Why does iron changes from BCC to FCC when heated over 910 degree Celsius?
Answers
Iron is an allotropic metal, which means it can exist in more than one type of crystal structure depending upon the temperature.
The melting point of iron is 1539⁰ C. Above this, it becomes liquid.
When iron first solidifies 1539⁰ c. It is in the δ form of B..C.C structure.
On cooling further, the phase change occurs at 1401⁰ C and the atoms rearrange themselves into the γ form which is F.C.C and non magnetic.
When the temperature reaches 910⁰ C , another phase change from F.C.C non magnetic to B.C.C non magnetic iron.
Answer:
I) for BCC , Z=2, 4r=a
3,MA
=56 edge length=a
1ρ
BBC= N A×a 13
2×56 =N A×64r
3112×3
3=K×3 3
II) for FCC, Z=4, 4r=a
2
2, MA
=56 edge length=a 2ρ
FFC= N A×a 2
34×56= N A×64r
32×112×2 2
= N A ×64r 3
224×2 2
=K×4 2
pFCC
ρ BCBC= 4 2
3 3=0.918