Metals II - C5

Question 1

How do dislocations reduce ductility, malleability, and increase brittleness in alloys?

Answer 1

In alloys different atoms of sizes or bonding pin down the lattice, increasing the yield strength.

Question 2

Alloys increase d…..y and c….n resistance

Answer 2

ductility, corrosion

Question 3

Why is the model of a whole plane of ions moving for the dislocation model wrong?

Answer 3

This requires more energy and therefore more stress, assuming the strength is 1000x greater.

Question 4

What is the correct dislocation model?

Answer 4

The material moves ion by ion past the incomplete plane so neighbouring layers are displaced to absorb the strain. This increases toughness. Less stress is needed. The mobile dislocation moves opposite to ions.

Question 5

At smaller distances, repulsive forces increase in size …. than attraction. These forces are balanced at ….

Answer 5

faster, equilibrium.

Question 6

What happens in elastic and plastic deformation?

Answer 6

ELASTIC - Bonds are stretched.

PLASTIC - Metallic bonds are broken by external force.

Question 7

What point defects are there?

Answer 7

VACANCY - lack of atom.
INTERSTITIAL - a small atom fills the gap of a full lattice.
SUBSTITUTIONAL - an atom replaces another.

Question 8

What are line defects?

Answer 8

A lack of atoms in a crystal plane causes layers to slip past each other, letting the layer move to the edge. Bonds in atoms break so they can reach the half row.

Question 9

What are the two surface defects?

Answer 9

Grain boundaries stop crystal planes from slipping and are more prone to corrosion.
External surfaces stop the lattice but are more reactive.

Question 10

A density is calculated using the microscopic tools for iron. The density measured is less than this. Why?

Answer 10

• The calculation assumes the spheres fit perfectly together.
• A measurement is done at lower temperature so the atoms are closer together.
• The volume of the calculated value is less than the volume for the actual value.

Question 11

When temperature rises, iron contracts. How does this affect its properties?

Answer 11

The ions are packed closer together. Therefore the iron has a greater density. As a result, the same mass is in a smaller volume. Iron is also stronger and harder.