ME 331 EXAM 2 QUESTIONS AND ANSWERS
Diffusion - Answers :Mass transport by atomic motion
Mechanisms for diffusion of different phases - Answers :Gases and Liquids- Random
motion
Solids- Vacancy or interstitial diffusion
What are the two diffusion mechanisms - Answers :Vacancy and Interstitial Diffusion
What does vacancy diffusion depend on? - Answers :Number of vacancies
Activation energy to exchange (break bonds)
What form of diffusion is easier/faster? Why? - Answers :Interstitial:
Interstitial atoms are smaller
There are more empty interstitial positions than vacancies
What are the differences between Fick's 1st law and 2nd law? - Answers :Fick's 1st law
requires steady state diffusion and is independent of time
Write out Fick's first law, second law, solution to 2nd law, diffusion coefficient equation -
Answers :Write out
Give an example of interstitial diffusion - Answers :Carbon Steel- Case hardening: The
diffusion of carbon atoms into the the host iron atoms at the surface make the surface
harder
Used for gears
Examples of vaccancy diffusion - Answers :Self Diffusion
Substitutional Diffusion
Doping - Answers :Process of adding impurities to a semiconductor to increase its
conductivity
Phosphorous into silicon
Is D the same for self diffusion? - Answers :No, it depends on bonding between atoms
What causes diffusion to be faster? - Answers :Open crystal structures
Materials with secondary bonding
smaller diffusing atoms
lower density materials
What causes diffusion to be slower? - Answers :Close-packed structures
Materials with covalent bonding
Larger diffusing atoms
, Higher density materials
Interdiffusion - Answers :diffusion of atoms of one metal into another metal due to
concentration gradient
Which is faster? Self-diffusion or Inter-diffusion? - Answers :Inter (2 atoms)
Which is faster? Substitutional diffusion or Interstitial diffusion? - Answers :Interstitial
Give an example of interdiffusion? - Answers :Tools in the garage binding together over
time- 2 materials bond together via interdiffusion of atoms
What are the 3 main types of failure? Give 1 ex of each. - Answers :Low temperature
ductile-brittle transition: Titanic fractured during night at cold water temperature
Fatigue failure due to cylic loading- Boeing 737
High temperature creep failure- Union oil refinery explosion
Do we usually prefer ductile or brittle fracture? - Answers :Ductile- there is deformation
(warning) before fracture and more strain energy is required for fracture to occur
What are the characteristics of ductile fracture? - Answers :One piece
Large deformation
What are the characteristics of brittle fracture? - Answers :Many pieces
Small deformation
What is fractography and what is it used for? - Answers :The study of fracture surfaces
on a microscopic level- it can be used to analyze type of fracture
Two types of brittle fracture (Define each) - Answers :Intergranular- Material separates
at the grain boundary (Most common) (Between grains)
Transgranular- Material separates partially through the grain (Within grains)
Why do real materials have much lower strength values compared to perfect materials?
- Answers :Flaws, which increase with increasing samples size, cause premature failure
Why do cracks cause failure? - Answers :Stress always concentrates more on crack tips
Flaws are also known as? - Answers :Stress concentrators
When do cracks propogate? - Answers :If the stress at the crack tip is larger than the
critical stress
fracture toughness is defined as - Answers :The resistance of a material to brittle
fracture in the presence of a crack or void
Diffusion - Answers :Mass transport by atomic motion
Mechanisms for diffusion of different phases - Answers :Gases and Liquids- Random
motion
Solids- Vacancy or interstitial diffusion
What are the two diffusion mechanisms - Answers :Vacancy and Interstitial Diffusion
What does vacancy diffusion depend on? - Answers :Number of vacancies
Activation energy to exchange (break bonds)
What form of diffusion is easier/faster? Why? - Answers :Interstitial:
Interstitial atoms are smaller
There are more empty interstitial positions than vacancies
What are the differences between Fick's 1st law and 2nd law? - Answers :Fick's 1st law
requires steady state diffusion and is independent of time
Write out Fick's first law, second law, solution to 2nd law, diffusion coefficient equation -
Answers :Write out
Give an example of interstitial diffusion - Answers :Carbon Steel- Case hardening: The
diffusion of carbon atoms into the the host iron atoms at the surface make the surface
harder
Used for gears
Examples of vaccancy diffusion - Answers :Self Diffusion
Substitutional Diffusion
Doping - Answers :Process of adding impurities to a semiconductor to increase its
conductivity
Phosphorous into silicon
Is D the same for self diffusion? - Answers :No, it depends on bonding between atoms
What causes diffusion to be faster? - Answers :Open crystal structures
Materials with secondary bonding
smaller diffusing atoms
lower density materials
What causes diffusion to be slower? - Answers :Close-packed structures
Materials with covalent bonding
Larger diffusing atoms
, Higher density materials
Interdiffusion - Answers :diffusion of atoms of one metal into another metal due to
concentration gradient
Which is faster? Self-diffusion or Inter-diffusion? - Answers :Inter (2 atoms)
Which is faster? Substitutional diffusion or Interstitial diffusion? - Answers :Interstitial
Give an example of interdiffusion? - Answers :Tools in the garage binding together over
time- 2 materials bond together via interdiffusion of atoms
What are the 3 main types of failure? Give 1 ex of each. - Answers :Low temperature
ductile-brittle transition: Titanic fractured during night at cold water temperature
Fatigue failure due to cylic loading- Boeing 737
High temperature creep failure- Union oil refinery explosion
Do we usually prefer ductile or brittle fracture? - Answers :Ductile- there is deformation
(warning) before fracture and more strain energy is required for fracture to occur
What are the characteristics of ductile fracture? - Answers :One piece
Large deformation
What are the characteristics of brittle fracture? - Answers :Many pieces
Small deformation
What is fractography and what is it used for? - Answers :The study of fracture surfaces
on a microscopic level- it can be used to analyze type of fracture
Two types of brittle fracture (Define each) - Answers :Intergranular- Material separates
at the grain boundary (Most common) (Between grains)
Transgranular- Material separates partially through the grain (Within grains)
Why do real materials have much lower strength values compared to perfect materials?
- Answers :Flaws, which increase with increasing samples size, cause premature failure
Why do cracks cause failure? - Answers :Stress always concentrates more on crack tips
Flaws are also known as? - Answers :Stress concentrators
When do cracks propogate? - Answers :If the stress at the crack tip is larger than the
critical stress
fracture toughness is defined as - Answers :The resistance of a material to brittle
fracture in the presence of a crack or void
