An Introduction to Ferrous and Non-Ferrous Alloys, Ceramics, and Polymers
Chapter 11: Ferrous Alloys
Iron-Based Principle Constituent
Ferrous alloys are iron-based and include cast irons and steels.
Advantages of Ferrous Alloys
- Cost-effective
- Diverse properties achievable by adding vacancy impurities
Types of Steel
Low Carbon Steels
- Less than 0.25 wt% carbon
- Strengthening is accomplished by cold work
- Microstructure consists of ferrite & pearlite (alpha & Fe3C)
- Relatively soft & weak but ductile & tough
- Machinable, weldable, least expensive to produce
HSLA – High Strength Low Alloy
- Contains other elements such as Cu, Vanadium, Ni, Molybdenum as high as 10%wt
- More resistant to corrosion than plain carbon steels
Medium Carbon Steels
- C: 0.25% to 0.60wt%
- Heat (austenitizing) ferrite to austenite
- Quenching
- Tempering (improve hardness then QL)
High Carbon Steels
- About 0.6-1.4%wtC
- Stronger but not as ductile
- Add Cr, Ni, Mo to improve strength & capacity to be heat treated
- High strength structure, railways, wheels, tracks
High Carbon Steels
- About 0.6-1.4wt%
- As carbon increases, harder & stronger but lose ductility & flexibility
- Hardest, strongest, least ductile
- Always used in a hardened tempered condition
Stainless Steel
- Highly resistant to corrosion (rusting)
- At least 11% wt Cr
- Can be enhanced with Ni & Mo
Types of Stainless Steel
- Martensitic stainless steel: capable of being heat treated
- Austenitic & Ferritic stainless steel: strengthened by cold work
- Austenitic stainless steel: most corrosion-resistant because of Cr & Ni
- Martensitic & Ferritic stainless steel: are magnetic but austenitic is not
Cast Iron
- Generally 3-4.5wt%C in addition to other alloying elements
- Can be easily melted & made from casting
- Melting pt 1150-1300C
- Fe3C → 3Fe(alpha)+graphite (decompose)
Types of Cast Iron
- Gray Iron
- Varies. C%: 2.5-4.0wt%. Si%: 1-3wt%.
- C exists as graphite surrounded by ferrite or pearlite matrix
- Relatively weak & brittle
- Ductile Iron
- Add Mg or Ce into gray iron before casting graphite
- Stronger & more ductile than gray iron
- White Iron & Malleable Iron
- Less than 1 wt% silicon
- Fracture surface is white
- Extremely hard & brittle
- Almost nonmachinable
- Generally used as an intermediary in the production
- Compacted Graphite Iron (CGI)
- C exists as graphite. 3.1-4wt%. Si 1.7-3wt%.
- Less than 20% spherical particles of C
- Mg or Ce also added
- Matrix is alpha or pearlite, tensile or yield strengths are similar to ductile iron
- Ductility between gray & ductile irons
- High thermal conductivity
- Better resistance to thermal shock (different parts expand at different rates & rapid temperature change), lower oxidation rate at high T
Disadvantages of Ferrous Alloys
- Heavy
- Low electrical conductivity
- Corrosion
Non-Ferrous Alloys
Non-ferrous alloys are not based on iron.
Types of Non-Ferrous Alloys
- Copper Alloys: Very conductive. Cu-Zn brass alloy. Bronze- Cu-Sn, Al, Si, Ni.
- Aluminum Alloys: Light, strong. In ambient atmosphere Al2O3 thin layer. Good conductivity.
- Magnesium Alloy: Light & strong. Lowest density of all.
- Titanium Alloy: Very strong. Not too heavy. Used in aerospace. In ambient environment, TiO2 thin layer is formed at the surface (dense, resistant to corrosion). Self-healing property. Chemically stable. Used in paint, toothpaste, cosmetics, solar cells, photocatalyst.
- Refractory Metals: Metals that can take high temperatures. Stable at high T. W, Niobium, Mo, Tantalum.
- Superalloys: Extreme properties. Noble alloys (very stable & expensive). As, Au, Pt, Pd used a lot in reactors & plants.
- Nickel Alloys: Strong, resistant to corrosion. Nuclear plants.
- Lead Alloys: Protection against x-ray. Pb: toxic heavy metal.
Processing (Fabrication)
- Forming Operations aka Hot Working: At elevated temperature below melting. Apply stress in order to change the shape of metal.
- Cold Working: At room temperature apply stress to change the shape.
- Casting: Melt the metal or alloy & pour the melt into the mold.
- Other Techniques:
- Powder Metallurgy: Lots of metal powders- press into desired shape.
- Welding: Heat & join two metal pieces together.
- 3-D Printing: Additive manufacturing. Only metals & polymers, cannot do ceramics. Works layer by layer.
- Thermal Treatment: Most economical of all- the finished shape is too big or complicated. Ductility is improved with thermal treatment.
Chapter 12: Structure & Properties of Ceramics
Types of Ceramics
- Traditional: Brick, pottery, glass
- New Ceramics: Used in electronics, aerospace, biomaterials, semiconductors
Ceramic Structure
- Cations & anions. Cation is smaller than anion.
- Stable ceramics come from when a cation is touching an anion. Unstable if stuck in between & no contact.
- Crystal structure determined from magnitude of electron charge & the relative size of anion & cation.
- Coordination # is # of cations surrounding one anion.
Examples of Ceramic Structures
- Rock Salt – NaCl: Both FCC & fit inside each other. Sodium at the center of the edge.
- KCl: Rock salt structure
- Silica Ceramics: Quartz-silica (SiO2) crystalline more expensive
- Glass-Silicas: Non-crystalline. SiO2 most abundant elements on earth. Sand comes from Si. Silica ceramics not expensive. Silicas are tetrahedral. Have network structures in the form of tetrahedral. Sodium silicates glass (doped)
- Carbon Polymorphic Forms: Diamond, graphite, & fullerenes (bucky balls). FCC has highest APF 0.74. Zinc blende consists of ZnS.
- Diamond: Has even more APF than 0.74. In addition to FCC another four atoms inside. FCC has four inside. Another four so eight total atoms. APF is larger than 0.74 or HCP. All covalently blended, hard, insulating. Coordination # is four.
- Graphite: Layered structure. Hexagons all connected by pi bonding.
- Bucky Balls: Discovered by Professor Smalley. C60 shaped like a soccer ball. Can take many forms, all these have different properties. Crystalline structure, pure crystalline solid.
- C Nanotubes: Graphene prepared with scotch tape & thin layer peeled off, very stiff. Nanotechnology, TV.
Defects in Ceramics
- A Frenkel defect occurs when one cation (small) moves to an interstitial. Overall charge is the same.
- Schottky defect when one cation & anion missing. For NaCl one Na missing one Cl missing. CaCl2, one Ca missing two Cl vacancy. Al2O3 every one Al 3/2 oxygen.
Mechanical Properties of Ceramics
- Brittle. Fracture before plastic deformation.
- KIC is the fracture toughness. Y is a parameter dependent of specimen & crack geometry. Sigma- applied stress. a- the length of a surface crack or half the length of the crack inside the specimen.
- Morphology of ceramics + optical electron microscopes. Flexural strength is performed by bending test because ceramic is brittle. Bend back & forth till it breaks.
- Porosity- many ceramics are porous. P up then decrease strength & toughness.
- Ceramics suffer from creep at elevated temperature.
Chapter 13: Application of Ceramics
Ceramics are a complement of metals & polymers.
Types of Ceramics
- The advanced ones are biomaterials & smart
- Glass Ceramics: Non-crystalline material state → heat into glass-ceramic. Crystallization. Makes it stronger. A nucleating agent (TiO2) is added to promote the state.
- Advantages: Strong, low thermal expansion & high-temperature capability. Good dielectric material properties (insulating). Better biological capabilities (replace teeth, etc)
- Clay Products:
- Structural clay product: brick, tile
- White wares: pottery: china
- Advantage of Clay Products: Abundant, low cost
- Refractories: The ceramics that can take high temperatures. Remain stable under extreme conditions when exposed to a severe environment. Usually composed of fire clay, silica (quartz).
- Abrasives: Used to cut/grind other materials (that are softer). Diamond is the hardest material but expensive. WC (tungsten carbide) SiC (silicon carbide) Al2O3 aluminum.
- Cement, Lime: When mixed with water they form a paste that subsequently sets & hardens.
- Advanced Ceramics: Electrical, magnetic, optical, biological applications. Optical fiber- high purity silica SiO2. Data storage units. Energy devices, electronic displays.
Fabrication & Processing of Ceramics
- For glass: melting point, working point, softening point, annealing point, strain point. As T goes up go up this spectrum viscosity increases. Glass transition temperature.
- Hydroplastic Forming: When clay mixed with water super soft & low yield strength. Becomes paste.
- Clay Alumino Silicates: Composed of Al2O3, SiO2 chemically bonded with H2O. Written as (Al2O3)m(SiO2)n x H2O
- Powder pressing into a dense mass. Uniaxial is → ←. Isostatic is from all directions. Hot pressing is one direction with heat.
Chapter 14: Polymer Structures
Introduction
Wood, silk, rubber, cotton, plastic, leather.
Hydrocarbon Molecules
- Triple bonds are more active than double & single.
- Isomersion- same composition but different structure. C4H10- isobutane, butane. Butane stronger. Van Der Waals- less sterically hindered so higher boiling pt.
Polymer Molecules
- Macromolecules. Structural or repeating units are monomers because CH2-CH2 repeats.
- R-OH alcohol. R-O-R ether. Carboxylic acid.
- The chemistry of polymer molecules are repeating carbon units. C-C-C-C-C. Addition of each opens a double bond.
- Polyethylene (PE) is the most common plastic. There’s also poly (vinyl chloride) PVC & polypropylene.
Types of Polymers
- Homopolymer: All the repeat units are the same
- Copolymer: Two or more than two repeating units
Molecular Weight of Polymers
- Average molecular weight. Mn = XiMi
- Mw=WiMiXi is the fraction of the total # of polymer chains within the corresponding size range. Mi is the mean molecular weight of size range. Wi is the weight fraction.
- Degree of Polymerization (DP) average # of repeat units in a chain. Mn/m. m is the molecular weight of the repeat unit.
- When Mw goes up, Van Der Waals goes up & the melting & boiling pts go up. Larger macromolecules are solid at room temp.
Molecular Shape
- Simplified- linear chains. The realistic version is zigzags. Intertwining & entanglement bending & twisting. Coiling kinks. However, double bonds restrict rotation. Bulky group restricts movement.
Molecular Structure
- Linear polymer, branched, cross-linked, network (ladder structure)
Configuration
- Isotactic configuration & syndiotactic, atactic, & isotactic configuration (draw into sheet). Cis config- same sides. Trans- opposite.
Types of Polymers Based on Thermal Behavior
- Thermoplastic Polymers: Becomes pliable upon heat. Hardens as T goes down.
- Thermosetting Polymer: Does not soften upon heat.
Copolymers
- Two or more than two repeat. 1. Random. 2. Alternating. 3. Graft 4. Block- clusters of the same repeating unit.
Crystallinity in Polymers
- Polymers are often semi-crystalline
- Percentage of Crystallinity:
- Pc: density of crystalline part. Pa: density of amorphous part. Ps: the specimen to be determined.
- Larger, complex macromolecules → amorphous. Linear→ crystalline. Branched → less crystalline.
- Alternating block copolymers→ crystalline. Random or graft is amorphous
- Semicrystalline Polymer: Spherulite structure ribbon-like chain folded lamellar crystalline. The amorphous & crystalline parts together form a spherical shape.
Defects in Polymers
- Different from ceramics- vacancy, impurity, chain ends, dangling & loose chains, dislocations.
Diffusion in Polymers
- Interstitial type mechanism, small or gas molecules diffuse into the polymer. The lifetime of the polymer is short. Polymers in water swell & eventually destroyed.
Chapter 15. Characteristics, Applications, & Processing of Polymers
Mechanical Behavior
- Polymers have long chains that bend, twist & coil. Only tiny force uncurls it. For metals, the elongation 100% polymers more than 1000%. More sensitive to temperature. For semi-crystalline polymers, there is necking. During necking, the polymer chains get more straightened. Viscoelasticity (highly). An amorphous polymer may behave like a glass at low temperatures, a rubbery solid at intermediate temperatures [above the glass transition temperature (section 15.12)], & a viscous liquid as the temperature is further raised.
- Elastic deformation is instantaneous- reversible. When applying stress the strain is changing right away. Viscous: there is a delay in the deformation, not reversible. Viscoelastic: delayed, partially reversible (study this part). Results in the diffusion of atoms inside.
- Fracture of polymer is both brittle & ductile for thermoplastic. During fracture, covalent bonds break. “Crazing” there are voids & the bridge connects them creating a rip.
- Impact strength of polymer (also toughness of polymer) can use the Charpy test. Polymers: more brittle at low temperature. More ductile at high temperature. Usually have a narrow brittle to ductile transition. Hardness use Rockwell test.
- Also experience fatigue (more sensitive to dynamic load then metals).
- Tear Strength- something to pay attention to- energy to take apart the polymer. Crystal & amorphous part. At the elastic deformation, chains in amorphous parts align & are elongated. Stage 2- c….align till the chains in the crystalline part starts to stretch (no bonds broken). Stage 3- adjacent chains in the lamella part start to slide against one another & start to tilt toward the tensile axis of the force. Stage 4 the crystalline segments separate from amorphous. Stage 5- all chains align, the chains & blocks are aligned with the tensile axis. Degree of crystallinity & molecular weight. E is stronger if more weight. (read about this)
- Elastic Polymer- Elastomers: Amorphous, composed of cross-linked chains that are highly twisted, kinked, & coiled.
- Vulcanization: Cross-linking process in elastomers. Chemical process for converting natural rubber or related polymers into more durable materials by heating them with sulfur or other equivalent curatives or accelerators.
Applications of Polymers
- Coating, adhesives, foams. Advanced polymers- medical, energy, semiconductor.
Processing of Polymers
- Radical. Add R group with radical. Condensation is like Fischer esterification or dehydration & water comes out. Spin-spin into fiber from a viscous melt after heating.