MEEN 360 T/F + Short Answers
A glass is metastable material that becomes rigid without crystallizing.
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A larger degree of polymerization generally results in polymers with larger strengths and toughnesses.
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At the eutectic composition, an alloy can solidify at a constant temperature.
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At the surface of a casting, grains tend to be randomly oriented.
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Ceramics are usually stronger in compression than tension.
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Ceramics may be crystalline, partially crystalline, or amorphous.
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Ceramics typically have larger hardnesses than metals.
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Certain materials can age harden at room temperature.
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Crystalline ceramics typically have dislocations that are essentially immobile.
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Crystallization of a polymer results in an increase in its density.
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Crystallized polyethylene (PE) has a higher density and strength than amorphous PE.
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Cu alloys typically exhibit both high electrical and thermal conductivity.
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Cu alloys usually exhibit both high electrical and thermal conductivity.
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Faster solidification results in smaller interlamellar spacing.
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For the same conditions, a casting with a relatively small volume and large surface area cools more rapidly.
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Heterogeneous nucleation requires less free energy for nucleation than homogeneous nucleation.
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Homogeneous nucleation requires more undercooling than heterogeneous nucleation.
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In the AISI system for designating steels, the first two numbers refer to the major alloying elements of the steel.
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Increasing the % cold work tends to lower the recrystallization temperature (Tr).
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Intermetallic compounds are usually hard and brittle.
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Investment casting can be used to produce complicated shapes.
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Mechanical properties tend to be anisotropic after both cold and hot working.
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Nb and W have high melting points (> 1925ºC), and are thus refractory metals.
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Ni and Co alloys have high strength.
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Over-aging can occur when the aging temperature is large enough such that incoherent precipitates form.
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Planar growth is promoted when the temperature of the liquid just ahead of a solid/liquid interface is larger than the melting temperature.
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Quenching hardens most steels while tempering increases the toughness.
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Shot peening introduces residual compression at the surface of a metal.
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Solidification usually occurs by heterogeneous nucleation.
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Spheroidizing improves the machinability of hypereutectoid steels.
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Surface finishes tend to be poorer after hot working than cold working.
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Tempering of martensitic steel increases its ductility.
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The solid, liquid, and gas form of an element can coexist simultaneously.
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Thermoplastics become relatively stiff but brittle at temperatures below the glass transition temperature (Tg).
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Thermosets are generally stronger but more brittle than thermoplastics.
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Ti alloys are highly resistant to fatigue damage.
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Steels have historically been used in ships. However, with increasing fuel costs, we want to find alternative lower weight materials. The requirements are high yield strength, high corrosion resistance, and low cost. Discuss the pros/cons of using Al alloys as a replacement for steels in shipbuilding.
The specific strength of Al is very high, which can result in fuel savings. Al is generally corrosion resistant. However, it is possible that it will suffer from galvanic corrosion in salt water (it is anodic compared to most other materials used in shipbuilding). Al has a low melting temperature, which may be an issue for certain components of the ship, e.g., if there are accidents related to fires. Al is also quite difficult to weld, which may present issues in shipbuilding. Fatigue may be a major issue as well and needs to be accounted for in design.
Degradation Temperature
The temperature above which a polymer burns, chars, or decomposes. Thermoplastics decompose in the liquid state while thermosets decompose in the solid state.
Is there an alloy that freezes at a constant temperature?
Yes, Eutectic
Can alloys remember their shape?
Yes.. Shape memory alloys NiTi !
Higher temperatures (but below the melting point) tend to make steels more ductile and less strong, likely due to: (a) Increased dislocation density (b) Increased mobility of dislocations (c) Exceeding the ductile-brittle transition temperature (DBTT) (d) Exceeding the glass transition temperature (Tg)
(a) F (Dislocation density does not change appreciably with temperature) (b) T (c) T (d) F (Steels do not have Tg)
Which of following is NOT a main reason for adding alloying elements to the steels? (a) Improved thermal conductivity (b) Solid solution strengthening (c) Precipitation strengthening (d) Improved corrosion resistance (e) Improved hardenability
(a) Improved thermal conductivity
The fracture toughness (Kc) of a steel sample could be increased by: (a) Heating the sample (b) Increasing the sample thickness (c) Decreasing the rate of loading
(a) T (b) F (Decreasing the thickness may lead to an increase in fracture toughness) (c) T
The following materials typically exhibit strength described by a Weibull distribution: (a) Ceramics (b) Glasses (c) Metals (d) Thermoplastic polymers above T_g
(a) T (b) T (c) F (Blunting of cracks from plastic deformation leads to a Gaussian distribution) (d) F (Blunting of cracks from plastic deformation leads to a Gaussian distribution)
If you wish to buy a plain low carbon steel, which of the following AISI steels should you pick? (a) 4340 (b) 1010 (c) 1080 (d) 52100
(b) 1010
Which of the following materials should NOT be used in human body (i.e., which is NOT biocompatible)? (a) Austenitic stainless steels (b) Be (c) Co-Cr (d) Ti (e) All of the above can be used
(b) Be
304 stainless steel is often used in medical applications due to its high ductility and nonmagnetic nature. Based on these characteristics, what category of stainless steel (SS) does it belong to? (a) Ferritic SS (b) Martensitic SS (c) Austenitic SS (d) Precipitation hardened SS
(c) Austenitic SS
Engine blocks made of cast iron need to provide large thermal conductivity, high damping capabilities, and large wear resistance. Which of the following cast irons best provides these characteristics? (a) White iron (b) Malleable iron (c) Gray iron (d) Pure Iron (e) Nodular Iron
(c) Gray iron
Crystalline ceramics are typically brittle and hard because: (a) they typically have small dislocation densities (b) they typically have large dislocation densities (c) their atomic structures result in a large resistance to dislocation motion (d) their bonds are typically weak (e) their free electrons are mobile
(c) Their atomic structures result in a large resistance to dislocation motion
Refractory metals are used with coatings to avoid oxidation at high temperatures. The coatings must: (a) Have a high melting temperature (b) Provide a diffusion barrier to oxygen (c) Have a coefficient of thermal expansion similar to that of the refractory metal (d) All of the above
(d) All of the above
A refractory material is one that: (a) readily forms a glass (b) is readily cast (c) has excellent toughness at high temperature (d) maintains its strength at high temperature (e) All of the above
(d) Maintains its strength at high temperature
A hypereutectoid steel often contains hard and brittle cementite along the grain boundaries of pearlite. Which of the following heat treatments will best mitigate this issue by changing the microstructure? (a) Annealing (b) Normalizing (c) Austenizing (d) Spheroidizing (e) Process-annealing
(d) Spheroidizing
Which of the following nonferrous alloys has the highest specific strength? (a) Al (b) Ni (c) Cu (d) Ti (e) Mg
(d) Ti
Which of the following are considered ceramics? (a) Diamond (b) Silica (c) Alumina (d) Ice (e) All of the above
(e) All of the above
What alloys are most commonly used for soldering?
-Lead and tin (Pb-Sn) -(Sn-Ag-Cu..) -Eutectic compositions
Copolymer
Addition polymers produced by joining two or more types of monomers. There are four types including alternating, random, block, and grafted.
Quenching
Cooling rapidly. Quenching is commonly used to prevent certain phase transformations.
Tacticity
Describes the spatial arrangements of atoms or groups of atoms in the polymer chain when polymers are formed from nonsymmetrical repeat units.
Austenitizing
Heating steel to a temperature to form homogeneous austenite. This process is performed by heating a hypoeutectoid steel above the A3 temperature or by heating a hypereutectoid above Acm (if pure austenite is desired) or more commonly by heating above A1 to form austenite and cementite.
Annealing
In steel, a heat treatment used to produce coarse pearlite by austenitizing, followed by slow cooling.
Normalizing
In steel, a heat treatment used to produce fine pearlite by austenitizing, followed by rapid cooling.
Crazing
Localized plastic deformation in a polymer. Occurs when local stresses cause separation between polymeric chains. The craze produces a translucent region, which looks like a crack but is capable of sustaining stress.
A glass experiences a sudden change in density when cooling from the melt.
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A larger Weibull modulus indicates a higher strength of a material.
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A larger Weibull modulus indicates a larger strength.
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A refractory material is one that readily forms a glass.
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Al alloys are usually highly resistant to fatigue damage.
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Al alloys usually suffer from a ductile to brittle transition at low temperature.
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Al and Ti are categorized as refractory metals.
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Ceramics have large tensile strengths but low compressive strengths.
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Dendritic growth is promoted when the temperature of the liquid just ahead of a solid/liquid interface is larger than the freezing temperature.
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For effective dispersion strengthening, the dispersed phase should be continuous.
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For effective dispersion strengthening, the dispersed phase should be needle-like, as opposed to round.
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Hot working strengthens metals.
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In ceramics, a large porosity results in large strength.
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Martensitic transformations only occur in the Fe-C system.
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Mg alloys are highly ductile.
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Precious metals typically have poor corrosion resistance.
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Pure metals can be age hardened.
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Sand casting creates good surface finishes and tolerances compared to other casting techniques.
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Slip casting is forming of a ceramic by pushing through a die of a constant cross-section.
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Stoichiometric intermetallic compounds exist over a range of compositions.
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The glass transition temperature refers to when a polymer melts.
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Thermoset plastics are easily recycled.
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Thermoset plastics are generally more ductile than thermoplastics.
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True stress is always equal to or smaller than engineering stress.
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The dividing point between steels and cast irons is 1% C.
F (2.11% C.)
Homogeneous nucleation commonly occurs on the surfaces of casts.
F (Heterogeneous nucleation commonly occurs on the surfaces of casts)
Homogeneous nucleation usually occurs in a well-inoculated liquid.
F (Heterogeneous nucleation usually occurs in a well-inoculated solution)
Both cold working and hot working generally strengthen metals.
F (Hot working generally does not strengthen materials.)
Hardenability refers to the ease with which a steel can be quenched to form pearlite.
F (Refers to ability to form martensite.)
Hot working only occurs at temperatures above room temperature.
F (Some metals have recrystallization temperatures below room temperature.)
What is a refractory metal? What are some examples?
A refractory metal is a metal with an exceptionally high melting temperature, above 1925ºC. Consequently, they have potential for high-temperature applications. Examples include W, Mo, Ta, and Nb.
Process annealing
A low-temperature heat treatment used in steels to eliminate residual stresses and/or all or part of the effects of cold working.
What is a precious metal? What are some examples?
A precious metal is a metal that is rare and/or has high monetary value. They are good conductors of electricity and resist corrosion. Examples include Au, Ag, Pd, Pt, and Rh.
What are coherent and incoherent precipitates? Which produces higher strength alloys?
Coherent precipitates have an atomic arrangement that matches (has a continuous relationship) its surrounding matrix. This arrangement displaces atoms in the matrix, producing strain fields in the matrix. Incoherent precipitates have an atomic arrangement that does not match (has no relationship to) its surrounding matrix. These precipitates do not disrupt the matrix. Coherent precipitates are stronger due to their disrupting the matrix.
Sand Casting
Pros: - Inexpensive - Can make large components - Easy to demold Cons: - Poor finish and tolerances
Die Casting
Pros: - Smooth finish - Dimensional accuracy - Fast casting Cons: - Mold costs - Cannot do complex shapes
Investment Casting
Pros: - Tight tolerance - Can make complex, thin-walled structures Cons: - Higher cost - Time consuming
What is the Bauschinger effect? Describe typical microstructural processes that lead to the Bauschinger effect in metals.
The Bauschinger effect refers to a phenomenon in which a material that has been plastically deformed under a tensile stress shows a reduction in compressive strength, and vice versa. Plastic deformation during cold working can result in piling up of dislocations at barriers such as grain boundaries. When stresses are applied in the reverse direction, the dislocations would energetically prefer to "unpile," so it requires lower stresses to move dislocations in the opposite direction. As a result, the material will plastically deform at a lower stress than in the absence of "piled-up" dislocations, thus lowering the yield strength.
