Crystal structures and lattice defects

Question 1

Name three materials with polycrystalline structures.

Answer 1

Steels, aluminium, titanium alloys

Question 2

What is a polycrystalline structure?

Answer 2

Collection of crystals called grains that are joined together, in each grain atoms are arranged into long-range patterns of atomic order with a repetitive 3D pattern

Question 3

Name three materials with hexagonal close packed (HCP) structure?

Answer 3

Alpha-titanium, zinc, zirconium

Question 4

Name three materials with face-centred cubic (FCC) structure?

Answer 4

Nickel, aluminium, gamma-iron

Question 5

Name four materials with body-centred cubic (BCC) structure?

Answer 5

Chromium, beta-titanium, alpha-iron, beta-iron

Question 6

What causes lattice defects?

Answer 6

-Crystals are not perfect
-Disruptions in their lattice arrangements
-Atoms may be positioned in a non-lattice site, or an atom may be missing at a given lattice size

Question 7

What are the different types of crystal defect?

Answer 7

-Point
-Line
-Surface
-Volume

Question 8

What are some examples of point defect?

Answer 8

Vacancies, interstitial and substitutional atoms

Question 9

How are vacancy point defects introduced?

Answer 9

Thermal vibrations of atoms or from imperfect packing during crystallisation

Question 10

How can a solute atom be incorporated into a host lattice?

Answer 10

Substitutionally, Interstitially

Question 11

What happens when an interstitial atom is inserted into a crystal lattice?

Answer 11

Neighbouring atoms become displaced radially away from the interstitial solute atom

Question 12

How to point defects affect a material stress/strain state?

Answer 12

They induce local distortions, who’s strain fields will have hydrostatic and deviatoric contributions which result in the straining of the crystal which induces a stress field

Question 13

What are line defects?

Answer 13

Dislocations that are characterised by a chain of atoms that are incorrectly places in the lattice

Question 14

Name three types of line defect/dislocation.

Answer 14

-Edge
-Screw
-Mixed

Question 15

How do dislocations interact with each other?

Answer 15

Through their elastic fields, this self induces stress and strains within the crystal

Question 16

What is glide (in terms of dislocations)?

Answer 16

Applied shear stresses induce a force on the dislocations, causing them to move on close-packed planes

Question 17

How are plastic deformations induced by dislocations?

Answer 17

Shear stresses lead to a displacement of the dislocation through repeated breaking an formation of bonds

Question 18

How are slip bands measured experimentally?

Answer 18

Using electron-back-scatter-diffraction (EBSD) and digital image correlation (DIC), where the slip bands are revealed as ‘lines’ of high strain

Question 19

Name three types of surface defect.

Answer 19

-Twins
-Staking faults
-Grain boundaries

Question 20

What physical change can alter grain boundaries?

Answer 20

Temperature

Question 21

Name three types of volume defect.

Answer 21

-Voids
-Cracks
-Foreign inclusions

Question 22

How are volume defects introduced into a crystal lattice?

Answer 22

Unintentionally during processing or fabrication