why does ductile-brittle transition not occur in FCC? 2

[solphemacho]

I know it is about vibration of atoms and dislocations but can you give me a brief explanation?

 

[CoryPad]

The bonding nature in FCC crystals is uniform, so the stress required to initiate and propagate dislocation slip is small. This also means that slip has low temperature dependence, so FCC metals retain their plastic strain capacity at low temperatures. You can read good information on this subject from this previous Eng-Tips thread:

thread330-40730


Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[TVP]

The ductile to brittle transition occurs because deformation by dislocation slip becomes more and more difficult as the temperature decreases. In the BCC crystal structure, deformation transitions from dislocation slip to cleavage. The energy to initiate cleavage fracture is substantially lower than the energy necessary for ductile fracture through microvoid coalescence and growth (microvoids nucleate during dislocation slip). Metals with the HCP crystal structure can also exhibit cleavage fracture.

The FCC crystal structure is generally immune to cleavage, although austenitic stainless steels can fail due to stress corrosion crack growth due to transgranular cleavage in certain aqueous solutions (e.g., NaCl solutions) [ref: ASM HANDBOOK Volume 19 Fatigue and Fracture]. Cleavage occurs along well-defined crystallographic planes ({100} facets and {112} tongues in BCC iron), which are not operable in FCC metals. For FCC crystals, slip occurs most often on {111} octahedral planes and in <110> directions that are parallel to cube face diagonals [ref: [/I]Deformation and Fracture Mechanics of Engineering Materials[/I] by R. W. Hertzberg]. In BCC crystals, slip occurs in <111> cube diagonal direction and on {110} dodecahedral planes. Slip occurs on {112} and {123} planes as well. Hence, dislocations that are operative in the BCC crystal can initiate cleavage, with cracks around the carbide particles providing a mechanism for continued propagation at relatively low stresses.