Stuctures

Question 1

define crystalline structure

Answer 1

repeating arrangement of atoms (molecules or ions)

Question 2

define amorphous structure

Answer 2

no organization (not a crystalline structure), random

Question 3

define polycrystalline materials

Answer 3

solids that consist of many small crystals

Question 4

describe how polycrystalline are formed

Answer 4

during solidification crystals start to form at multiple locations they continue to grow until they meet each other, with their orientation is random.

Question 5

define unit cell

Answer 5

the smallest repeating unit of the pattern in a crystalline structure

Question 6

what structure does an HCP

Answer 6

Forms a closely packed hexagonal unit cell 737, Hexagonal Closely Packed (HCP)

Question 7

what structure does FCC have

Answer 7

One atom in the center of each face and each corner 545, Face Centred Cubic (FCC)

Question 8

what structure does BCC have

Answer 8

One atom at centre and one at each corner 414, Body Centred Cubic (BCC)

Question 9

explain Edge Dislocations

Answer 9

• extra portion of a plane of atoms(half-plane), which end within the crystal
• can be generated at grain boundaries and crystal defects
• the shear stress must act across the slip plane

Question 10

different types of point defect

Answer 10

• Vacancies, vacant atomic sites in a lattice
• Self-interstitials, extra atoms positioned between lattice sites
• Impurities, the presence of foreign atoms in pure lattice

Question 11

what are grain boundaries

Answer 11

• caused when crystal structures with different orientations meet
• forming intermittent bonding
• reduce thermal andelectrical conductivity

Question 12

explain Work Hardening

Answer 12

If a material is deformed plastically and then stress is released, it becomes harder to deform the second time a stress is applied. Due to the formations of dislocations.
- No change in Young’sModulus
- Yield strength and Strength increase
- Ductility decreases

Question 13

explain Solid solution hardening

Answer 13

a process used to increase the strength and hardness of a metal alloy by introducing foreign atoms into its crystal lattice structure, they dissolve into the solid solution of the base metal.

Question 14

Is a higher shear stress require for a closely or sparsely packed plane

Answer 14

closely

Question 15

name the two types of solid solution hardening

Answer 15

• substitutional, the substitution of an atom with another element to change its properties
• interstitial, the addiction of foreign atoms within the atomic structure

Question 16

different effects of solid solution hardening when comparing small and larger atoms

Answer 16

Small:
- increased tensile strain
- diffuses to regions of compression

Larger:
- increased compressive strain
- diffuses to regions of tension

Question 17

Why does solid solution hardening reduce ductility

Answer 17

• it prevents dislocations from moving

Question 18

explain precipitation hardening

Answer 18

different combinations of elements in alloys form separate phases, precipitates
- stop dislocation movement