MAT E 273 - EXAM 2

The overall composition that produces the softest steel upon slow cooling

0.035 wt% C

Yield Point

The stress at which dislocation motion and/or twinning is assumed to become the dominant response upon loading a material

Dislocation motion is generally _____ in BCC metals versus HCP metals

less difficult

eutectic

liquid transforms to two solid phases

K

+1 ion is relatively big compared to typical components of modern silicate glasses

Li

+1 ion is very small, making it useful for battery applications where the diffusion of positive charge through an electrolyte needs to be fast

What is the approximate solid-solubility of water in NaCl?

0 wt% water; water is insoluble in NaCl

A steel with this overall composition will feature mostly pearlite and proeutectoid ferrite, where the majority of the stel (by mass) is pearlite, if cooled relatively slowly

0.70 wt% C

A steel specimen with this overall composition will produce a microstrucutre with no proeutectoid phase, for relatively slow cooling conditions The composition of austenite taht decomposes into ferrite and cementite when a steel is cooled relatively slowly

0.76 wt% C

Conversion: mm to cm

1 cm = 10 mm

4 Processes that tend to strengthen a typical pure metal

1) Cold-working the pure metal 2) Adding solute to forma solid solution 3) Precipitation a secondary phase 4) Increasing the dislocation density (dislocations count per unit volume)

4 Helpful ways to decrease creep deformation

1) Decrease load that is applied 2) Decrease service temperature 3) Coat material with insulating layer (ex: ceramic) 4) Substitute with material that offers high elastic modulus

4 Not-commonly observed property/behavior for crystalline ceramic materials

1) High strength under tensile loads 2) High electrical conductivity 3) Low melting temperature 4) High toughness

6 Ways temperature change can influence what aspects of a phase transformation

1) Identity of the phase(s) that form 2) Rate of transformation 3) Amount of time required for the transformation to actually commence 4) Elapsed time of the transformation itself 5) Microstructure(s) that form(s) 6) Properties of the material after transformation

8 Possibilities related to annealing and cooling a polycrystalline material

1) Recrystallization of grains 2) Dislocation annihilation 3) Overall microstructure change during an annealing process 4) Crystal structure change(s) during an annealing process 5) Crystal structure change(s) during the cooling stage after an annealing process 6) Overall microstructure change(s) during the cooling stage after an annealing process 7) Chemical reaction with the annealing atmosphere components 8) Diffusion of atmosphere components into the specimen

3 Definitely true if a crystalline compound AB12 is the stable state for a single specific overall composition over a variety of temperatures at 1 atmosphere

1) System is below its melting or solidus temperature 2) Compound under conditions are lower Gibbs free energy compared to other phases or phase combinations 3) Since just the compound itself is said to be stable, the system would be single-phase in equilibrium

Load types

1) Tension 2) Compression 3) Shear 4) Torsion

3 Similar Mathematical Descriptions for Brittle Fracture

1) Theoretical strength of a material to the magnified stress at a flaw 2) "Energy cost" associated with crating exposed surfaces 3) A material property that describes resistance to brittle failure

Conversion: cm to m

100 cm = 1 m

What is a reasonable value for the elastic modulus of a typical structural metal?

300 GPa

What is a reasonable value for the yield stress (yield point) of a typical structural metal?

400 MPa

If a 1040 steel is cooled relatively slowly from 1000oC (after holding at this temperature long enough to ensure the specimen begins as completely γ when cooling commences), some of the γ begins to decompose around _____ C as it transforms into the ____ phase.

800; Ferrite

Metals with FCC packing (like aluminum) are often more ductile than metals with HCP packing (like magnesium), because the FCC structure has ____

A higher number of slip systems than HCP

Elasticity

A measure of a material's ability to absorb mechanical energy in the form of stretched bonds

Ductility

A measure of a material's ability to deform plastically priory to failure

Hardness

A measure of a material's ability to withstand local deformation or wear. This value is correlated with the yield point

tensile strength

A measure of how much stress from pulling, or tension, a material can withstand before breaking.

Re

A metal added to nickel-based superalloys to stabilize the precipitation of the secondary phase (gamma-prime) that strengthens such alloys The pure metal with second-highest melting point.

Mg

A metal that is quite similar to aluminum, but is less ductile and highly susceptible to corrosion

Ti

A relatively low-density but high-strength metal that has excellent corrosion resistance as a cool solid, but not as a very hot solid or liquid A metal whose oxide is more commonly produced (approximately 60x more by weight). The identity of this oxide mineral is rutile.

Cr

A shiny metal that is completely brittle below 300 degrees C A metal added to steels to drastically improve their corrosion resistance.

Boiling points

Between liquid and vapor phases

Melting points

Between solid and liquid phases

Sublimation points

Between solid and vapor phases

Highest to lowest DUCTILITY is ranked on a graph from _______

Bottom to top (Spheroidite, coarse pearlite, fine pearlite, bainite, tempered martensite, martensite)

Recalling that most brass for structural use is a single-phase alloy of Cu and Zn, what is the primary reason why brass is stronger than copper?

Brass leverages solution hardening

A ______ pearlite structure is produced when austenite is allowed to transform into pearlite at a temperature that is relatively _____ the eutectoid temperature, and such a structure will be relatively soft and ductile when compared to other a fully pearlitic structural possibilities.

Coarse; close to

Stainless steel derives its corrosion resistance from the presence of what alloying addition (which forms a protective and self-healing oxide on the surface of the steel)?

Cr (chromium)

Completely brittle failure occurs while the loading is _____.

Elastic

[True/False] Small and sharp features on a metal specimen generally experience stress levels that are much lower than the nominal / macroscopic stress that is applied to the bulk specimen

False

[True/False] The shape, position, and identities of "microstructure fields" on the IT curves for a steel are independent of the overall composition of the steel. In other words, the austenite decomposition kinetics and as-cooled microstructure possibilities for a steel do not depend on the overall composition of the steel.

False

[True/False] To be "truly ductile", according to most design engineers, a metal must offer at least 5% or greater tensile plastic elongation

False

Martensite is formed whenever austenite (all or just a part of a specimen's microstructure) is cooled extremely _____

Fast

The final microstructure will feature proeutectoid ______ and _____ as microconstituents after cooling below the eutectoid temperature.

Ferrite; pearlite

Brittle failure

Flat surface after separation

The fracture surface of a brittle material is often ____ and ____.

Flat; smooth

_______ is a material property that pertains to local resistance to plastic deformation, such as scratching or denting. It is often measured by poking a material with a very hard tip (like diamond) and measuring the size and depth of the resulting crater that forms.

Hardness

A ____ value of elastic modulus implies a high magnitude of slope on the force-separation curve ( bonding curve, F versus r ) at the equilibrium bond distance (ro) for the material of interest.

High

How to know if Hooke's Law can be used

If yield strength is GREATER than tensile stress, it can be used If yield strength is LESS than tensile stress, it cannot be used

The ______ of a system dictates the rate(s) and mechanism(s) of phase transformations, thereby dictating the microstructure(s) that a system adopts for a given transformation path (transformation temperature, heating / cooling rate, complex transformation pathway, etc.). Furthermore, this also dictates the time over which these microstructure(s) tend to exist.

Kinetics

Dislocations move in response to _____ within a crystal

Local shear stresses

Without any other information to consider, it is reasonable to assume that a material with a low elastic modulus will have a _____ melting point and a material with a high elastic modulus will have a ____ melting point.

Low; high

Cs

Lowest-melting alkali metal (most susceptible to creep deformation at room temp)

What affects creep deformation if at a lower level?

Melting Point (Tm)

Ductile failure

More of a tip after separation if high ductility, teeth-like break if moderately ductile

Is it possible to have multiple phases equilibrated together (simultaneously present) for a temperature-pressure state (point on the diagram) that is NOT on a phase boundary (line or curve on the diagram) of a single-component system?

No, because you can't have ice and water at the same time

Fatigue

Occurs when a specimen is subjected to dynamic or cyclic stresses - Single largest cause of failure in metals

A material property that allows one to convert between perpendicular and oppositely-signed elastic strains is called __________ .

Poisson's ratio

Why is the yield strength preferred over proportional limit?

Proportional limit is where the effects of elasticity are still reversable, where the yield point tells where deformation begins (more useful knowledge)

Macroscopic stress applied to a real specimen is magnified at ____ features, especially at microscopic internal voids, as well as surface scratches.

Sharp

Which provides higher yield stress: Larger distance between grain structures or smaller distance between grain structures?

Smaller distance between grain structures

Crack propagation in ductile materials is termed _____ crack growth, since _____ stress is required for the crack to propagate further.

Stable; additional

Elongation

The fractional increase in a material's length due to stress in tension or thermal expansion.

W

The highest melting point pure metal A metal that forms a very hard ceramic carbide for cutting tools and wear resistant coatings

Engineering stress

The instantaneous load applied to a specimen divided by its cross-sectional area before any deformation.

Liquidus

The line in a binary phase diagram above which only liquid occurs

Ultimate Tensile Strength

The maximum stress that a material can withstand before it breaks

Which material cold-works "the fastest" (the most rapidly with strain) upon initial plastic deformation?

The one located higher on the yield strength portion of the graph

If the state point lies on a phase boundary between single phase fields, what phase(s) are stable?

The phases associated with the boundary

Solidus

The temperature during solidification of an alloy at which the last of the liquid phase solidifies.

At best, the _____ of a system tell us the phase(s) that are stable as well as their composition(s) & fraction(s), for a given set of state variables such as temperature, pressure, and overall composition.

Thermodynamics

Highest to lowest HARDNESS is ranked on a graph from _______

Top to bottom (Martensite, tempered martensite, bainite, fine pearlite, coarse pearlite, spheroidite)

[True/False] Ductile specimens are less affected by poor quality surface finish, since the propagation of microscopic flaws as cracks is generally less likely compared to plastic deformation.

True

[True/False] Even though metal parts are often intended to endure only elastic (reversible) strain, they should generally offer at least some ductility to account for local stresses in a specimen that may exceed the metal's yield stress

True

How do changes in temperature affect the elastic modulus of a material?

When you increase the temperature, the atoms of the object gain significant amount of Kinetic energy. Kinetic energy is a measure of the ability to move. This causes atoms to move apart causing an increase in the area. When the area increases, the stress (Force / Area ) decreases. E = Stress / Strain. As the numerator falls, the value of E also falls. This is why Young's modulus decreases with increase in Temperature.

Elastic Modulus

a quantity that is correlated with the bond strength of a material

Changes in pressure are generally less effective on ____

condensed matter

The formation of pearlite, spheroidite, and bainite are examples of ______ transformation, whereas the formation of martensite is referred to as a ______ transformation.

diffusion-controlled; diffusionless

While dislocations can exist in ionic and covalent ceramic materials, their motion in these materials is _____ compared to metals

extremely difficult

Dislocations prefer to slip on ____-density planes and in ____-density directions within a crystal

high

Dislocation density (count per volume) generally ______ during plastic deformation at room temperature

increases

Hooke's Law applies in the ____ regime of the stress-strain curve for materials that exhibit a linear elastic response.

initial

Martensite is a/n _______ phase of steels, so it _____ appear on the Fe-Fe3C phase diagram.

non-equilibrium; does not

eutectoid

one solid phase transforms to two other solid phases

For most materials, a tensile stress results in a _____ strain, whereas a compressive stress results in a _____ strain.

positive, negative

elastic modulus

stiffness of a material

Dislocations feature _____ that interact with other ___ in a crystal (such as those produced by other crystalline defects)

strain fields

failure strength

stress at which a material breaks or fails

Engineering strain

the change in length of sample divided by the original length of the sample

changes in pressure are generally more effective on ____

vapor states