Dear Baradari,

X-Ray Diffraction is to analyse the presence and amounts  of phases in the sample by use of the crystal structure information of those phases by use of diffraction principles.

Your question should be about X-ray absorption and emission spectroscopy and in that case the position of the atom is immaterial.

rRegards

As far as I know interstitial carbon atoms are distributed inside the phase crystalline structure voids such as for example in the quenched carbon steel.  When austenite is transformed into martensite carbon atoms become  trapped inside the BCC lattice and thus change the lattice parameter so that initially cubic  lattice cell becomes tetragonal.  In this  case XRD is sensitive to  the cоntent of carbon thus giving a martensite reflection split.

Thanks Şakir, thanks Sergei

I think it is better that I ask my question in another form. In some cases, like austenite to bainite transformation, dislocation density inside a specific phase (say retained austenite or bainitic ferrite) becomes very large and as a result, a huge amount of carbon atoms may be trapped at dislocations. Now when we calculate the carbon content of the phase using quantitative XRD, does the result include these carbon atoms?

As Sakir hinted, XRD feels only the effect of SOLUTE C on lattice parameter. When trapped by defect cores, Carbon will give only secondary indirect effect, reflecting a weak change elastic fields generated by dislocations. At the same time, the main effect on a XRD profile is due to defect (eg dislocation) density as such. Anyway the influence of such carbon in this case is hardly distinguishable  on the background of principal effect due to elastic fields of dislocations.

effect. 

I agree with Zisman. Only C atoms in lattice can induce lattice parameter change, which can be determined by XRD. However, I think the content of carbon atoms trapped at dislocations can not be determined by XRD.

Dear Sadjad

X-ray is a ‘‘bulk’’ analysis in the sense that C content estimation by means of this technique is an average of regions of the material that may contain carbon-enriched regions such as dislocations and phase interfaces, which are very distinctive features of microstructures you asked which is my special field. Therefore, this technique is unable to disclose the large variation of austenite C content from region to region. 

Check out the following papers for more details:

1- B. Avishan, C. Garcia-Mateo, L. Morales-Rivas, S. Yazdani and F. G. Caballero: ‘Strengthening and mechanical stability mechanisms in nanostructured bainite’, J. Mater. Sci., 2013, 68, 6121– 6132.

2- Caballero FG, Miller MK, Babu SS, Garcia-Mateo C (2007), Atomic scale observations of bainite transformation in a high, carbon high silicon steel. Acta Mater 55(1):381

3- Caballero FG, Garcia-Mateo C, Santofimia MJ, Miller MK, Garcı´a de Andre´s C (2009) New experimental evidence on the incomplete transformation phenomenon in steel. Acta Mater 57(1):8

Dear Zisman, Dear Wang and Dear Behzad, Thank you all for your helpful answers.

Dear Behzad,

Caballero et al. [reference No.2 you mentioned above] and Seol et al. [Acta Mater. 60 (2012)] calculated the carbon concentration in retained austenite using XRD and APT techniques. A relatively large deviation was found between the values of two methods. However, they provide two opposing explanations for this deviation. They are as follows:

Caballero et al: "This is because the atom-probe estimate was a simple counting of atoms within the selected volume of the matrix of ferrite or austenite that did not contain any carbon-enriched regions such as dislocations and boundaries and not an average of a larger volume that may be enriched in carbon, as in the X-ray estimate."

Seol et al:"The deviation between the XRD and APT data can be explained by the fact that the C content determined by XRD under the simplified assumption that neither cementite formation nor C segregation at dislocations takes place."

I am confused, which of these is the right explanation?

I think, C% obtained by XRD does not contain C segregating at dislocations and boundaries. Because  XRD method is based on lattice parameter which is a founction of solute content. However, I thought, solute at dislocations and boundaries could not induce the lattice parameter change.

Lets say when you measure the C content of austenite or ferrite in bainitic microstructure, the measured value covers both carbon enriched and carbon poor regions and so you can not measure the carbon concentration trapped at dislocations.

Dear Wang and dear Behzad, thank you again!

Dear Baradani,

The source of confusion is due to under estimation or misunderstanding of the transformation mechanisms.

In martensitic transformation, cooling (quenching is so effective that carbon atom mobility , even by a fraction of of  lattice parameter or atomic size, is forbidden (crystallographicallymartensite is a supersaturated ferrite). One cannot talk of any segrregation of C atoms to the boundary or dislocations. The dislocations generated are geometrically necessary. Thats to say all the carbon in the austenite will end up in martensite.

If one considers the bainite morphology that forms, (then carbon mobility, diffusion and segregation plays an important role) upper and lower bainite distinction should definitely be considered because their formation mechanisms are different. See publications by "BADESHIA". From possibilities point of view, using a very crude analogy, upper bainite is more like an austenite to pearlite dissociation, while lower banite is more  like tempered martensite transformation. 

Since, most probably, in your material there are 4 or 5 phases (morphologies) that have formed concurrently, the problem and the question is much more complicated than a segregation problem with a simple "yes" or "no" answer.

Good luck, regards.

Dear friends,

Fortunately, I found a recently published work by Prof. Bhadeshia, which is related to this topic. Follow the link below to download the paper, if you are interested in it.

http://www.msm.cam.ac.uk/phase-trans/2015/anomalies.html