Chapter 3 - Structure Of Crystalline Solids

Question 1

What are the properties of materials directly related to? Why is it relevant?

Answer 1

Their crystal structure

• Fracture and plasticity depend on crystal structure
• as well as corrosion, cracking and degradation
  -Strengthening and stabilisation at an atomic level

Question 2

What are the seven group of cubic structures and define their unit cell geometry

Answer 2

1. cubic a=b=c or α=β=γ=90 degrees
2. hexagonal a=b≠c α=β=90o, γ=120o

3.Tetragonal = a=b≠c α=β=90o

4. Rhombohedral a=b=c or α=β=γ ≠ 90
5. Orthohombic a ≠ b ≠ c α=β=γ=90o
6. Monoclinic a ≠ b ≠ c α= γ = 90o ≠ β

7 Triclinic a ≠ b ≠ c α ≠ β ≠ γ ≠ 90o

Question 3

What is a point coordinate?

Answer 3

The position of a point within a unit cell is specified in terms of its coordinates as fraction multiples of the unit cell length

Question 4

Crystallographic points af what is the center on a unit cell?

Answer 4

Right handed coordinate system consist of of three axes situated at one of the corners and coinciding with cell edges

Point coordinates for unit cell corners are 1111 while the center is a/2 b/2, c/2

Question 5

What is families of directions?

Answer 5

Directions in cubic crystals with the same indices without regard to order or sign are equivalent and all belong to the same family but is not true for other crystal systems

Question 6

What is linear density

Answer 6

Linear density of atoms = number of atoms/unit length of direction vector

Linear density of AL in [110] direction

LD = 2/root 2a = 3.5nm-1

Question 7

How many indices are used to describe crystallographic direction [uvtw] in hexagonal crystal structures? What is the coordinate system?

Answer 7

A four axis miller braved coordinate system. Four indices are used to describe crystallographic directions [uvtw] in hexagonal crystal structures. Four indices corresponding to a1, a2, a3 and Z

Question 8

What is planar density?

Answer 8

Number of atoms centered on a plane/ area of a plane

I.e 1/a^2

Question 9

Why does slip occur? (Plastic deformation)

Answer 9

Slip occurs on the most densely packed crystallographic planes in the directions of the greatest atomic packing

Question 10

Crystals as building blocks for ?

Answer 10

Turbine blades, crystals for abrasives, semiconductors i.e quartz fractures more easily along some crystal planes than others

Question 11

What are polycrystals?

Answer 11

Aggregates of lots of little crystals
Each grain is a single crystal
Component properties are not directional
Grains range from 1nm to 2cm

Question 12

What is anisotropy?

Answer 12

Where physical properties of single crystals of some substances depend in the crystallographic direction in which measurements are taken. Directionality of properties is associated with atomic/ionic spacing in the crystallographic direction investigated

Degree if an isotope increases with decreasing structural symmetry- triclinic structures and highly anisotropic

Question 13

What the difference between single and poly crystals

Answer 13

Single
- properties vary with direction anisotropic

Polycrystals
- may or may not vary with direction
-if grains are randomly oriented isotropic
-if grains are oriented material is textured/anisotropic