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Mechanical properties and fracture

Which material shows a distinct ductile-to-brittle transition at low temperatures?

  1. AAustenitic stainless steel (FCC)
  2. BAluminum alloys (FCC)
  3. CFerritic carbon steel (BCC)
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Answer: C. Ferritic carbon steel (BCC)

In BCC metals, the stress needed to move dislocations (the Peierls stress) rises sharply as temperature falls, so at low temperatures cleavage fracture occurs before yielding and the absorbed energy in the Charpy impact test drops abruptly over a certain temperature range. This is the ductile-to-brittle transition, and ferritic carbon steel is the classic example. FCC austenitic stainless steels and aluminum alloys have many slip systems and dislocations remain mobile at low temperatures, so they show no distinct transition and are used for cryogenic applications such as liquefied gas tanks. The transition temperature is lowered by grain refinement and raised by neutron irradiation and similar damage.

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