Microstructure of Metals and Alloys

Question 1

What are the three categories of microstructures of metals and alloys?

Answer 1

Basic crystal structures, alloys, Single crystal and polycrystals

Question 2

What are the three basic crystal structures?

Answer 2

Body-centered cubic (bcc), Face-centered cubic (Fcc), Hexagonal close packed cubic (hcp)

Question 3

What are the two alloy forms of microstructures?

Answer 3

Solid solution and intermetallic compound.

Question 4

What are atoms made of?

Answer 4

A positively charged nucleus and enough negatively charged electrons to balance the charge.

Question 5

What determines the atomic number and element?

Answer 5

Number of protons (which is equal to the number of electrons).

Question 6

What are the two types of bonds found in molecules?

Answer 6

Primary and secondary bonds. Primary bonds lead to the formation of molecules, while secondary bonds are found between attracting molecules.

Question 7

What is the difference between an atom and a molecule.

Answer 7

An atom is the smallest unit of matter that still has all of the properties of its element, while a molecule is a structure that contains multiple atoms bonded together

Question 8

Primary Bonds

Answer 8

Characterized by strong atom-to-atom attractions that involve the exchange of valence electrons.

Question 9

What are the three types of primary bonds.

Answer 9

Ionic bonds, covalent bonds, and metallic bonding.

Question 10

Ionic Bond

Answer 10

The atoms of one element give up their outer electrons which are then attracted to the atoms of another element. This increases their valence electron count to 8.

Question 11

Covalent Bond

Answer 11

Electrons are shared (instead of transferred) between the outermost shells to create a stable configuration.

Question 12

Metallic Bonding

Answer 12

Takes place in pure metals and metallic alloys. The electrons in the outer shell are shared by all atoms to create a general electron cloud. This electron cloud allows for electrical conductivity.

Question 13

Secondary Bonds

Answer 13

Bonds that do not involve the transfer or sharing of electrons. This makes them weaker than primary bonds.

Question 14

What are the three types of secondary bonds?

Answer 14

Dipole forces, london forces, and hydrogen bonding.

Question 15

Dipole Forces

Answer 15

These forces can be found in a molecule made up of two atoms that have an equal and opposite charge.

Question 16

London Forces

Answer 16

These forces can be found in non-polar molecules. Due to the rapid motion of electrons in orbit they form temporary dipoles when more electrons happen to be on one side of the molecule than the other.

Question 17

Hydrogen Bonding

Answer 17

Occurs in molecules with hydrogen stoms that are covalently bonded to another atom.

Question 18

Crystalline

Answer 18

Atoms are located in regular or reoccuring positions in three dimensions.

Question 19

Non-Crystalline

Answer 19

An absence of long-range order in the molecular structure.

Question 20

Body-centered cubic (BCC)

Answer 20

Made up of atoms organized in a cube with one atom in each corner and one atom in the center.

Has a repeating pattern of 8.

Question 20

Face-centered cubic (FCC)

Answer 20

Made up of atoms organized in a cube with one atom in each corner and one atom in the center of each face. Is more space efficient than BCC.

Has a repeating pattern of 12.

Question 21

Hexagonal close-packed

Answer 21

In a hexagonal shape.

Has a repeating pattern of 12.

Question 22

Diamond cubic

Answer 22

Has a repeating pattern of 8. Is found materials like semiconductors and diamonds.

Does not follow the Bravais lattic definition.

Question 23

Allotropism

Answer 23

When materials undergo a change in crystal structure at different temperatures. An example is Iron (Fe)

Question 24

Imperfections

Answer 24

Arise naturally due to the solidifying material’s inability to continue the unit cell’s replication indefnitely without interruption.

Can also be introduced on purpose during the manufacturing process (adding an alloying ingredient in a metal to increase its strength).

Question 25

Point defects

Answer 25

Imperfections in an atom or a few atoms.
E.g. vacancy, ion-pair vacancy, intersitialcy, displaced ion

Question 26

Line defects

Answer 26

A connected group of point defects that forms a line in the lattice structure.

E.g. edge dislocation, screw dislocation

Question 27

Surface defects

Answer 27

Imperfections that extend in two directions to form a boundary.

E.g. grain boundary

Question 28

Slip

Answer 28

Deformation mechanism involving relative movement of atoms on opposite sides of lattice slip plane. Mostly in BCC and HCP.

Enabled by dislocation irregularities.

Question 29

Twinning

Answer 29

Deformation mechanism involving plastic deformation where atoms on one side of the twinning plane shift and form a mirror image on the other side. Most important in HCP (because does not slip easily).

Question 30

What are grain boundaries and what affects grain size?

Answer 30

Interference of crystals in nucleation sites that form at interface surface.

Size is inversely related to cooling rate: fast=small =higher strength & hardness. slow=large

Question 31

Strain hardening

Answer 31

Process that makes metal stronger and harder due to plastic deformation. Occurs at a temperature low enough that atoms cannot recrystalize.

Question 32

Recovery

Answer 32

Stress is relieved in highly deformed regions.

Takes place below re-crystallization temperature. (<0.3~0.5 Tm)

Question 33

Re-crystallization

Answer 33

New strain free grains are formed, replacing the old grains.

(=0.3~0.5 Tm)

Question 34

Grain growth

Answer 34

Grains begin to grow and exceed the original size and form a rought surface appearance.

Above re-crystallization temperature. (>0.3~0.5 Tm)

Question 35

Cold working

Answer 35

Plastic deformation takng place below 0.3Tm

Question 36

Warm working

Answer 36

Plastic deformation taking place between 0.3Tm-0.5Tm.

Question 37

Hot working

Answer 37

Plastic deformation taking place above 0.5Tm.

Usually carried out above the re-crystallization temperature and below 0.75Tm.

Question 38

Annealing

Answer 38

Heating metal to a certain temperature, holding (soaking), and then slowly cooling.

Reduces hardness/brittleness, alters microstructures, re-crystallizes cold-worked metals, relieves residual stresses.

Question 39

Hot Shortness

Answer 39

When alloys fracture in a brittle manner along grain boundaries when deformed at temperatures near the melting point

Question 40

Dislocation

Answer 40

Linear irregularity within a crystal structure that contains an abrupt change in the arrangement of atoms. The movement of dislocations allow atoms to slide over each other at low stress levels and is known as glide or slip.