Question

Explain why C diffuses faster in BCC Fe compared with FCC Fe; whereas C solubility in BCC Fe is much less than that in FCC Fe

Answer 1

The gamma phase is called austenite. Austenite is a high temperature phase and has a Face Centred Cubic (FCC) structure [which is a close packed structure].

The alpha phase is called ferrite. Ferrite is a common constituent in steels and has a Body Centred Cubic (BCC) structure [which is less densely packed than FCC].

• austenite > ferrite+cementite.
• gamma > alpha + Fe₃C.

Austenite has face centered cubic (FCC) crystal structure and ferrite has body centered cubic (BCC) crystal structure.

Due to its larger size, carbon atoms occupies octahedral interstitial sites in these crystals. Octahedral interstitial site of BCC ferrite is much smaller for carbon than that of FCC austenite.

This is why solubility of carbon in ferrite(BCC) is much smaller than solubility of carbon in austenite (FCC).