L5 - Elastic And Plastic Mechanical Properties

Question 1

What are the types of defects?

Answer 1

• Point defects
• Line defects (dislocations)
• Volume defects

Question 2

Why are line defects important?

Answer 2

• are what assist plastic deformation to occur
• ‘slip’ requires the breaking and reforming of rows of atoms at a time
• less energy needed than slipping in a perfect plane

Question 3

What are volume defects?

Answer 3

• Grain Boundaries
• Particle Inclusions:
• internal grain boundary with differing crystal structure
• Cracks and voids

Question 4

What are point defects?

Answer 4

• Vacancies
• Interstitial atoms
• Self-Intersitial atoms
  
  • atom in the wrong position
• Substitutional atoms

Question 5

What affect does crystallographic structure have on the propagation of dislocations?

Answer 5

• close packed structures, especially close packed planes allow slip to occur easily along them
• FCC and to an extent HCP allow easy dislocation movement

Question 6

How many slip systems do different crystallographic structures have?

Answer 6

• FCC has 12
• 4 planes with 3 directions each
• BCC has 12-24
• not close packed, thus not instantly available
• needs to be heated
• HCP has 3-6
• less than FCC
• most dislocations do not occur on close packed planes either

Question 7

What is twinning?

Answer 7

• when dislocation motion can no longer occur
• results in crystallographic structure being sheared
  
  • no bonds are broken but orientation of atoms relative to each other is change
• Stress needed for twinning is greater and less temperature sensitive than slip
• occurs in BCC and HCP at low temp, rare in FCC

Question 8

How does yield stress vary between crystallographic structures?

Answer 8

• FCC: temperature independent
• slip is easy due to number and availability of slip systems
• BCC: temperature dependent
• no close packed planes, thus requires thermal activation
• decrease with increase in temperature
• HCP: temperature dependent
• limited close-packed slip systems
• decrease with increase in temperature

Question 9

What are the rules relating to alloying?

Answer 9

• Atomic size should be within 15% difference from original atoms
• Should be comfortable in being in the original crystallographic structure
• Electronegativity should be similar to avoid reactions
• Valency should be lower than that of the original atoms
  
  • metals dissolve more easily into those with higher valency

Question 10

How does Solid Solution strengthening work?

Answer 10

• is an alloying process
• along dislocations, there are compressive and tensile stresses
• inserting small solute atoms can reduce compressive stress
• via substitutional or interstitial diffusion
• vice versa for larger atoms

Question 11

How does Interstitial Solution strengthening work?

Answer 11

• interstitial atom is placed along dislocation
• creates a compressive stress field
• interacts with compressive stress field from dislocation
• results in dislocation being pinned
• results in higher initial yield stress
• yield stress decreases once interstitial atom is pulled away