Why is fcc more ductile than bcc

  1. HI all

    Why is fcc more ductile than bcc although bcc has greater number of slip planes than fcc?
  2. jcsd
  3. Astronuc

    Staff: Mentor

    Re: Ductility


    See also - http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Structure/deformation.htm


    See figure 2a (fcc) and 2b (bcc) in the following. Note the angle between slip systems.



    This may be the most helpful -
    See page 134-135 of The Science and Engineering of Materials By Donald R. Askeland, Pradeep P. Fulay, Wendelin J. Wright
    http://books.google.com/books?id=qz...4#v=onepage&q=Ductility slip fcc bcc&f=false
  4. Re: Ductility

    Alloying elements are by far more important than crystal lattice to determine ductility.
    Take pure aluminium, it has virtually no limit to ductility. The sputtering targets I used got a notch by pressing one's nail on them.
    But alloyed with 8% zinc (AA7049), aluminium loses much ductility, with only 8% guaranteed elongation at break.
  5. Re: Ductility

    One example of very ductile body-centred cubic is Armco iron:

    It's used annealed and slowly cooled, ferritic (BCC), for its soft ferromagnetic properties, and also its resistance to corrosion.
    Medium grades guarantee <0.01% of C, P, S and even Mn and Si. It's essentially plain ferritic pure iron.

    With 200MPa yield strength, 40% elongation and 70% reduction of area at break, it is excellent at cold-forming. Such figures are absolutely similar to austenitic (FCC) iron-based alloys.

    Hence my claim that essentially the alloying elements (C, P, S...) determine ductility.
  6. Astronuc

    Staff: Mentor

    Re: Ductility

    P and S are generally considered impurities in most alloys, particularly structural materials. Both can increase notch sensitivity, or conversely reduce fracture toughness, particularly at cold temperatures.
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