with zinc. Widely used as an alloy with copper (brass). Production of Zinc Principal ore: zinc blende or sphalerite (ZnS). Process: Ore concentrated by crushing, grinding with water to create slurry. Slurry agitated to float mineral particles, which are skimmed off. Concentrated ZnS roasted to form zinc oxide (ZnO). Zn liberated from ZnO by electrolysis or thermochemical reactions. Lead and Tin Often considered together due to low melting points and use in soldering alloys. Lead: Dense, low melting point, low strength, low hardness, high ductility, good corrosion resistance. Applications: solder, bearings, ammunition, type metals, x-ray shielding, storage batteries, vibration damping. Tin: Lower melting point than lead, low strength, low hardness, good ductility. Applications: solder, bronze, "tin cans" for food storage. Refractory Metals Metals capable of enduring high temperatures, maintaining high strength and hardness. Most important: Molybdenum, Tungsten. Other: Columbium, Tantalum. Molybdenum Properties: high melting point, stiff, strong, good high temperature strength. Used as pure metal ($99.9+\%$ Mo) and alloyed. Applications: heat shields, heating elements, electrodes for resistance welding, dies for high temperature work (die casting molds), rocket/jet engine parts. Alloying ingredient in steels (e.g., high speed steel) and superalloys. Tungsten Properties: highest melting point among metals, densest, stiffest (highest modulus of elasticity), hardest of all pure metals. Applications: high operating temperatures (filament wire in light bulbs, rocket/jet engine parts, arc welding electrodes). Alloying element in tool steels, heat resistant alloys, tungsten carbide. Precious Metals Gold, platinum, and silver. Called noble metals because chemically inert. Available in limited supplies. Used for coinage and to underwrite paper currency. Widely used in jewelry and other applications exploiting high value. Properties: high density, good ductility, high electrical conductivity, corrosion resistance, moderate melting temperatures. Superalloys High-performance alloys for strength and resistance to surface degradation at high service temperatures. Contain substantial amounts of three or more metals. Commercially important due to high cost and unique properties. Importance: Room temperature strength properties are good. Excellent high temperature performance: tensile strength, hot hardness, creep resistance, corrosion resistance at very elevated temperatures. Operating temperatures often $\sim 1100^\circ C (2000^\circ F)$. Applications: gas turbines (jet/rocket engines), steam turbines, nuclear power plants (efficiency increases with higher temperatures). Three Groups: Iron-based: Iron $ Nickel-based: Better high temperature strength than alloy steels, alloyed with Cr, Co, Fe, Mo, Ti. Cobalt-based: $\sim 40\%$ Co, $\sim 20\%$ Cr, alloyed with Ni, Mo, W. Strengthening in virtually all superalloys is by precipitation hardening. Manufacturing Processes for Metals Shaped by basic processes: casting, powder metallurgy, deformation, material removal. Joined by welding, brazing, soldering, mechanical fastening. Heat treating enhances properties. Finishing processes (e.g., electroplating, painting) improve appearance and/or provide corrosion protection. How to Enhance Mechanical Properties Alloying: Increases strength of metals. Cold working: Strain hardening during deformation increases strength (reduces ductility). Strengthening is a byproduct of forming operation. Heat treatment: Heating and cooling cycles alter microstructure to beneficially change mechanical properties. Operates by altering metal's microstructure, which determines properties.