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Explain the principle of slip in terms of BCC and FCC crystal structures - Leaving Cert Engineering - Question i - 2007
Question i
Explain the principle of slip in terms of BCC and FCC crystal structures.  
Answer
The body-centered cubic (BCC) structure is characterized by a non-close packed arrangement of atoms. This less efficient packing leads to a lower atomic density compared to other structures. In a BCC structure, the atoms occupy the corners of a cube with one atom in the center. Due to this arrangement, slip – the process of dislocation movement – is less favorable, making BCC metals generally more brittle.
Step 2
Explain the properties of FCC crystal structure
Answer
In contrast, the face-centered cubic (FCC) structure is a close packed arrangement. Atoms are positioned at each corner of the cube and the centers of each face. This close packing allows for greater atomic movement and easier activation of slip systems, resulting in higher ductility. FCC metals can undergo significant deformation before fracturing, making them more malleable.
Step 3
Comparison of slip in BCC and FCC
Answer
The key difference in the slip behavior of BCC and FCC structures lies in their atomic arrangements and packing efficiencies. The BCC structure restricts slip due to fewer slip systems available, leading to brittleness. Conversely, the FCC structure, with its multiple slip systems, facilitates dislocation movement, allowing FCC metals to exhibit more ductility.