06 Metals and Ceramics 3

Characterise Aluminium (7)

• Third more common material on Earth

• Low density

• Good strength (70 - 100 MPa) (Good tensile strength to density ratio: Al > Fe (200 MPa))

• High resistance to weather (corrosion resistance due to Al₂O₃)

• Good formability and machinability

• High electrical conductivity (Good electrical conductivity to density ratio: Al > Cu)

• High thermal conductivity

Characterise Al wrt its alloys (2)

• Wrought alloys: Mn, Mg, Cu

• Cast alloys: Si, Cu, Mg

Characterise Copper (7)

• Available as Cu ore

• Very high electrical conductivity (only after Ag)

• High thermal conductivity

• Strength increased through cold deformation (but still low tensile strength (less than 440 MPa))

• Formable

• Weldable and solderable

• High corrosion resistant (patina)

Applications of Copper (5)

• Electrical applications

• Mechanical engineering

• Chemical engineering

• Civil engineering

• Coins

Which metals is copper usually alloyed with? (4)

• Zinc (Brass, more than 50 wt % Cu): higher strength, higher corrosion

• Tin (Bronze more than 60 wt % Cu): higher strength, hardness, corrosion resistance

• Nickel: weldable, high restance against erosion, corrosion, cavitation, high stength (more brittle)

• Al

• Mn

• Si

Characterise Magnesium (12)

• High availability

• Lowest density among metals (1.74 g cm^-3)

• Low strength

• Low E

• Medium cost

• Excellent machinability

• Good mechanical damping

• Good castability

• Low corrosion resistance

• High reactivity

• Not used for construction

• Hexagonal Centered Packed: poor formability, low toughness, high notch-sensitivity

Where is Mg used? (3)

• Gearboxes, housings, etc

• Helicopters

• Airplanes

Which metals is magnesium usually alloyed with? (4)

• Aluminium, zinc: increase toughness, decrease notch-sensitivity

• Manganese: increase corrosion resistance

• Cerium, thorium: increase strength at high temperature (up to 300°C)

• Zirconium: grain refinement, formability and strength

Characterise Titanium (9)

• Fourth most common material in Earth’s crust

• Cost effective obtainment since mid 20th century

• Pure and with up to 20 wt % of other elements

• High stength (up to 1400 MPa)

• Low density

• High cost

• HIgh working temperatures

• High corrosion resistance (oxide layer)

• Ductile and forgeable

• Varying lattice structure

Use in Aerospace of Ti (5)

• High performance engines

• Landing gear components

• Airframe structure

• Spacecraft and rocket structures

• Helicopter structures

Discuss inclusion of oxygen in Ti alloys (3)

• Dobled tensile strength

• Reduced formability

• Reduced corrosion resistance

Which additives is α-titanium normally alloyed with? (4)

• Aluminium

• Oxygen

• Nitrogen

• Carbon

Characterise α-titanium (4)

• Low to medium strength

• Low density

• Good weldability

• High warm and cold stability of mechanical properties

Characterise β-titanium (4)

• Higher strength

• High vibrational resistance

• Higher cold-formability

• Higher density

Characterise α + β-titanium (4)

• Two phase alloy (two different unit cells)

• High specific strength

• Hardenable

• Lower strength than β-titanium but better specific strength

Characterise Nickel (7)

• 0.02% of Earth’s outer silicate crust

• FCC (high formability)

• High strength

• Very high density

• High cost

• Very high working (service) temperatures

• Ferromagnetic

Uses of Nickel (4)

• Stable properties at high service temperatures

• HP compressor blades

• Combustion chamber

• Turbine blades

Which materials is Nickel most alloyed with? (5)

• Copper

• Chromium

• Cobalt

• Iron

• Molybdenum

What are Nickel-based superalloys? (7)

• Alloys with extremely high temperature stability

• At least 50 wt % Ni

• Inconel

• Used for turbine engines

• High strength

• Long fatigue life

• High creep, stress rupture and corrosion resistance at high temperatures

How are ceramics classified? (3)

• Consumer ceramics (Silicate + Oxide)

• Functional ceramics (Silicate + Oxide + Non-oxide )

• Structural ceramics (Oxide + Non-oxide)

How are ceramic materials created?

• Formed out of Ceramic Raw Mass

• Formed element sintered to reach finished element

What forming processes are there for ceramics? (5)

• Hot isostatic

• Cold

• Hot

• Dry

• Slip casting (non-oxide ceramics)

• Plasticizing (often addition of thermoplastics) → extrusion/injection molding

What is pre-sintering?

• “Heat treatment“ to a lower temperatures than final sintering

What is green machining?

• Process that increases manufcaturing precision

• Allows complex modifications of geometry without influencing forming process

What is sintering?

• Heating the material up up to a temperature dependant of the material

What is hard machining?

• Post-processing machining to achieve higher dimensional precission and surface quality

How is the structure of silicate ceramics after sintering?

• Glassy and crystalline

How is the structure of oxide and non-oxide ceramics after sintering?

• Polycrystalline

What happens after sintering wrt the overall structure? (2)

• Pores and inclusions limit grain growth in the atomic structure

• Pores, inclusions and manufacturing defects dominate properties of ceramics

Which kind of bonds is displayed by the atoms of a cermaic material?

• Fully covalent

• Partial ionic

• Fully ionic

What is a coordination number?

• Number of anions surrounding one cation

What are the usual coordination numbers for ceramics?

• 4

• 6

• 8

What is the ralation between the ratio of radii and the coordination number?

• The higher the ration of radii, the higher the number of possible cation-anion connections (coordination number)

General Disadvantages of Ceramics (7)

• Mechanical properties inferior to metals

• Disposition to brittle fracture with low energy absorption

• High scatter of mechanical strength

• Strength depedant of item volume and surface

• Decrease of strength at static load (static fatigue)

• Very low thermal conductivity

• Low thermal expansion

What is the reason for the mechanical and physical disadvantages of ceramics? (4)

• Inhomegenities

• Pores

• Inclusions

• Micro cracks

Physical and mechanical properties of ceramics (3)

• High hardness

• Low electric conductivity

• Ofter high corrosion resistance

How is a ceramic charaterized?

• With a flexural strength test (actually measures the maximum shear but whatever)

• Formulae in Cheat Sheet

Why is it difficult to apply a regular tensile test to a ceramic material? (3)

• Difficult specimen preparation (complicated geometry)

• Brittle materials difficult to grip

• High precission required

How is the probability of failure of a ceramic material modeled?

• Via the Weibull distribution and using a flexure tension test

• Formula in Cheat Cheet

Which rules are to be followed for constructions with ceramics? (5)

• Avoid sharp edges

• Avoid abrupt cahnges of cross section

• Pressure load prefered over tensile load

• Avoid high edge pressure

• Consider different thermal expansion coefficients

Characterise Al₂O₃ (6)

• Common

• Good mechanical and chemical properties

• High hardness

• Disposition to creeping at high temperatures

• High thermal expansion

• Medium thermal conductivity

Characterise Si₃N₄ (5)

• Group of ceramics: dense silicon nitrides

• Suitable for high temperature applications up to 1400 °C

• Polyphase material with amorphous or semi-crystalline proportions up to 30%

• Tailorable

• Los resistance against aggresiva chemical solutions

Characterise B₄C (8)

• Boron Carbide

• Third hardest material in the world

• Extreme high hardness at high temps (stable up to 1500 °C)

• Non-oxide and non-metallic

• Only machinable with diamond tools

• Thermal conductivity decreases with increasing temperature

• Los oxidation ressitance

• High chemical resistance

What does TBC stand for?

• Thermal Barrier Coating

What is TBC used for? (5)

• Thermal isulation via ceramic coating

• Metallic bond coat

• Wear and corrosion resistant due to high proportion of Al

• Applied by plasma sprying or electron beam vapor deposition

• Normally used for gas turbines

SQ: For which applciations are nickel-based superalloys typically used in aerospace engineering?

• Turbine engines

• Wherever thermo stability is needed

• Higher stability of material leads to higher operation temperature of engine and thus to higher thrust

SQ:What is the main advantage of α-titanium over β-titanium?

• Low density

• Good weldability

• High warm and cold stability of mechanical properties

SQ: Why is Al often preferred over Mg for lightweight design, even though it has higher density?

• Al displays better mechanical properties, specially tensile strength

• Mg is more reactive than Al

SQ: Why is copper not commonly used for aerospace structure? In which engineering discipline is copper used pedominantly instead?

• High denisty

• Inferior strength to weight ratio

• Cost

• Electrical engineering

SQ: How are ceramic materials classified?

• Silicate ceramics

• Oxide ceramics

• Non-oxide ceramics

SQ: Name the differences between silicate ceercamic, oxide and non-oxide ceramics according to their crystal structure after sintering

• Silicate: glassy and crystal phases

• Oxide and non-oxide: polycrystalline structure

SQ: What should be considered in the design process of ceramic products?

• Ceramics raw material

• Final geometry

• Quantity of pieces

SQ: What characteristic of ceramic materials is of special interest for aerspace applications?

• Thermal insulation

SQ: Why is the mean value of strength insufficient for characterization of ceramic materials?

• Due to considerable deviations due to pores, inclusions, inhomogenities, micro cracks, brittleness

• Failure at 0.1% strain

SQ: Describe the setup of a thermal barrier coating

• Main structure

• Bond Coat (high Al proportions)

• Oxide Layer (Al)

• TBC (Zircon or Yridium ceramics)

SQ: Why are turbine engine parts with high temperature requirements typically not made form massive ceramic materials?

• Due to inferior mechanical properties of ceramics, like brittleness or poor eleongation to failure

SQ:Describe the crystal structure of predominantly ionic ceramics

• Oppositely charged ions to achieve neutral crystall over the whole material body

• Accomodation within lattice structure driven by and radii ration (cation to anion radii ratio) and coordination numbers

SQ: What is the difference between green machining and hard machining of ceramics?

• Green machining: presintered material

• Hard machining: sintered material