Materials Science

Question 1

Ionic bond

Answer 1

where one or more electrons completely transfers from an atom of one element to the atom of another. The force of attraction due to the opposite polarity of the charge holds the element together.

Question 2

Covalent bond

Answer 2

the bond formed by shared electrons when an atom needs electrons to complete its outer shell it shares those electrons with its neighboring atom. The electrons become a part of both atoms, filling both atoms’ electron shells.

Question 3

Metallic bond

Answer 3

the atoms do not share or exchange electrons to bond together. Many electrons, roughly one for each atom, are more or less free to move throughout the metal; each electron can interact with many of the fixed atoms.

Question 4

Molecular bond

Answer 4

a temporary weak charge exists when electrons of neutral atoms spend more time in one region of their orbit than in another region. The molecule weakly attracts other molecules. This molecular bond also called a van der Waals bond

Question 5

Hydrogen bond

Answer 5

similar to the molecular bond, a hydrogen bond occurs because of the ease with which hydrogen atoms are willing to give up an electron to atoms of oxygen, fluorine, or nitrogen.

Question 6

Describe the following types and features of solids:

Amorphous

Answer 6

Amorphous materials have an irregular arrangement of atoms or molecules; they exhibit properties of solids. Amorphous solids do not have a repeating crystalline structure. These materials have definite shape and volume and diffuse slowly; however, they lack sharply defined melting points. As solids, they resemble liquids that flow slowly at room temperature. Glass and paraffin are examples of amorphous materials. Other examples of amorphous materials include thin gels and thin films.

Question 7

Describe the following types and features of solids:

Crystalline solids

Answer 7

Arrays of atoms in regular patterns create crystal structures in metals and other solids. Crystalline structures have repeating units of atoms, ions, and molecules. A crystal structure has atoms arranged in a pattern that repeats periodically in a three-dimensional geometric lattice. Forces associated with chemical bonding result in this repetition and produce properties such as strength, ductility, density, conductivity, and shape. Ductility is the metal’s ability to bend

Question 8

Describe the following types and features of solids:

Grain structures

Answer 8

Examining a thin section of a common metal under a microscope illustrates the molecular structure similar to that shown below in the figure. Each of the light areas is a grain, or crystal, which is the region of space occupied by a continuous crystal lattice. Grain boundaries are the dark lines surrounding the grains. The term grain structure refers to the arrangement of the grains in a metal. Each grain has a particular crystal structure determined by the type of metal and its composition.

Question 9

Body-Centered Cubic (BCC):

Answer 9

The unit cell consists of eight atoms at the corners of a cube and one atom at the body center of the cube in a body-centered cubic (BCC) arrangement of atoms.
These BCC metals have two properties in common, high-strength and low-ductility

Question 10

Face-Centered Cubic (FCC):

Answer 10

In a face-centered cubic (FCC) arrangement of atoms, the unit cell consists of eight atoms at the corners of a cube and one atom at the center of each of the faces of the cube.
These FCC metals generally have lower strength and higher ductility than BCC metals.

Question 11

Hexagonal Close-Packed (HCP):

Answer 11

The unit cell consists of three layers of atoms in a hexagonal close-packed (HCP) arrangement of atoms. The top and bottom layers each contain six atoms at the corners of a hexagon and one atom at the center of each hexagon. The middle layer contains three atoms nestled between the atoms of the top and bottom layers, therefore, the name close-packed.
HCP metals are not as ductile as FCC metals.

Question 12

Point Imperfections… List 3 types

Answer 12

Vacancy Defects
Substitutional Defects
Interstitial Defects

Question 13

Describe Vacancy Defects

Answer 13

Vacancy defects, the simplest defect, result from a missing atom in a lattice position. This defect results from imperfect packing during the crystallization process, or may be due to increased thermal vibrations of the atoms from elevated temperatures.

Question 14

Describe Substitutional Defects

Answer 14

Substitutional defects result from an impurity present at a lattice position. An alloying material added to the metal, such as carbon (carbon steel) creates an impurity at a lattice position.

Question 15

Describe Interstitial Defects

Answer 15

Interstitial refers to locations between atoms in a lattice structure. They result from an impurity located at an interstitial site or one of the lattice atoms being in an interstitial position instead of its lattice position. Interstitial impurities called network modifiers act as point defects in amorphous solids.

Question 16

Describe Edge Dislocations

Answer 16

Edge dislocations consist of an extra row or plane of atoms in the crystal structure, shown below in the figure. The imperfection may extend in a straight line all the way through the crystal, or it may follow an irregular path. The edge dislocation may be short, extending only a small distance into the crystal causing a slip of one atomic distance along the glide plane (direction the edge imperfection is moving).

Question 17

Describe Screw Dislocations

Answer 17

Screw dislocations develop by a tearing of the crystal parallel to the slip direction. A screw dislocation makes a complete circuit, shows a slip pattern similar in shape to that of a screw thread, whether left- or right-handed. It is necessary for some of the atomic bonds to re-form continuously such that after yielding to this location, the crystal returns to the original form in order for another screw dislocation to occur.

Question 18

Describe Mixed Dislocations

Answer 18

The orientation of dislocations varies from pure edge to pure screw, and at some intermediate point, dislocations may possess characteristics of each.

Question 19

Describe Macroscopic (Bulk) Material Defects

Answer 19

: Bulk defects are three-dimensional macroscopic material defects. They generally occur on a much larger scale than microscopic defects, usually introduced into a material during refinement from its raw state or during the material’s fabrication processes.
The most common bulk defect arises from inclusion of foreign particles in the prime material. Called inclusions, they undesirably alter the material’s structural properties. Examples of inclusions include oxide particles in a pure metal or a bit of clay in a glass structure.

Question 20

Describe the common characteristics of alloys

Answer 20

Alloys are usually stronger than pure metals, although generally with reduced electrical and thermal conductivity. Strength is one of the most important criteria for judging many structural materials. Therefore, for industrial construction normally the preferred choice is alloy over pure metals

Question 21

Identify the desirable properties of type 304 stainless steel

Answer 21

Type 304 stainless steel, which contains 18 to 20 percent chromium and 8 to 10.5 percent nickel, is extremely tough and corrosion resistant. Used extensively in applications where corrosion is a concern, Type 304 Stainless Steel resists most, but not all, types of corrosion.

Question 22

Strength

Answer 22

Strength is the ability of a material to resist deformation. The strength requirements of a structure equal the maximum load that can be borne before failure occurs

Question 23

Ultimate tensile strength

Answer 23

The ultimate tensile strength (UTS) is the maximum resistance a material presents to fracture. It is equivalent to the maximum load capability of one square inch of cross-sectional area with the load applied as simple tension.

Question 24

Yield strength

Answer 24

Yield strength is the term for identifying the stress where plastic deformation starts. The yield strength is the stress where a predetermined amount of permanent deformation occurs.

Question 25

Ductility

Answer 25

Ductility is the ability of a material to deform easily on the application of a tensile force, or the ability of a material to withstand plastic deformation without rupturing. Ductility also considers factors such as bendability and crushability.

Question 26

Malleability

Answer 26

Where ductility is a material’s ability to deform easily on the application of a tensile force, malleability is a metal’s ability to exhibit large deformation or plastic response when subjected to compressive force.

Question 27

Toughness

Answer 27

Toughness describes the way a material reacts under sudden impacts. Toughness is the work required to deform one cubic inch of metal until it fractures.

Question 28

Hardness

Answer 28

Hardness is the property of a material enabling its resistance to plastic deformation, penetration, indentation, and scratching. Hardness is important from an engineering standpoint because resistance to wear by friction or erosion from steam, oil, water flow, and so forth, generally increases with hardness.

Question 29

Heat treatment

Answer 29

Large carbon steel components undergo heat treatment that takes advantage of metallic crystalline structures and their effects on the metal to gain certain desirable properties. Toughness and ductility decrease as hardness and tensile strength increase in heat-treated steel. Heat treatment is unsuitable for increasing the hardness and strength of type 304 stainless steel because of its crystalline structure.

Question 30

Cooling (Quenching)

Answer 30

Varying the rate of quenching or cooling the metal, allows control of the grain size and grain patterns in the metal material during manufacture. Grain characteristics produce different levels of hardness and tensile strength. Generally, the faster a metal cools, the smaller the grain size. Smaller grain size yields a harder metal.
The cooling rate used in quenching depends on the method of cooling and the size of the metal. Uniform cooling is important because it prevents distortion. Steel components typically use oil or water for quenching.

Question 31

Annealing

Answer 31

Annealing is another common heat-treating process for carbon steel components. The annealing process is where component heating occurs slowly to an elevated temperature then held there for a long time and cooled. Annealing results in the following effects:
• Softens the steel and improves ductility.
• Relieves internal stresses caused by previous processes such as heat treatment, welding, or machining.
• Refines the metal’s grain structure.

Question 32

Cold working

Answer 32

Cold working is plastic deformation in a particular temperature region and over a specific time interval such that the strain, or work hardening, is not relieved.
Cold working a metal decreases the metal’s ductility. The decreased ductility results from the cold working process, which repeatedly deforms the metal. Slip occurs essentially on primary glide planes and the resulting dislocations form coplanar arrays in the early stages of plastic deformation. Cross slip takes place as deformation proceeds. The cold worked structure forms high dislocation density regions that eventually develop into networks. The grain size decreases with strain at low deformation; however as deformation continues, the grains reach a fixed size. Altering the metal’s grain size during the cold working process causes the decrease in ductility.

Question 33

Hot working

Answer 33

Hot working refers to the process where metal deformation happens above the re-crystallization temperature and prevents strain hardening from occurring.

Question 34

Describe General corrosion and methods for controlling

Answer 34

General corrosion caused by water, steel, or iron often results from a chemical reaction where the steel surface oxidizes, forming iron oxide, or rust.
Using materials with corrosion-resistant composition, such as stainless steel, nickel, chromium, and molybdenum alloys
Using protective coatings, such as paints and epoxies prevents corrosion:
Corrosion is electrochemical by nature, and the corrosion resistance of stainless steel results from surface oxide films that interfere with the electrochemical process.
Applying surface metallic and nonmetallic coatings or linings protects against corrosion and allows the material to retain its structural strength. For example, a carbon steel pressure vessel lined with stainless steel cladding.

Question 35

Describe Galvanic corrosion and methods for controlling

Answer 35

Galvanic corrosion occurs when two dissimilar metals with different electrical potentials are in electrical contact in an electrolyte. It may also take place within one metal with heterogeneities (dissimilarities such as impurity inclusions, grains of different sizes, differences in grain composition, or differences in mechanical stress).
Galvanic corrosion is a concern in both design and material selection. Material selection is important because different metals may contact each other and form galvanic cells. Design is important to minimize system low flow conditions and resultant areas of corrosion buildup. Loose corrosion products transport through systems and deposit in low-flow areas, causing additional flow restrictions. Substances formed by corrosion and exposed to radiation become highly radioactive, further increasing radiation levels and contamination issues in nuclear plants.

Question 36

Stress corrosion cracking

Answer 36

Stress corrosion cracking (SCC), a form of intergranular attack corrosion occurring at the grain boundaries of a metal under tensile stress, is one of the most serious metallurgical problems.

Question 37

Chloride stress corrosion

Answer 37

Chloride stress corrosion, an intergranular type of corrosion in austenitic stainless steel under tensile stress in the presence of oxygen, chloride ions, and high temperature, is of tremendous concern to the nuclear industry.

Question 38

Caustic stress corrosion

Answer 38

Carbon steels are susceptible to caustic stress corrosion. Similarly to other types of localized corrosion, caustic steel corrosion begins when cracks form and grow along the grain boundaries combined with extensive crack branching. High-tensile stress external to the steel or within the steel’s fabrication cause caustic stress corrosion.

Question 39

Describe hydrogen embrittlement

Answer 39

Hydrogen embrittlement is the process whereby steel loses its ductility and strength due to tiny cracks resulting from the internal pressure of hydrogen (H2) or methane gas (CH4) that form at the grain boundaries. Hydrogen embrittlement is a particular concern in the nuclear industry because of the susceptibility of zirconium alloys to this type of corrosion.

Question 40

Fatigue failure

Answer 40

Fatigue causes the majority of engineering failures. Fatigue failure is a material’s tendency to fracture by means of progressive brittle cracking under repeated alternating or cyclic stresses at levels considerably below the normal strength.

Question 41

Work hardening

Answer 41

Work hardening occurs when straining a metal beyond the yield point in the ductile region. Increasing stress produces additional plastic deformation and causes the metal to become stronger and more difficult to deform.

Question 42

Creep

Answer 42

Structural materials develop the full strain they will exhibit as soon as a load is applied at room temperature. This is not necessarily the case at high temperatures (for example, stainless steel above 1,000 °F). Many materials continue to deform at a slow rate at elevated temperatures and constant stress or load, which demonstrates creep behavior.