Of course, I mean direct measurements rather than usual studies of carbide powder extracted from the steel matrix by its dissolution.
In case of retained austenite appropriate reference data are available; specifically, theoretical diffraction intensities calculated from first principles for both pure alpha- and gamma phases. I could not find similar coefficients or appropriate quotations for numerous carbides. According to literature, one can only IDENTIFY them in terms of diffraction data.
I guess you mean the problem of phase quantification in case of small amounts? This is certainly a problem since the "magical" 2% limit is not valid since carbides even have lower scattering coefficients compared to the surrounding metal. You already mentioned neutron diffraction. I have no real idea, but to me it looks like that the scattering power of carbon is not much better (not like for hydrogen).
In order to get at least a first approximation you only can increase the signal to noise ratio by "longer" measurements or other tricks to increase the number of counts and reduce the noisy background signal.
Did you also mean that you could not find diffraction data from the literature for other carbides? What specific carbide phases were you referring to? If you could list them out, maybe someone can help you find them or point you to the right direction. For quantification of (crystalline) carbides using XRD/neutron diffraction, you need to know their lattice parameters, etc. but if the contents of these carbides within the steel sample are very low, then, as quantitative XRD/neutron diffraction analysis will not give accurate results. I was thinking: why not try some imaging analysis e.g. SEM, then use some sort of image processing software to work out the wt%? That might work...
@ G. Qian: The image analysis only sounds better....but imagine, if you use an (magnified) image covering only one carbide the phase content is 100%. On the other hand, during preparation you do not know what you are doing with your material. In worst case, you break or solve them out so that the image does not show any carbide anymore. Therefore (from my point of view) do not trust ANY image analysis as long as you do not know the result already. :-) You can prove everything! I know all arguments against my opinion, but be fair... there is no better technique than XRD or better neutron diffraction since it prevent the impact of the subjective factor "scientist". Using images, the selection of the area of interest is deciding, and also this is not an objective technique.
Dear colleagues, sorry for late reply, had no access to inet…. As to the matter: 1) No problem with the beam intensity and hence distinct peaks if they exist of course (scattering limitation). I asked for theoretical intensities of pure carbides; by analogy with retained austenite, such coefficients are necessary to express the sought fractions in terms of measured intensities of both the matrix and sought phases. These figures are calculated from first principles and tabulated for the austenite and ferrite, however I heard nothing of similar data for carbides. I mean at least plain cementite and two common types of chromium carbides. Probably, nobody implements such calculations since considers related carbide peaks insufficiently strong in real steels. 2) Certainly, any image analysis as well as other alternatives (to neutrons) will in no way allow to quantify carbides because of their small dimensions and subtle integral quantity in steels. 3) By the way, even the approach to retained austenite with the known reference coefficients is not ideal as far as such data reject any improvement by comparison to independent data.
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