Unit 5 - Crystal Structure and Microstructure

Question 1

What are crystalline materials?

Answer 1

Atoms are arranged in a regular and repeating pattern

Question 2

What are polycrystalline materials?

Answer 2

Solids which are composed of many crystalllites

Question 3

What is an amorphous material?

Answer 3

a solid that lacks the long range order characteristic of a crystal

Question 4

what is crystallography?

Answer 4

The study of crystals and crystal structures

Question 5

Define Crystalline

Answer 5

Atoms or molecules are arranged in regular periodic manner. The degree of crystallinity influences hardness, density, transparency and diffusion

Question 6

Define Polycrystalline

Answer 6

solids that are composed of many crystallites of varying size and orientation

Question 7

Define Amorphous

Answer 7

a solid which lacks the long range order characteristic of a crystal

Question 8

what is a unit cell

Answer 8

simplest repeating unit

Question 9

Describe the three basic lattices which exist for metallic crystal structures

Answer 9

cubic a=b=c all 90
tetragonal a=b not c all 90
hexagonal a=b not two 90 and one 60

Question 10

what affects the yield stress in metals crystal structure

Answer 10

Imperfections in the crystal structure, the dislocations can move within each grain causing the yield stress of metals to be lower than the theoretical limit

Question 11

5 imperfections which act as source of slip

Answer 11

grain boundaries
surface defects
thermal effect at high temps
missing atoms or foreign atoms in the lattice
misalignment of rows of atoms in the crystal structure

Question 12

what is an edge dislocation

Answer 12

when there is an extra half row or plane of atoms distorting nearby planes of atoms

Question 13

what is a screw dislocation

Answer 13

it occurs when the atoms are displaced along the line of the cut

Question 14

3 methods of strengthening

Answer 14

Alloying
work hardening
grain refinement