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Why is fcc more ductile than bcc

by pukb
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Nov14-11, 10:02 AM
P: 92
HI all

Why is fcc more ductile than bcc although bcc has greater number of slip planes than fcc?
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Nov28-11, 05:37 AM
Astronuc's Avatar
P: 21,880
Crystalline structure is important because it contributes to the properties of a material. For example, it is easier for planes of atoms to slide by each other if those planes are closely packed. Therefore, lattice structures with closely packed planes allow more plastic deformation than those that are not closely packed. Additionally, cubic lattice structures allow slippage to occur more easily than non-cubic lattices. This is because their symmetry provides closely packed planes in several directions. A face-centered cubic crystal structure will exhibit more ductility (deform more readily under load before breaking) than a body-centered cubic structure. The bcc lattice, although cubic, is not closely packed and forms strong metals. Alpha-iron and tungsten have the bcc form. The fcc lattice is both cubic and closely packed and forms more ductile materials. Gamma-iron, silver, gold, and lead have fcc structures. Finally, HCP lattices are closely packed, but not cubic. HCP metals like cobalt and zinc are not as ductile as the fcc metals.

See also -

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

In fcc metals, the flow stress, i.e. the force required to move dislocations, is not strongly
temperature dependent. Therefore, dislocation movement remains high even at low
temperatures and the material remains relatively ductile.

In contrast to fcc metal crystals, the yield stress or critical resolved shear stress of bcc
single crystals is markedly temperature dependent, in particular at low temperatures. The
temperature sensitivity of the yield stress of bcc crystals has been attributed to the
presence of interstitial impurities on the one hand, and to a temperature dependent
Peierls-Nabarro force on the other. However, the crack propagation stress is relatively
independent of temperature. Thus the mode of failure changes from plastic flow at high
temperature to brittle fracture at low temperature.

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
Nov28-11, 06:45 AM
P: 661
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.

Dec12-11, 07:38 PM
P: 661
Why is fcc more ductile than bcc

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.
Dec13-11, 09:09 AM
Astronuc's Avatar
P: 21,880
Quote Quote by Enthalpy View Post
Hence my claim that essentially the alloying elements (C, P, S...) determine 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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