Chapter 4

Question 1

What is solidification?

Answer 1

Initiated by the casting of molten (liquid) material. First the nuclei of the cold phase form, then crystals grow until their boundaries meet each other, and the crystals become grains.

Question 2

What shapes can grains have?

Answer 2

• Equiaxed (roughly the same dimension in all directions, due to rapid cooling)
• Columnar (elongated in one direction, often due to slow cooling and heat flow)

Question 3

What are grain boundaries?

Answer 3

Regions between grains that have crystallographic misalignment, and atoms near the boundaries have high mobility and chemical reactivity.

Question 4

What is a crystalline defect?

Answer 4

A lattice irregularity with dimensions on the order of an atomic diameter.

Question 5

What are the three types of imperfections?

Answer 5

• Vacancies, interstitial atoms, substitutional impurity atoms (point defects 0D)
• Dislocations (linear defects 1D)
• Grain boundaries (interfacial defects 2D)

Question 6

What are vacancies?

Answer 6

Vacant atomic sites (holes in atomic lattice) that cause inward distortion.

Question 7

What are self-interstitials?

Answer 7

Atoms that are positioned in interstitial sites (the small gaps between atoms) that cause outward distortion.

Question 8

What is the relationship between vacancies and temperature?

Answer 8

The number of vacancies increases as temperature increases.

Question 9

What is Boltzmann’s constant k?

Answer 9

A thermal energy constant having the value of 1.38 × 10−23 J/atom ⋅ K or 8.62 × 10−5 eV/atom ⋅ K.

Question 10

What are the two types of solid solutions?

Answer 10

• Substitutional
• Interstitial

Question 11

What is a host atom?

Answer 11

The primary atom in a solution also called a solvent

Question 12

What is a solute?

Answer 12

An atom of lower concentration in a solution, dissolved in the solvent

Question 13

What are second phase particles?

Answer 13

Different compositions/structures within a solid solution (often when there is a greater solute concentration).

Question 14

What are the conditions for Hume-Rothery rules?

Answer 14

1. difference in atomic radius < 15%
2. proximity in the periodic table (electronegativity)
3. same crystal structure for pure metals
4. valences (metals tend to dissolve other metals of higher valences)

Question 15

What are dislocations and what types are there?

Answer 15

One-dimensional defects around which atoms are misaligned. There are three types: edge, screw, and mixed.

Question 16

What is an edge dislocation?

Answer 16

There is an extra half-plane of atoms inserted into a crystal structure, Burger’s vector is perpendicular to the dislocation line.

Question 17

What is a Burger’s vector b?

Answer 17

A measure of lattice distortion

Question 18

What is a screw dislocation?

Answer 18

A spinal planar ramp resulting from shear deformation, Burger’s vector is parallel to the dislocation line.

Question 19

What is an alloy?

Answer 19

A metallic substance that is composed of two or more elements.

Question 20

When does permanent (plastic) deformation occur?

Answer 20

From dislocation motion caused by applied stresses.

Question 21

What are interfacial (planar) defects?

Answer 21

Grain boundaries, twin boundaries/planes, and stacking faults

Question 22

What is weight percent?

Answer 22

A concentration specification on the basis of weight (or mass) of a particular element relative to the total alloy weight (or mass).

Question 23

What are twin boundaries/planes?

Answer 23

Mirrors that reflect atom positions from one side of a twin plane to the other.

Question 24

What are stacking faults?

Answer 24

Errors in the planar stacking sequence
Ex: in an FCC metal, the normal sequence is ABCABC, so a stacking fault would be ABCABABC

Question 25

What is a catalyst?

Answer 25

Something that increases the rate of a chemical reaction without being consumed. Catalytic reactions often occur at surface defect sites.

Question 26

What is optical microscopy?

Answer 26

Uses light, effective up to 2000x magnification. Potentially requires polishing to remove surface features and etching to change reflectance based on grain orientation.

Question 27

Why do grain boundaries appear as dark lines under microscopes?

Answer 27

They are more susceptible to etching.

Question 28

When is polarized light used?

Answer 28

In metallographic scopes to increase contrast or with transparent samples.

Question 29

What is a scanning electron microscope (SEM)?

Answer 29

A microscope that produces an image by using an electron beam that scans the surface of a specimen; an image is produced by reflected electron beams. Examination of surface and/or microstructural features at high magnifications is possible.

Question 30

What is a scanning probe microscope (SPM)?

Answer 30

A microscope that does not produce an image using light radiation. Rather, a very small and sharp probe raster scans across the specimen surface; out-of-surface plane deflections in response to electronic or other interactions with the probe are monitored, from which a topographical map of the specimen surface (on a nanometer scale) is produced.

Question 31

What is a transmission electron microscope (TEM)?

Answer 31

A microscope that produces an image by using electron beams that are transmitted (pass through) the specimen. Examination of internal features at high magnifications is possible.