Would you expect the diffusion of C in ? iron (BCC) to be larger or smaller than
ID: 702526 • Letter: W
Question
Would you expect the diffusion of C in ? iron (BCC) to be larger or smaller than for ? iron (FCC). The figure below should help you answer this question. (8pts) Can you give a reason why the diffusion of C carbon is different between BCC and FCC iron? (2pts) (Hint: What are differences you know between BCC and FCC crystal structures?)
Temperature (°C) 1500 1200 1000 800 600 500 400 300 10-8 10-10 C in y-Fe C in ?-Fe 1012 Zn in Cu 5 10-14 Fe in ?-Fe un Al in Al -16 ? 10 Fe in ?-Fe Cu in Cu 10-18 10-20 0.5 1.0 1.5 2.0 Reciprocal temperature (1000/K)Explanation / Answer
BCC is body centered cubic and FCC is face centered cubic. Iron-carbon eutectic phase diagram, showing various forms of FexCy substances. Iron allotropes, showing the differences in lattice structure. The alpha iron (?) is a body-centered cubic (BCC) and the gamma iron (?) is a face-centered cubic (FCC). Form the figure it can be observed that alpha iron (?) i.e. BCC iron has more diffusion coefficent than gamma iron (?) i.e. diffusion coeffiient for a given temperature. So, For a given temperature carbon diffuses through both iron types however it is more for BCC iron than the FCC iron.
Body centered cubic structure is more loosly pack than that of face centered cubic structure.beacuse the body-centered cubic unit cell has atoms at each of the eight corners of a cube and one atom in the center of the cube.It is said to have a coordination number of 8.The bcc unit cell consists of a net total of two atoms. one in the center and eight eighths from corners atoms.
The face centered cubic structure has atoms located at each of the corners and the centers of all the cubic faces.Each of the corner atoms is the corner of another cube so the corner atoms are shared among eight unit cells. Additionally, each of its six face centered atoms is shared with an adjacent atom. because 12 of its atoms are shared, it is said to have a coordination number 12.
so we can easily say when atoms are loosly pack in a structure diffusion is easy.
according to the structure of BCC and FCC we can also calculate packing factor.
packing factor=The volume of atoms in a cell/total volume of a cell.
packing factor of BCC=0.68
packing factor of FCC in iron=0.74
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