materials lab exam #2 UT AUSTIN

What determines the length of the austenizing heat treatment?

Size

What determines the temperature required to begin the austenization process?

Composition

How is pearlite formed?

Austenite at an isothermal heat treatment directly below eutectoid temperature.

What happens to the diffusion distance as the transformation temperature decreases?

It decreases.

Is coarse or fine pearlite stronger?

Fine pearlite is stronger than coarse pearlite.

How is coarse pearlite formed?

Higher temperatures over a long period of time.

How is bainite formed?

Lower temperatures of 250-550 °C.

Does bainite have a lemmar structure and why?

No, because self-diffusion of iron stops.

What does bainite look like?

Cementite rods in a ferrite matrix.

How is martensite formed?

1076 steel cooled to room temperature from the eutectoid temperature very quickly.

What are the steps for tempering martensite?

Steel specimen is austenized and cooled to room temperature quickly, then martensitic steel is reheated.

How is spheroidite formed?

Heating near the eutectoid temperature for around 24 hours.

What is the microstructure of spheroidite?

Spheres of Fe3C in a ferrite matrix.

What does the area under the stress-strain curve represent?

Internal strain energy.

What is the relationship between internal strain energy and dislocation density?

As dislocation density increases, more energy is stored.

How does cold work impact electrical conductivity and why?

Reduces conductivity because it scatters electrons.

How does cold work impact corrosion and why?

Cold work makes a metal more susceptible to corrosion due to dislocations representing atoms with unsatisfied chemical bonds.

What are the microstructural changes during plastic deformation at room temperature?

Grain shape, size, orientation change; dislocation density increases (strain hardening).

What is annealing and what does it do?

Cold worked metal is reheated to restore ductility, toughness, and conductivity.

What processes does annealing involve?

Recovery and recrystallization.

What does annealing do to the grains?

It increases grain size.

What do larger grains mean and how do they form?

More ductile and not as strong; forms with more diffusion.

What do smaller grains mean and how do they form?

Stronger but not as ductile; forms with less diffusion.

What does recovery in annealing do?

Dislocations are able to move due to an increase in diffusion coefficient, improving electrical conductivity.

What does recrystallization in annealing do?

Material loses internal strain energy; stress-free grains grow, replacing strained grains.

What happens to the extent of recrystallization as the annealing temperature increases?

It increases the extent of recrystallization.

Define recrystallization temperature.

Lowest temperature at which recrystallization can be completed in exactly one hour.

Do alloys or pure metals have a higher recrystallization temperature and why?

Alloys have a higher recrystallization temperature because secondary atoms reduce atomic diffusion.

How does increasing the amount of cold work change recrystallization?

It improves the rate of recrystallization by increasing the sites for grain nucleation.

What is hot working?

Plastic deformation at higher temperatures where strain hardening doesn't occur.

How does grain growth occur?

Heat treatment is carried past recrystallization, causing larger grains to grow at the expense of smaller.

What happens with a reduction in grain boundary area?

Average grain size increases with elevated temperature, leading to coarse grains and reduced strength.

How are polymers synthesized?

From small molecules called monomers.

What is another name for a monomer?

Repeat unit.

What is initiation in polymer synthesis?

A free radical removes a double bond and leaves one covalent bond.

What is propagation in polymer synthesis?

Additional molecules are added to the free bond, causing the polymer chain to lengthen.

What are the two ways of termination in polymer synthesis?

Combination and disproportionation.

What is combination in the termination process of polymer synthesis?

Combining two chains with free radicals to form a single longer chain.

What is disproportionation in the termination process of polymer synthesis?

Two chains react to form two independent chains.

What is polymer crystallinity?

Degree of structural order in a solid.

How is the degree of crystallinity defined?

Mass fraction or volume fraction of a material that has a crystalline structure.

Are metals or polymers more crystalline?

Metals, they are almost 100% crystalline.

As crystallinity increases, what properties increase?

Strength, hardness, and elastic properties.

As crystallinity increases, what properties decrease?

Ductility.

Does HDPE or LDPE have higher crystallinity and why?

HDPE has higher crystallinity because it has less branching compared to LDPE.

What is the fundamental feature of a polymer?

Chain of atoms bound to each other with strong covalent bonds.

What is possible between adjacent chains with secondary bonds?

Plastic deformation and irreversible sliding due to weak secondary bonds.

What happens if adjacent chains have a covalent bond between them?

Irreversible sliding is prevented, preventing plastic deformation.

What is cross linking?

It is a process that connects polymer chains.

What properties does an elastomer have?

It can undergo large amounts of reversible deformation.

What are the defining characteristics of an elastomer?

Amorphous and made up of long polymer chains that are highly twisted and coiled which are crosslinked.

What happens when an elastomer undergoes stress and is released?

The chains twist, straighten or uncoil and then return to their initial shape.

What is the term for the "necking" that develops in polymers under stress?

Constriction.

What occurs during cold drawing in polymers?

The constriction propagates along the reduced section.

What is the relationship between crystallinity and transparency?

As crystallinity increases, the material becomes more opaque.

What happens to crystallinity as plastic deformation temperature increases?

It increases.

What happens to the crystalline structure during the tension test of a semi-crystalline polymer?

The crystallinity increases as plastic deformation increases.

What does aligned crystallinity look like?

Transparent.

What does random crystallinity look like?

Opaque.

How is viscoelasticity described?

Combination of fast elastic deformation with thermally activated viscous flow.

To which types of materials is viscosity confirmed?

Semi-crystalline or amorphous materials.

What is required to activate viscoelastic deformation?

Thermal activation and small shear stresses.

Under what conditions is viscoelastic deformation favored?

Low-strain rates and high temperatures.

When does a viscoelastic material show creep and stress relaxation?

When a constant load is maintained and constant extension is maintained.

What is the fracture mode in crosslinked polymers?

Brittle and flat fracture surface is observed.

What phenomenon is observed in thermoplastic polymers?

Crazing.

What happens when crazing occurs?

The fracture starts with the appearance of a tear on one of the edges, perpendicular to the tensile axis.

What follows crazing in polymers?

Microvoids and fibrillar bridges.

What behavior do long chain polymers exhibit?

Brittle elastic behavior.

What is the glass transition temperature?

The temperature at which polymers that are brittle at room temperature become more ductile.

Where is there no strain during beam bending?

The neutral surface/axis.

Which bending method is more accurate, three point or four point bending?

Four point bending because the compressive load is more evenly distributed.

Define composite materials.

They consist of two or more constituents that remain separate in the final structure.

What are the three major categories of composites?

Particle, fiber, and structural reinforced.

Define particle reinforced composite.

Dispersed phase in the form of particles spread in the continuous phase.

Define fiber reinforced composite.

Dispersed phase in the form of fibers.

What is isostrain?

All fibers are aligned in the same direction as the applied load.

What is isostress?

All fibers are aligned perpendicular to the direction of the load.

In the elastic modulus versus volume fraction graph, what is the upper bound?

Isostrain.

In the elastic modulus versus volume fraction graph, what is the lower bound?

Isostress.

Why would a fiber-reinforced composite experience a change in slope in the elastic region?

The elastic limit of the matrix is smaller than the fibers.

Why is the theoretical fracture strength of ceramics higher than the actual value?

Small flaws are present in the material.

Why is it easier for a flaw to grow under tension than in compression?

In compression, the flaw is closed up instead of opened up.

Why is it important to temper martensite?

Tempering martensite increases ductility and toughness.

Why does water quenching produce a harder steel than air cooling?

Water has a higher heat capacity than air, extracting heat rapidly.

Why do materials become harder under cold working?

Dislocation density increases, and dislocations impede on each other.

Will 10% or 50% cold worked material have a lower recrystallization temperature?

50% cold worked material will have a lower recrystallization temperature.

Why does copper recrystallize at a lower temperature than brass?

Foreign atoms interfere with home atoms, requiring higher energy to form new grains.

What happens to the modulus and ductility when increasing the crystallinity of a polymer?

Modulus increases and ductility decreases.

Why does testing a polymer at a faster speed make it appear brittle?

Polymer chains don't have time to untangle or rearrange.

What is the difference in molecular structure between a thermoplastic and a thermoset?

Thermosets have cross-linking.

Which fiber orientation provides the highest strength in a single direction?

Uniaxial (longitudinal).

If a polymer is brittle at room temperature, is the glass transition temperature above or below room temperature?

Above.

Why does adding a bulky side group to a polymer increase the glass transition temperature?

Bulky groups impede chain rotation, requiring more energy for movement.

What acts as a physical cross-link in a semi-crystalline polymer?

The crystallites, which anchor the amorphous chains.

What is austenite?

A single phase microconstitutent.

What does tempered martensite consist of?

Fine ferrite and cementite.

What are the worst conditions for viscoelastic flow?

High strain and low test temperature.

What parameter is not needed to calculate material flexural stress?

Sample yield strength.

Are commercial fibers stiffer in isostress or isostrain?

They are stiffer in isostrain.