MAT E 202 POST-MIDTERM | QUESTIONS & VERIFIED
ANSWERS
What is interdiffusion in an alloy? - Answers - In an alloy atoms tend to migrate from
regions of high concentrations to regions of low concentrations.
What is self-diffusion in a solid? - Answers - In an element solid atoms migrate.
What are the conditions for diffusion of one atom from one lattice site to another? -
Answers - 1. Must have an empty adjacent site.
2. Atom must have sufficient energy to break bonds with neighbor atoms and cause
some lattice distortion during displacement.
What are the properties of vacancy diffusion? - Answers - Atoms exchange with
vacancies and applies to substitutional impurities in atoms. The rate depends on the
number of vacancies, frequency of jumping, and activation energy to exchange.
Vacancy diffusion is a thermally activated process and also depends on the number of
vacancies present.
What are the properties of interstitials diffusion? - Answers - The small interstitial atom
jumps from one interstitial site to another. This applies to interstitial impurities and is
more rapid than vacandy diffusion.
What are some examples in which diffusion is faster or slower for? - Answers - Faster:
1. Open crystal structures
2. Lower melting temperature materials
3. Materials with secondary bonding
4. Smaller diffusing atoms
5. Cations
6. Lower density materials
Slower:
1. Closed-packed structures
2. Higher melting temperature materials
3. Materials with covalent bonding
4. Large diffusing atoms
5. Anions
6. Higher density materials
Describe the mechanism of plastic deformation. - Answers - Plastic deformation occurs
through dislocation motion (slip) and is a process of incremental breaking of bonds. A
shear stress parallel to a slip plane is responsible for the motion of dislocations. Slip will
occur along the path of least resistance (closed-packed direction).
, How is single crystal slip produced? - Answers - Plastic deformation in single crystal is
produced by relative motion of one part of material against another.
What are the mechanism of plastic deformation in different types of materials? -
Answers - Metals - dislocation is easier, non-directional bonding, closed packed
directions for slips.
Covalent ceramics - motion is hard, directional bonding.
Ionic ceramics - motion is hard, need to avoid electrochemical attractions.
What is the mechanism of plastic deformation in cubic and hexagonal metals? -
Answers - Plastic deformation by plastic shear or slip where one plane of atoms slides
over adjacent plane by dislocations.
What are the characteristics of a slip plane and slip direction in a slip system? -
Answers - Slip plane - plane allowing easiest slippage, wide interplanar spacings,
highest planar packing factor
Slip direction - direction of movement, highest linear packing factor
How many independent slip systems is required for a material to exhibit good ductility? -
Answers - 5.
What is the condition for dislocation motion of resolved shear stress? - Answers -
Resolve shear stress is greater than the critical resolved shear stress. (Note: crystal
orientation can make it easy or hard to move dislocation).
Describe slip in polycrystals. - Answers - Slip in polycrystals occur at higher stresses as
grain boundaries pin deformation. Slip plane and directions change from one direction
to another. Resolved shear stress will vary from one crystal to another.
How do grain boundaries effect slip? - Answers - Grain boundaries act as barriers to
slip. Barrier strength increases with increasing angle of misorientation. Smaller grain
sizes will result in more barriers to slip.
How does adding substitutional impurities effect dislocation motion? - Answers -
Impurity atoms distort the lattice and generate stress. This stress can produce a barrier
to dislocation motion. Small impurities tend to concentrate at dislocations and large
impurities concentrate at dislocations on the low density side.
What is solution strengthening? - Answers - Adding solutes to the metal to create stress
fields which interact with dislocation stress fields. This results in pinning the
dislocations.
ANSWERS
What is interdiffusion in an alloy? - Answers - In an alloy atoms tend to migrate from
regions of high concentrations to regions of low concentrations.
What is self-diffusion in a solid? - Answers - In an element solid atoms migrate.
What are the conditions for diffusion of one atom from one lattice site to another? -
Answers - 1. Must have an empty adjacent site.
2. Atom must have sufficient energy to break bonds with neighbor atoms and cause
some lattice distortion during displacement.
What are the properties of vacancy diffusion? - Answers - Atoms exchange with
vacancies and applies to substitutional impurities in atoms. The rate depends on the
number of vacancies, frequency of jumping, and activation energy to exchange.
Vacancy diffusion is a thermally activated process and also depends on the number of
vacancies present.
What are the properties of interstitials diffusion? - Answers - The small interstitial atom
jumps from one interstitial site to another. This applies to interstitial impurities and is
more rapid than vacandy diffusion.
What are some examples in which diffusion is faster or slower for? - Answers - Faster:
1. Open crystal structures
2. Lower melting temperature materials
3. Materials with secondary bonding
4. Smaller diffusing atoms
5. Cations
6. Lower density materials
Slower:
1. Closed-packed structures
2. Higher melting temperature materials
3. Materials with covalent bonding
4. Large diffusing atoms
5. Anions
6. Higher density materials
Describe the mechanism of plastic deformation. - Answers - Plastic deformation occurs
through dislocation motion (slip) and is a process of incremental breaking of bonds. A
shear stress parallel to a slip plane is responsible for the motion of dislocations. Slip will
occur along the path of least resistance (closed-packed direction).
, How is single crystal slip produced? - Answers - Plastic deformation in single crystal is
produced by relative motion of one part of material against another.
What are the mechanism of plastic deformation in different types of materials? -
Answers - Metals - dislocation is easier, non-directional bonding, closed packed
directions for slips.
Covalent ceramics - motion is hard, directional bonding.
Ionic ceramics - motion is hard, need to avoid electrochemical attractions.
What is the mechanism of plastic deformation in cubic and hexagonal metals? -
Answers - Plastic deformation by plastic shear or slip where one plane of atoms slides
over adjacent plane by dislocations.
What are the characteristics of a slip plane and slip direction in a slip system? -
Answers - Slip plane - plane allowing easiest slippage, wide interplanar spacings,
highest planar packing factor
Slip direction - direction of movement, highest linear packing factor
How many independent slip systems is required for a material to exhibit good ductility? -
Answers - 5.
What is the condition for dislocation motion of resolved shear stress? - Answers -
Resolve shear stress is greater than the critical resolved shear stress. (Note: crystal
orientation can make it easy or hard to move dislocation).
Describe slip in polycrystals. - Answers - Slip in polycrystals occur at higher stresses as
grain boundaries pin deformation. Slip plane and directions change from one direction
to another. Resolved shear stress will vary from one crystal to another.
How do grain boundaries effect slip? - Answers - Grain boundaries act as barriers to
slip. Barrier strength increases with increasing angle of misorientation. Smaller grain
sizes will result in more barriers to slip.
How does adding substitutional impurities effect dislocation motion? - Answers -
Impurity atoms distort the lattice and generate stress. This stress can produce a barrier
to dislocation motion. Small impurities tend to concentrate at dislocations and large
impurities concentrate at dislocations on the low density side.
What is solution strengthening? - Answers - Adding solutes to the metal to create stress
fields which interact with dislocation stress fields. This results in pinning the
dislocations.
