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19 - Diffusionless Transformations

from Part III - Types of Phase Transformations

Published online by Cambridge University Press:  24 April 2020

Brent Fultz
Affiliation:
California Institute of Technology
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Summary

Diffusionless transformations occur when atoms in a crystal move cooperatively and nearly simultaneously, distorting the crystal into a new shape. The martensite transformation is the most famous diffusionless transformation, owing to its importance in steel metallurgy. In a martensitic transformation the change in crystal structure occurs by shears and dilatations, and the atom displacements accommodate the shape of the new crystal. The atoms do not move with independent degrees of freedom, so the change in configurational entropy is negligible or small. The entropy of a martensitic transformation is primarily vibrational (sometimes with electronic entropy, or magnetic entropy for many iron alloys). This chapter begins with a review of dislocations, and how their glide motions can give crystallographic shear. Some macroscopic and microscopic features of martensite are then described, followed by a two-dimensional analog for a crystallographic theory that predicts the martensite “habit plane” (the orientation of a martensite plate in its parent crystal). Displacive phase transitions are explained more formally with Landau theories having anharmonic potentials and vibrational entropy. Phonons are discussed from the viewpoint of soft modes and instabilities of bcc structures that may be relevant to diffusionless transformations.

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Publisher: Cambridge University Press
Print publication year: 2020

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  • Diffusionless Transformations
  • Brent Fultz, California Institute of Technology
  • Book: Phase Transitions in Materials
  • Online publication: 24 April 2020
  • Chapter DOI: https://doi.org/10.1017/9781108641449.023
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  • Diffusionless Transformations
  • Brent Fultz, California Institute of Technology
  • Book: Phase Transitions in Materials
  • Online publication: 24 April 2020
  • Chapter DOI: https://doi.org/10.1017/9781108641449.023
Available formats
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To send content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about sending content to Google Drive.

  • Diffusionless Transformations
  • Brent Fultz, California Institute of Technology
  • Book: Phase Transitions in Materials
  • Online publication: 24 April 2020
  • Chapter DOI: https://doi.org/10.1017/9781108641449.023
Available formats
×