Lecture 15 - Ferrous Alloys
Describe the differences between steel types in terms of carbon.
What are the limitations of ferrous alloys? Why?
Decarburized steels have <0.005% Carbon. Carbon steels have <2% Carbon. Cast Irons have 2-4.5% Carbon
The limitations of ferrous alloys are:
Relatively poor electrical conductivity due to the crystal structure of the iron and its alloying element impeding the flow of electrons.
Generally poor corrosion resistance due to the fact that iron and its alloys are susceptible to oxidation and corrosion.
Relatively high densities -> Iron heavy
Describe some properties of aluminum alloys.
How are aluminum alloys classified? Describe these classifications.
Ductile. Low hardness. Low melting temp. Low density relative to iron. Prone to eventual fatigue failure.
Classified into 2 categories: Wrought alloys (shaped by plastic deformation) and Cast alloys. Further classified by whether the alloy is heat-treatable or non-heat-treatable.
Most Wrought alloys are not age hardenable. Generally strengthened by strain hardening, grain-size control & solid-solution strengthening
Cast alloys contain enough Si to enable the eutectic reaction, giving low melting points and good fluidity and castability. Properties controlled by dispersion strengthening, solid-solution strengthening, and solidification.
Other: T4 = Solution heat treated and naturally aged. T6 = Solution heat treated and artificially aged.
What are ceramic materials? Describe some of the properties of ceramic materials.
Compounds between metallic and/or nonmetallic elements that exhibit strong primary bonding (ionic and/or covalent bonds; some mixed bonding and metallic bonding). A key thing to note is that the ionic percentage increases with the difference in electronegativity.
High strength under compression. Highest hardness among engineering materials. No ductility under tension and low fracture toughness. Due to lack of free electrons, they have poor thermal and electrical conductivity.
What are polymers?
What are some pros and cons of polymers?
Chain molecules made up of monomers (primarily carbon atoms along length of the chain).
Pros: Inexpensive, light, flexible, easy to shape, thermally and electrically insulating, high toughness, resistant to chemical attacks.
Cons: Low stiffness, limited temp. range, uncertainty in life expectancy (susceptible to environmental degradation), toxic.
Describe the temperature effects on thermoplastics, starting from a high temperature. Discuss polymer crystallinity as well.
Starts off as a liquid, where there is easy movement of chains. As it cools below melting temperature, amorphous solid regions and crystalline solid regions start to occur. The amorphous solid regions are characterized by the movement of chains under stress, and the crystalline movement is characterized by the difficulty chains have moving. Crystallinity is also characterized by the alignment of the chains. Below the glass temperature, the amorphous solids will turn into this glassy region where only local movement of chain segments occur.
As crystallinity goes up, so does tensile strength and E (due to stronger Van der Waals because of closer proximity of chains). Annealing causes these crystalline regions to grow, however it is extremely rare for a polymer to reach 100% crystallinity as it is too difficult to get all the chains alligned.
As you increase the carbon content (and/or add alloying elements) in steels, what is the general trend? Why?
Increasing strength and cost. Decrease in ductility.
Introduction of carbon into the matrix impedes slippage (dislocation movement). Same with alloying elements.
Describe Magnesium alloys, Titanium alloys, and Superalloys.
Mg: Low density, melting temp., strength, ductility, and corrosion resistance. High strength to weight ratio. Solid solution strengthening with Al. Precipitation hardened with Mn, Zn, Zr, and Th. Good damping properties for a metal. Biocompatible.
Ti: Most corrosion resistant metal. Less dense than steel for comparable strength. High melting temperature. Highly reactive in oxygen. Biocompatible.
Superalloys: Metal alloys used in high temperature applications. Usually Nickel, Cobalt, Iron.
If bonding is primarily ionic in a ceramic material, describe the crystal structure.
Compare and contrast the properties of graphite and diamond in terms of their crystal structures and applications. How do their bonding types influence their properties?
Cation-anion structures in which the anion is typically larger. Stable structures form when anions
surrounding a cation all contact the anion (stability depends on coordination number and varies with radius ratio), and the net charge in the structure is zero. Ceramic structures are packed in similar manners to metals - FCC or HCP, where larger anions form close packed structures thus creating interstitial spaces in which the cation lies.
Graphite features a layered structure with weak van der Waals forces, making it soft and conductive, while diamond's tetrahedral structure of covalent bonds results in extreme hardness and insulating properties.
From lowest strength to highest strength, give the molecular structures of polymers. What is always present in each structure?
What is the degree of polymerization and how is it related to thermoplastics?
Linear structure (thermoplastic), Branched structure, Cross-linked structure, Network structure (thermosetting). Secondary Bonding (Van der Waals).
The degree of polymerization represents the average length of a polymer chain (avg. MW of polymer/MW of repeat unit). An increase in the degree of polymerization, lead to an increase in tensile strength, creep resistance, melting temperature, and impact and wear toughness.
What are stainless steels? Are there any subgroups of stainless steels, and if so, name them and their properties.
Stainless Steels are steels that normally contain at least 11% Chromium (Cr) that forms a protective Chromium Oxide layer, giving the steel excellent corrosion resistance.
Ferritic S-Steels: <30% Cr, <0.12% C. Good strength, moderate ductility, ferromagnetic, not heat treatable, and inexpensive.
Martensitic S-Steels: <17% Cr, 0.1-1% C. High strength and corrosion resistance.
Austenitic S-Steels: Addition of nickel means austenite at room temp. (nearly no ferrite). Very good ductility, may be cold worked. Not ferromagnetic. Expensive and corrodes at high temps.
What are the 4 simple heat treatments used for steels?
Process Anneal – Negate effects of cold work (recovery/recrystallization to reduce dislocation density). Done below eutectoid temperature.
Full Anneal – Heat treatment used to soften low-medium carbon steels. Done above the eutectoid temp. to form Austenite, then steel is slowly cooled to room temperature (over several hours) to form coarse pearlite with a uniform grain structure
Normalizing – Refines grains to a more uniform grain distribution. Heated form Austenite. Moderate cooling rate to produce fine pearlite.
Spheroidizing – Makes very soft steels.
Briefly list and explain the different forms of ceramic synthesis and processing methods.
Compaction: Powder is compacted thus removing porosity.
Tape Casting: Thin ceramic tapes are made from a slurry of ceramic material.
Extrusion & Injection Molding: Self-explanatory
Slip Casting: Ceramic liquid (slip) is allowed to form a layer (cast) along the mold's walls.
Sintering: The process of forming a solid mass of material through heat and pressure without melting.
Briefly explain copolymers. What are the different types of copolymers?
Copolymers are polymers made from two or more different monomer units. These monomers are chemically bonded together in the polymer chain, resulting in a single material with properties that can be tailored by adjusting the composition of the monomers. Copolymers often exhibit a combination of characteristics from each monomer, allowing for a wide range of applications with specific performance requirements.
Random, Alternating, Block, Graft.
What are Cast Irons?
Cast irons are iron‐carbon‐silicon alloys
that pass through the eutectic reaction
during solidification. Low melting temperature, easy to cast (hence the name).
Silicon encourages stable eutectoid reaction, encouraging graphite formation. Cementite undergoes a very slow process from ferrite to graphite.
What is precipitation hardening? How is it comparable to the hardening process in steel?
What is precipitation strengthening in aluminum alloys? What is not desirable in precipitation strengthening and why?
Precipitation hardening is the process in which metals are strengthened by dissolving alloying elements, quenching to trap them, and then aged to form particles that hinder dislocation movement (enhancing strength). Precipitation hardening and hardening mechanisms in steel both utilize heat treatment processes to enhance material strength. They involve quenching to freeze the microstructure in a high-energy state, followed by aging or tempering to refine the microstructure and relieve internal stresses. However, in precipitation hardening, the strengthening mechanism primarily involves the formation of coherent precipitates within the material, while in steel hardening, it often relies on the transformation of austenite to martensite during quenching, followed by tempering to refine the microstructure and relieve internal stresses.
The formation of very small brittle intermetallics that hinder dislocation movement. Larger brittle intermetallic particles because they are less effective in strengthening and can act as crack initiators.
Are ceramics susceptible to flaws?
As the porosity of ceramic materials increases, what happens to the compressive strength of the material?
Yes. However, they are strong in compression (it inhibits crack propagation). Note that they are weak in tension.
Because cracks originate at the pores, this would mean that the compressive strength decreases.
What are the three major polymer groups? Describe some of their properties.
Thermoplastics: Composed of long chains formed by joining monomers. Behaves in a ductile (plastic) manner. Can be amorphous or crystalline. Upon heating, softens, and melts. Chains have weak van der Waals (secondary) bonds. At elevated temperatures, secondary bonding weakens. Easily recycled
Thermosets: Long chains of molecules strongly (covalently) cross‐linked to form a 3‐D
network structure. Curing is a necessary step (heat, heat and pressure, chemical reaction). Stronger, but more brittle than thermoplastics. Resistant to creep. Decomposes on heating instead of melting. Not easily recyclable due to cross‐linking
Elastomers: Extensive elastic deformations possible; returns to original form when load is released. Elongation of chain molecules Ex. Rubber
Name the different types of cast irons and their properties.
White: <1%wt Si. Hard, Brittle, more cementite.
Gray: Graphite flakes. Weak/brittle under tension, stronger under compression. Two types of gray CI, Pearlitic and Ferritic.
Ductile/Nodular: Magnesium or Cerium alloying elements. Graphite in nodules (not flakes but round nodes). Matrix is often Pearlite -> better ductility.
Malleable Cast Iron: Graphite in rosettes. More ductile. Formed by annealing white cast iron.
Compacted graphite: Rounded but interconnected
graphite. Strength/ductility > gray iron. Good thermal conductivity and vibration damping properties.
List all the fabrication techniques in metal fabrication.
Explain the differences between the techniques.
For forming operations: Forging, Rolling, Extrusion, Drawing.
For casting: Sand casting, Die casting, Investment casting (rest are not talked about)
Other: Powder metallurgy, Direct Energy Deposit, Powder bed fusion/selective laser sintering.
Forging is when rough material is formed to a final shape. Hot work or cold work. For casting, a mold is filled with molten metal, and it is relatively inexpensive. However, it generally gives weaker products with internal defects. It is a good option for brittle materials.
Powder metallurgy uses sintering (densification through diffusion at high temps.). Direct energy has a laser that melts metal powder/wire as it is deposited.
What are silicate ceramics? How so amorphous silica structures occur and how do they influence the properties of a ceramic material?
Discuss the differences between crystalline materials and glass materials. (Hint: think of the specific volume vs. temperature graph)
Silicate ceramics are ceramic materials primarily made of silicon dioxide. Si-O bonds are strong and directional (significantly covalent). Leads to a strong, high melting materials (1710 o C). They are characterized by tetrahedral ion structures. Amorphous structure can occur by impurities which interfere with crystalline formation. Softens above glass temperature (Tg).
CM: Crystallizes at melting temp. Abrupt change in specific volume at melting temp.
Glass: Does not crystallize (this is why it is transparent, because no crystals to scatter light). Becomes increasingly viscous.
How does vulcanization affect elastomers such as natural rubber?
Vulcanization strengthens rubber by cross-linking polymer chains, improving mechanical properties like strength and durability. It also enhances thermal stability, chemical resistance, and reduces swelling. This process maintains rubber's elasticity and flexibility while expanding its applications.