Crystal Defects

Question 1

Explain the substitution defect

Answer 1

Substitution Defect –An impurity atom replaces an atom of the base metal. This can distort the lattice if the impurity atom is too big or too small for the lattice (crystal structure).

Question 2

Define a substitution defect

Answer 2

A substitutional; defect is where an atom of a different type fills a position in the lattice.

Question 3

How does a substitutional defect strengthen the metal?

Answer 3

If the foreign atom is a different size than the base metal atom, the lattice is distorted, and the resulting stresses make it more difficult for dislocations to move across the lattice, and therefore the metal is strengthened.

Question 4

List the 6 metal defects

Answer 4

Dislocation/line defect
Substitution defect
Vacant site defect
Interstitial defect
Self interstitial atom
Cluster of impurity atoms

Question 5

Explain the interstitial defect

Answer 5

Interstitial Defect - An additional impurity atom embeds itself in between the atoms of the base metal. This distorts the lattice.

Question 6

Define the interstitial defect

Answer 6

An interstitial defect is where an impurity or alloying atom sits in the space between the lattice atoms.

Question 7

How does an interstitial defect strengthen the metal?

Answer 7

This distorts the lattice and creates internal stresses that prevent dislocations moving across the lattice, and therefore strengthens the metal.

Question 8

Explain the vacant site defect

Answer 8

Atom missing from lattice. The density of the metal is reduced. This can result in a weaker and less conductive metal.

Question 9

Define a vacant site defect

Answer 9

a vacant site defect is where an atom is missing in the lattice

Question 10

How does a vacant site defect strengthen the metal?

Answer 10

This distorts the lattice structure because other atoms are forced inward toward the vacant space.
These extra stresses in the lattice make it more difficult for dislocations to move across the lattice, and so the metal is strengthened.

Question 11

Explain how clusters of impurity atoms affect the metal

Answer 11

Cluster of impurity atoms -This distorts the crystal structure.

Question 12

Explain a dislocation defect

Answer 12

Dislocation -A line of atoms is missing.

A dislocation creates some stress but a larger weak point in the lattice - where a small amount of force can move the dislocation along the lattice.

Question 13

Define a dislocation

Answer 13

a dislocation is an incomplete line in a lattice. Dislocations are the main reason that metals are ductile.

Question 14

What is the easiest way to create slip between atoms

Answer 14

The moving of dislocations is the easiest way to create slip between atoms.

Question 15

What is the main way that plastic deformation happens in metals?

Answer 15

Dislocations are the main way that plastic deformation happens in metals.

Question 16

What do dislocations do to metals?

Answer 16

They increase ductility and lower the yield strength and hardness.

Question 17

What is the exception to dislocations weakening a metal?

Answer 17

If you have a very large amount of dislocations, they have the opposite effect and they harden and strengthen the metal.

Question 18

What happens when there are too many dislocations in a metal?

Answer 18

If there are too many dislocations in a metal, the dislocations cannot move because they bump into too many other dislocations.

Question 19

Where do line defects happen?

Answer 19

Line defects happen in work hardening. Cold-working (rolling, hammering, etc) creates lots of dislocations in the metal, preventing the movements of dislocations and making the metal hard and brittle.

Question 20

Explain what a self-interstitial atom defect is.

Answer 20

Self-interstitial atom - The atom of the parent material embeds itself in between the other atoms. This distorts the crystal structure.

Question 21

When do metals not contain any dislocations?

Answer 21

Metals do not contain any dislocations when first cooled or annealed.

Question 22

What must be noted about the dislocation/line defect?

Answer 22

When there are too many dislocations, the metal is strengthened.
When they are not in excess, they weaken the piece as slip can easily occur between atoms.

Question 23

How do dislocations/line defects move? What happens when force is applied?

Answer 23

Using force the line defect can be “pushed” to the edge of the grain. However, when one dislocation meets another dislocation there is nowhere to go and slip cannot occur. This makes the metal harder.