Metal defects and polycrystilline structures

Question 1

what is a lattice vacancy/ point defect?

Answer 1

a site in the lattice where an atom is missing

Question 2

How common are lattice vacancies (per atom and cm^3)?

Answer 2

1 per 10^15 atoms
1x10^8 per cm^3
increases as temp increases

Question 3

What is thermal agitation and what frequency does it occur at?

Answer 3

atoms with temperature ocillate at a frequency of 10^12 Hz

Question 4

How do lattice vacancies move and how does this affect structure?

Answer 4

thermal agitation allows atoms to move into vacancies leave a new vacancy (thus the vacancy moves)
allows atoms to diffuse through the material and the material to rearange itself

Question 5

What is a line defect (dislocations)

Answer 5

One dimensional defects
in the lattice due to
mismatch between layers

Question 6

How do dislocations aid in slip?

Answer 6

the mismatch of layers allows the layers to slide over each other without all the bonds breaking (thus lower energy and force)

Question 7

how does strain hardening increase hardness

Answer 7

• more dislocations are created
• dislocations create tension and compression in the connected bonds
• when dislocations meet they cancel if oppositely orientated or repel in like orientatged
• this prevents slip form occuring easily

Question 8

What is isotropy?

Answer 8

Having properties that are the same in every
direction

Question 9

What is anisotropy?

Answer 9

Having properties that vary with direction

Question 10

What is a polycrystalline structure

Answer 10

Made
up of many separate
crystals (called grains)

Question 11

how does a polycrystaline structure form

Answer 11

.1 Above melting point atoms randomly collide. Where
several come together at once crystal embryos are
temporarily formed

2. Once the liquid temperature drops below melting point
   embryos become nuclei: small collections of atoms
   bound together
3. As new atoms collide with nuclei they bond to the
   nuclei and the nuclei grow into crystals

Question 12

Why are polycrystals isotropic when each crystal is anisotropic?

Answer 12

Because nuclei formed independently the resulting
crystals have random alignment of their crystal lattices cancelling out into an isotropic material

Question 13

How does crystal size affect yield strength?

Answer 13

Dislocations and vanacies cannot cross crystal boundaries
smaller crystals restrict the slip of the overall material

Question 14

What does the hall-petch equation describe?

Answer 14

the reltionship between crystal strength and crystal grain size

Yield_crystal = Yeild_grain + K/d^1/2
K is strengthening constant
d is mean grain size