Dislocation pile ups are generally observed at grain boundaries. Based on crystal system, how the dislocation pile ups are affected. Is it more in fcc materials or bcc materials? why?
1. Except for particular (hcp) crystals, distinct pile-ups of many dislocations (as suggested by classical models for microcracks) have not been observed in fact.
2. Observed or not, pile-ups should easier form in FCC than BCC. Indeed,
while the former have TWO slip {111} planes per each Burgers vector , in the latter THREE slip planes {110} are for each Burgers vector. Moreover alternative planes are also available to the same vector in many BCC crystals. So, CROSS SLIP is easier in BCC and hence rearrangement of the pile-up in more relaxed configuration is more probable.
I think because of the existence of the Peierls drag forces due to the bond angle distortion that occurs at the core of a moving dislocation in BCC transition metals, which have also developed a directional bonding components one may not observe dislocation pile up. That is also the case for Si, Ge and C, which have diamond cubic structures. But the high temperatures thermal lattice vibrations with sufficiently high amğlituıdes that the effect of the Peierls forces becomes nullified., other factors such as impurities and grains boundaries limit the dislocation motion that may cause the formation of pile ups. Bonding in FCC and HCP metals and ionic crystals is sufficiently nondirectional that drag due to the Peierls force is insignificantly small, where dislocation pile against the grain boıundaries becomes major strengthening mechanisms.
The dislocation pile up phenomenon is dependent on a number of factors:
1-Type of materials.
Some Material has BCC,FCC,and HCP, and so on.
2-Number of slip systems(slip direction & slip plane) in a particular crystal lattice.
The lesser the number of slip systems the more likely the dislocation pile up to occur.
3-presence of impurities.
The presence of impurities increases the chance of dislocation pile up.
4-Packing factor of the crystal lattice.
The higher the packing factor of the crystal lattice the less likely for the dislocation pile up to occur.
For instance, BCC lattice in α-Fe has {110} slip plane , slip direction and 12 slip systems. In the mean time, FCC lattice in Cu has {111} slip plane , slip direction and 12 slip systems. So, according to the above mentioned factors, the BCC lattice in α-Fe is more subject to have dislocation pile up than the Cu because the BCC lattice in α-Fe has less packing factor than the Cu. This is due to the fact that the α-Fe is more susceptible to host impurities than the Cu.
So, the dislocation pile up phenomenon is dependent on the combination of the above factors.
Bold numbers imply negative numbers.
Dear Santhosh,
To answer you question we need to understand the importance of slip systems and also may have to think through energy required to create a dislocation in FCC and BCC.
As Dr. Phani mentioned properly in his last remark, the energetic of dislocation motion, which involves lattice drag forces as well as the intrinsic energy barriers such as Peierls hills and valleys should be considered very seriously. Those considerations especially in BCC metals and alloys have been forcing us to postulate new mechanisms to explain even the secondary features of the plastic deformations such as the kink-pair theory, and their solitary wave like behaviour under the applied stress systems.
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