It is hexagonal (P6322). Here are the cell parameters:
a = b = 4.767, c= 4.354 (90, 90, 120); Volume 85.69 Å3, Z = 2
Cell coordinates:
Fe 1/3, 0, 0
C 1/3, 2/3, 1/4
reference: Yakel, H.L., "Crystal structures of stable and metastable iron-containing carbides", Int. Met. Rev. (1985), 30, 17-40 [http://dx.doi.org/10.1179/095066085790150162]
Bainite in steels. H. K. D. H. Bhadeshia
http://www.msm.cam.ac.uk/phase-trans/2001/slides.html
Bainite is a microstructure and noo crystallographic phase. Therefore you cannot find bainite in a crystal structure database.
What Daniel Lundberg describes is a iron carbide.
At the temperature of about 300-400 C, austenite in many steels decomposed to lower bainite, a type of BCC iron ferrite with finely dispersed carbide cementite. particles in it.. There is a a clear evidence from relief effects seen on metallographic surfaces that the lower bainite forms by a shear process.
exactly...so that you have bainite which consists of two phases: ferrite and cementite. However, the cementite I know os orthorhombic:
COL ICSD Collection Code 38308
DATE Recorded Dec 31, 1984; updated Nov 10, 1997
NAME Iron carbide
MINR Cohenite - synthetic
MINR (Cementite)
FORM Fe3 C
= C Fe3
TITL Etudes structurales de composes de type cementite: effet de
l'hydrogene sur Fe3 C suivi par diffraction neutronique
Spectrometrie moessbauer sur Fe Co2 B et Co3 B dopes au 57Fe
REF Journal of Solid State Chemistry
JSSCB 51 (1984) 246-252
AUT Fruchart D, ChaudouetÿP, FruchartÿR, RouaultÿA, SenateurÿJÿP
CELL a=5.092 b=6.741 c=4.527 à=90.0 á=90.0 ç=90.0
V=155.4 Z=4
SGR P n m a (62) - orthorhombic
CLAS mmm (Hermann-Mauguin) - D2h (Schoenflies)
PRS oP16
ANX NO3
PARM Atom__No OxStat Wyck -----X----- -----Y----- -----Z----- -SOF-
Fe 1 0.000 8d 0.1834(7) 0.0689(7) 0.3344(8)
Fe 2 0.000 4c 0.0388(11) 1/4 0.8422(16)
C 1 0.000 4c 0.8764(22) 1/4 0.4426(17)
WYCK d c2
ITF Fe 1 B=0.43(2)
ITF Fe 2 B=1.0(3)
ITF C 1 B=0.9(2)
REM NDP (neutron diffraction from a powder)
REM M PDF 34-1
REM TEM 498 (15 K above Curie point)
REM M Lattice constants at RT. At 498 K about 4.54, 6.765, 5.10
TEST No R value given in the paper. (Code 51)
Naturally you are correct, Gert. I guess I was too quick in my search effort. Thanks for the correction.
Thank you Dr.Noize, I missed to point out the Xtal structure of Fe3C .cementite, which is othorhombic..
Below the pearlite nose, where the free energy changing driving FCC to BCC transformation increases, ferrite plays an increasingly predominant role part the transformation. The continuous lamellar structure of pearlite is no longer possible. Instead, ferrite plates grow out init austenite, pushing carbon away from them until carbide particles are nucleated in the carbon rich region between the ferrite plates. This the upper bainite.
http://www.msm.cam.ac.uk/phase-trans/2013/controversy_Fielding_MST_2013.pdf
I looked that review article mention Dr. Al-Mosawi. No matter what is really going on during decomposition of of austenite to bainite, there is a redistribution of carbon in the system since the two product phases ferrite and cementite have different compositions than the parent phase austenite. This redistribution may be short range, which are the case in the formation of pearlite out of eutectoid austenite as long as the inter lamellar spacing is not to large, or the long range as in the case of pro-eutectoid reactions. The separation of pro-eutectoid ferrite or cementite, usually takes place at the austenite grain boundaries. The composition of the separating structure is now different than the parent austenite so that long-range migration of carbon is necessary to change the composition in the two regions of the decomposing grain.
According to some authors such T. Ko and S. A. Cottrell, (1952) the lower bainite forms by shear transformation similar to martensite, the progress of which is delayed by the diffusion of carbon. Carbides nucleate and form in the (slightly saturated) ferrite behind the interface, as they draw carbon away from the adjoining austenite, allow this interface to move forward by the crystallographic shear mechanisms.
As outlined by previous answers, Bainite is a name given to the arrangement of various microstructural features; primarily bainitic ferrite and carbides and some times retained austenite (in case of carbide free bainite). Ferrites and carbides are arranged in a nonlamellar fashion as opposed to pearlite where the arrangement is lamellar. As far as crystal structure is concerned, bainitic ferrite is close to BCC while carbides are mostly orthorhombic.
HI Ajit,
You can find enclosed the parallel between the formation mechanisms of perlite (Ferrite+Carbide (Fe3C=cemetite) )and the bainite (ferrite + carbide).
The documents are in french (Sorry !) I hope that it 's clear!
If you need more information visit the HKDH Bhadeshia site, at Cambridge.
Best regards,
Abdelkrim
Dr. Behera; I think we had enough discussions about Bainite at this platform. Please, take anyone of them you would not make mistake!
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