DoITPoMS

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Questions

Quick questions

You should be able to answer these questions without too much difficulty after studying this TLP. If not, then you should go through it again!

  1. Which of the following statements is false?

    a A bubble raft can demonstrate vacancies and solute atom defects.
    b Plastic deformation in a bubble raft occurs by dislocation motion.
    c Screw dislocations can be seen in the bubble raft.
    d Bubbles in a well-formed bubble raft are close-packed.

  2. Which of the following statements best describes the nature of dislocations in an amorphous material?

    a Amorphous materials cannot contain dislocations.
    b The dislocation density in an amorphous material is normally less than the dislocation density in a crystalline material with the same composition.
    c The dislocation density in an amorphous material is normally greater than the dislocation density in a crystalline material with the same composition.
    d A dislocation in an amorphous material must be of the edge type.

  3. What are the conventional units of dislocation density?

    a m-2
    b m-3
    c kg m-3
    d kg m-2

  4. How would you make a stack of ham sandwiches look like a screw dislocation?

Deeper questions

The following questions require some thought and reaching the answer may require you to think beyond the contents of this TLP.

  1. Do the explanations of these experimental observations involve the concept of dislocations? (Answer yes or no for each)

    Yes No a The yield stress of a material is lower than that calculated from assuming a perfect lattice
    Yes No b In some cases, the stress required to continue plastic deformation increases as deformation proceeds
    Yes No c The yield stress of most metals decreases as the temperature increases
    Yes No d Ceramics tend to be brittle, whereas metals tend to be ductile

  2. The energy per unit length, U, associated with an edge dislocation is given by

    U ~ 0.5(Gb2)

    where b is the magnitude of the Burgers vector b and G is the shear modulus. Estimate the energy per unit length of a dislocation in silver.

    Data for silver: Crystal system is cubic F, a = 0.409 nm. b lies parallel to <110> directions. Shear modulus G = 28.8 GPa.

  3. Determine whether the following dislocations in sodium chloride are edge, screw or mixed. Identify the slip plane in which the dislocation lies. (Sodium chloride is cubic F and slips on {110} slip systems.)

     Burgers vector b parallel to:Line vector l parallel to:
    a[110][110]
    b[001][110]
    c[100][111]

  4. Which of the following statements is true? (answer yes or no for each)

    Yes No a Using high-resolution microscopy techniques, it is possible to probe the structure of a dislocation.
    Yes No b The number of dislocations in a sample of material affects the ease of diffusion of impurities through the sample.
    Yes No c The diffusion of impurity atoms in a sample of a material affects the ease of motion of dislocations under an applied stress.

Open-ended questions

The following questions are not provided with answers, but intended to provide food for thought and points for further discussion with other students and teachers.

  1. Is dislocation glide always the mechanism of plastic flow? How might other mechanisms operate?

  2. Why might the presence of dislocations in materials used for electronic components (such as integrated circuits) be a problem? How might the problem be reduced or solved?

  3. Plastically deforming a material requires energy input. In what ways is this energy dissipated? Is any of it stored in the material?


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