Why do certain metals form a bcc structure which has a lower packing factor than fcc or hcp? Ideally, close packed systems have higher stability due to greater coordination numbers right?
Most metal do form a close-packed structure. But packing alone cannot explain the stability of a structure. For example, both CCP and HCP have the same packing fraction. But still some metal form CCP whereas some others HCP. Also, BCC, although not strictly close-packed, still has very high packing fraction, 68%, only 4% less than 74% of CCP nad HCP structures.
@@introductiontomaterialsscience Even we are considering the free energy concept then we can explain why the metal choose only that particular crystal structure [i.e Cu only form FCC, not BCC and HCP, so the free energy formation of the FCC Cu is more stable[i.e very less free energy than BCC cu and HCP cu]].
I think reverse is the case. Close-packing explains why metals deform easily. The close-packed planes act as slip plane and clos-packed direction act like slip direction.
The concept of stacking sequence of close-packed planes is usually applied only to CCP and HCP crystals. Monatomic BCC crystals do not have real close-packed planes. The highest density planes in BCC are {110} planes. In terms of stacking of these planes, the stacking sequence is ABAB.
Sir as you said HCP [packing sequence ABABABAB ...] Where as FCC/CCP [ABCABCABC......] here FCC have extra third layer as at the C crevice [where HCP not]. So this is the case FCC should have the higher packing efficiency than the HCP [i.e 74%] but both the packing have the same PE why ?
It is not correct to think that CCP has an extra third layer. Both FCC and HCP have the third layer. In CCP the third layer is placed at C whereas in HCP the third layer is placed at B. Thus both have same number of atoms occupying the same space resulting in the same packing efficiency.
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Thank you so much prof., for this wonderful lecture. 👏👏 Following all your lectures. What an amazing skill of teaching, you have!!
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Respected sir, Where can i get relevant notes or text book for these topics, i wish to same kind of discussion tn text also
Why do certain metals form a bcc structure which has a lower packing factor than fcc or hcp?
Ideally, close packed systems have higher stability due to greater coordination numbers right?
Most metal do form a close-packed structure. But packing alone cannot explain the stability of a structure. For example, both CCP and HCP have the same packing fraction. But still some metal form CCP whereas some others HCP. Also, BCC, although not strictly close-packed, still has very high packing fraction, 68%, only 4% less than 74% of CCP nad HCP structures.
@@introductiontomaterialsscience Even we are considering the free energy concept then we can explain why the metal choose only that particular crystal structure [i.e Cu only form FCC, not BCC and HCP, so the free energy formation of the FCC Cu is more stable[i.e very less free energy than BCC cu and HCP cu]].
@@udaykumarkondaveeti7698 Yes, it is a satisfactory answer at one level. But why FCC structure in Cu has the lowest free energy is not easy to answer.
hi, sir, why the kissing number drops from 6 to 2 changing from hexagonal to square? I thought the kissing number for square is 4, isn't is?
You are right, for square lattice it should be 4 and not 2.
HELLO sir, Can we get the ppt or pdf format of these notes?
@Introduction to Materials Science and Engineering
Does close packing explain the strength of metal alloys? Could close packing be used, macroscopically, to build super strong materials?
I think reverse is the case. Close-packing explains why metals deform easily. The close-packed planes act as slip plane and clos-packed direction act like slip direction.
Sir from this what can we conclude as stacking sequence of bcc crystal lattice?
The concept of stacking sequence of close-packed planes is usually applied only to CCP and HCP crystals. Monatomic BCC crystals do not have real close-packed planes. The highest density planes in BCC are {110} planes. In terms of stacking of these planes, the stacking sequence is ABAB.
@@rajeshprasadlectures thank you sir for clarifying my doubt
Sir as you said HCP [packing sequence ABABABAB ...] Where as FCC/CCP [ABCABCABC......] here FCC have extra third layer as at the C crevice [where HCP not]. So this is the case FCC should have the higher packing efficiency than the HCP [i.e 74%] but both the packing have the same PE why ?
It is not correct to think that CCP has an extra third layer. Both FCC and HCP have the third layer. In CCP the third layer is placed at C whereas in HCP the third layer is placed at B. Thus both have same number of atoms occupying the same space resulting in the same packing efficiency.
Thank you Sir, Nicely explained
Sir, which software you used to draw these layers ?
PowerPoint
ty sir
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