Final T/F

Question 1

Elastic modulus is proportional to the curvature of U(r) at the bottom of the well.

Answer 1

True, the elastic modulus is related to the second derivative of the potential energy curve U(r) with respect to displacement (r). The curvature at the bottom of the potential well is related to the stiffness of the material, which is quantified by the elastic modulus.

Question 2

Ductility in metals is determined by crystal structure, slip systems, and bonding.

Answer 2

True, ductility in metals is influenced by various factors, including the crystal structure, the presence of slip systems (crystallographic planes and directions along which dislocations move), and the nature of atomic bonding. These factors collectively determine how a metal can deform plastically without fracturing.

Question 3

Mechanical properties do not depend on crystallographic orientation.

Answer 3

False, mechanical properties, such as strength and ductility, often depend on the crystallographic orientation of a material. Anisotropic materials exhibit different mechanical properties in different crystallographic directions due to variations in atomic arrangement.

Question 4

Low-temperature ductile to brittle transitions occur in BCC metals due to thermally activated dislocation motion.

Answer 4

True, BCC (Body-Centered Cubic) metals can undergo a transition from ductile to brittle behavior at low temperatures due to a lack of thermally activated dislocation motion. This transition is often associated with a shift from dislocation-mediated plastic deformation to cleavage fracture.

Question 5

Elastomers have a lower degree of crosslinking than do thermosets.

Answer 5

True, elastomers typically have a lower degree of crosslinking compared to thermosetting polymers (thermosets). Crosslinks restrict polymer chain movement and contribute to the elasticity of elastomers.

Question 6

The nose on a TTT curve indicates the maximum growth rate.

Answer 6

False, the nose on a Time-Temperature-Transformation (TTT) curve indicates the onset of a phase transformation, not the maximum growth rate. The maximum growth rate is typically observed at a specific temperature within the transformation range.

Question 7

Defects are always present in materials.

Answer 7

True

Question 8

Over-aging a precipitation-hardenable alloy results in too many precipitates for optimal strength.

Answer 8

False, over-aging a precipitation-hardenable alloy means that the alloy has been heat-treated for too long or at too high a temperature. This can lead to a reduction in the number of precipitates and a decrease in strength, not an increase.

Question 9

8620 steel exhibits higher hardenability than 8660 steel and lower maximum HRC.

Answer 9

False, the statement is contradictory. Hardenability refers to the ability of a steel to be hardened by heat treatment, and it is generally determined by the alloy composition and cooling rate. The HRC (Rockwell hardness) of a steel is a measure of its hardness. The comparison provided in the statement is not accurate without additional context.

Question 10

The strength of ceramics is inversely proportional to the volume of the component.

Answer 10

False, the strength of ceramics is not necessarily inversely proportional to the volume of the component. Ceramics can have varying strengths depending on factors like composition, microstructure, and the presence of defects.

Question 11

Ceramics are much weaker in compression than in tension.

Answer 11

True, ceramics are generally stronger in tension than in compression due to their brittle nature. They are more prone to failure when subjected to compressive stress.

Question 12

Hot working metals causes an increase in dislocation density, thus higher strength.

Answer 12

True, hot working of metals, such as forging or rolling at elevated temperatures, introduces a high density of dislocations into the material, which can lead to an increase in strength and other desirable properties.

Question 13

As grains get smaller in a polycrystalline metal, the yield strength gets larger.

Answer 13

True, in general, as the grain size in a polycrystalline metal decreases, the material’s yield strength tends to increase. This is known as the Hall-Petch relationship.

Question 14

Welding highly hardenable steel is difficult because of the tendency to form pearlite.

Answer 14

False, highly hardenable steels are typically used for applications where high hardness and strength are desired. Welding such steels can be challenging due to issues like cracking, but the formation of pearlite is not the primary concern. Pearlite is a microstructural constituent formed during the cooling of eutectoid steel, not highly hardenable steel.

Question 15

Fick’s first law cannot be used if the concentration gradient changes with time.

Answer 15

False, fick’s first law, which describes diffusion, can still be used if the concentration gradient changes with time. However, in such cases, more complex mathematical models may be required to account for the changing gradient.

Question 16

Like thermoplasts, thermosets exhibit viscous flow.

Answer 16

False, unlike thermoplastics, thermosetting polymers do not exhibit viscous flow upon heating. Thermosets undergo irreversible crosslinking reactions when heated, which results in them becoming rigid and unable to flow or deform like thermoplastics.

Question 17

The yield stress is the stress at which dislocations begin to move in metals.

Answer 17

True, the yield stress in metals is the stress at which plastic deformation begins, typically when dislocations start to move. It marks the transition from elastic deformation to plastic deformation.

Question 18

Galvanic protection of metals requires external application of a current.

Answer 18

False, galvanic protection (cathodic protection) of metals involves creating a galvanic cell where the metal to be protected acts as the cathode. It does not necessarily require an external current source, as the potential difference between the protected metal and a sacrificial anode can drive the protective process.

Question 19

Viscoelastic response occurs in polymers at or near the glass transition temperature.

Answer 19

True, viscoelastic behavior, which combines both elastic and viscous responses, is commonly observed in polymers at or near their glass transition temperature (Tg).

Question 20

The diffusion constant increases exponentially with temperature.

Answer 20

True, the diffusion constant, typically represented as D in Fick’s law, increases exponentially with temperature as described by the Arrhenius equation. Higher temperatures lead to faster diffusion rates.

Question 21

Heterogeneous nucleation is always more favorable than homogeneous nucleation.

Answer 21

False, heterogeneous nucleation, which occurs at nucleation sites or on the surfaces of foreign particles, is often more favorable than homogeneous nucleation, which occurs in the bulk of the material. However, it is not always the case, as the favorability depends on various factors, including the nature of the material and the specific conditions.

Question 22

Aging Al 2014 at higher temperatures will decrease the peak yield strength.

Answer 22

True, aging Al 2014 at higher temperatures can lead to coarsening of the precipitates, reducing their effectiveness in strengthening the alloy and decreasing the peak yield strength.