??Both FCC and HCP crystal structures have atomic packing factors of 0.74 (maximum possible value)
??Both FCC and HCP crystal structures may be generated by the stacking of close-packed planes
??The difference between the two structures is in the stacking sequence
FCC: Stacking Sequence ABCABCABC...
Third plane is placed above the “holes” of the first plane not covered by the second plane
HCP: Stacking Sequence ABABAB...
Third plane is placed directly above the first plane of atoms
Density Computations :
A knowledge of the crystal structure of a metallic solid permits computation of its theoretical density ? through the relationship:
Thus, the formula for the density is:
Polymorphism and Allotropy:
Some materials may exist in more than one crystal structure, this is called polymorphism. If the material is an elemental solid, it is called allotropy.
Allotropic Transformation :
: Iron
?BCC at high temperatures (? - ferrite)
FCC at moderate temperatures (austenite)?
BCC at low temperatures (? - ferrite) ?
? Change in specific volume
Another example of allotropy is carbon, which can exist as diamond, graphite, and amorphous carbon (large, hollow fullerenes) .
Two kinds of fullerenes are shown here: buckminsterfullerene (buckyball) and carbon nanotube.
Where:
n : No. of atoms associated with each unit cell.
A: Atomic weight
Vc :Volume of the unit cell
NA: Avogadro number = 6.023 X1023 atoms/mol
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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