Re: [Phys-L] Iron density question
C O O LER -> BCC ... ferrite .....austenite FCC -> H O T
T E R/ DenserSolubility
0.02%............................................ 2.14%
This is a precis of a Q & A on ResearchGate
The solubility of C in ferrite is only 0.025% whereas in Austenite this is 2%
even though the atomic packing factor in austenite (fcc) (4 atom per unit cell)
is higher than ferrite (bcc) (2 atoms per unit cell). The calculation of the
amount of empty space in unfilled space in f.c.c is 25% and in b.c.c lattice is
32%, so it may seem strange that the solubility of C in austenite is much
greater than in ferrite. The reason is that the largest hole in b.c.c ferrite
is halfway between the center of the face and the space between the two corner
atoms. Two of the four possible positions for a Carbon atom are on the front
face of a body-centered cube. The largest interstitial sphere that would just
fit has a radius of 0.36(10)-8 cm. The largest hole in f.c.c austenite is
midway along the edge between two corner atoms. The largest interstitial sphere
that would just fit has a radius of 0.52(10)-8 cm. therefore, austenite will
have a greater solubility for carbon than ferrite..Although the void space in
BCC (32%) is greater than FCC (26%), the size of the cavities where carbon can
be located interstitially among iron atoms in BCC is much smaller compared to
FCC. Therefore, the solubility in BCC is significantly reduced (0.02), whereas
in FCC, it is much higher (2.14).
FCC has 8 and 4 octahedral and tetrahedral voids per unit cell respectively.
whereas BCC has 12 and 6 respectively. octahedral in FCC is bigger than
tetrahedral. C occupies octahedral with less distortion. Octahedral in BCC is
smaller than tetrahedral. even in that case C occupies octahedral void due to
lesser distortion (only top atom and bottom need to be distorted). No. of voids
in BCC are more than FCC whereas size of voids in BCC are significantly smaller
than FCC. For this reason austenite have higher solubility of C than ferrite.
FCC interstitial radius(0.41R) is larger than BCC (0.291R). R is the radius of
atom.so more carbon atoms are soluble in FCC then BCC.
Why is solubility of C in ferrite only 0.025% whereas in Austenite this is 2% ?
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Why is solubility of C in ferrite only 0.025% whereas in Austenite this ...
Read 12 answers by scientists with 1 recommendation from their colleagues to
the question asked by Shambhu Nath ...
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Credit to respective authors.
I conclude that purely mechanic level arguments serve to illustrate the
concept.There is another transformation at still higher temperature, by the way.
On Sunday, November 5, 2023 at 03:06:52 PM CST, bernard cleyet via Phys-l
<phys-l@mail.phys-l.org> wrote:
I just returned from the NorCal/Nevada AAPT section meeting, where i
demonstrated the allotropic change in Iron using a video clip from the dining
room table.
If unfamiliar, here:
http://www.cleyet.org/allotrope.mov ;
It clearly shows the change in density. The wire likely wasn’t very pure,
which, I assume, makes the change less sharp and “intense”.
So my question is: Is QM necessary to explain or can simple electrostatics
explain? If simple electrostatics does, then I’d expect the FCC form to be
less dense than the BCC form, no?
Note: on cooling it passed from more dense to less dense. Also why does one
structure dissolve carbon more? Seems obvious.
bc … to lazy to search which is BCC, etc.; austenite or ferrite, and he uses
scare quotes when he can’t remember the correct word.
p.s. I don’t remember the text explaining when I taught engineering materials
at the local community college.
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