Metals are usually (poly)crystalline; although formation of amorphous metals is possible by rapid cooling.
Since the atomic bonding in metals is non-directional ??no restriction on numbers or positions of nearest-neighbor atoms ??large number of nearest neighbors and dense atomic packing
The most common types of unit cells are the faced centered cubic (FCC), the body-centered cubic (FCC) and the hexagonal close-packed (HCP).
In the studying of the crystalline structures , we interest with four parameters :
1- a/R relationship .
2- No. of atoms per unit cell.
3-Coordination number(CN): the number of nearest- neighbor or touching atoms that surround specific atom.
4-Atomic packing factor (APF): it is also called packing efficiency, and represents the fraction of atoms volume in a unit cell.
Face-Centered Cubic (FCC) Crystal Structure :
?Atoms are located at each of the corners and on the centers of all the faces of cubic unit cell.
Number of atoms per unit cell, n = 4.
6 face atoms shared by two cells: 6 x 1/2 = 3
8 corner atoms shared by eight cells: 8 x 1/8 = 1
Body-Centered Cubic (BCC) Crystal Structure:
Atom at each corner and at center of cubic unit cell
Number of atoms per unit cell, n = 2
Center atom (1) shared by no other cells: 1 x 1 = 1
8 corner atoms shared by eight cells: 8 x 1/8 = 1
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 .
|