Dear Prof. Raabe, so much thanks for your kind reply.

Yeah, you are right; oftimes, STEM are applied to characterize the dispered pareticles in the metal matrix, for example SiC in Al alloys; Also, TEM coupled with SAED or EDS analysis, I believe, is able to indentify the phase in the area of interest. 

In my experiment, I want to show the grain size distributions of the two phase (bcc and fcc, they are both in nano-scale) in dual phase nanosized steels, 

XRD or SAED can tell us the coexistence of bcc and fcc phase, sometimes, one can also obtain the mean grain sizes of bcc and bcc using the XRD broadening profiles if the nanograin size is possible for doing XRD estimation.

One method, I want to have a try, is to use the dark field model in TEM, that is, the diffraction beam used for DF images is marked by diffraction rings of fcc phase first to indentify the grain siz distribution in fcc phase, and then is bcc phase with the same way.

If you have nanocrystalline phases XRD is probably not the best technique because of peak broadening. As long as you have such simply structures like bcc and fcc it is perhaps OK but if there are ordered structures of fcc and bcc you can only trust TEM (having the problem of low statistical significance of this technique).

EBSD and TKD (or t-EBSD) you cannot trust since there is no system which is only capable to simulate the intensities correctly. Only Bruker has a software (Dynamics) but it is not integrated in a useful way, i.e. you can do it but it is a very long and stony (manual) way. However, if you know, which phase you want to distinguish, i.e. you don't really ask whether there is another phase than bcc and fcc, you an certainly give TKD a chance. A better option is ASTAR which works in STEM. But also there TKD can produce better results when you stack of nano grains. TEM will produce an averaged  signal whereas TKD is working with the signal of the last few nm only. It might happen, that you get a better and more sufficient result than by automated diffraction in TEM.    

I would mention that TEM-based and other very LOCAL techniques will not give a statistically significant (representative) result. If you focus on the fraction of certain phase (at the cost of particle dimensions and distributions) diffraction methods are appropriate, however the diffraction volume (thickness) by XRD may happen still small to percept low fractions.  The neutron diffraction that increases the diffracting volumes by ten orders (!!) is the solution, though there is a problem of availability of course. 

Hi Alex, in case of nano-sized materials diffraction techniques are not very accurate because of peak broadening (even in case of synchrotron radiation). I guess this is not different for neutron diffraction, right?

It depends on the neutron flux. If the flux is particularly high, as in several research reactors working in impulse mode, even the least (in terms of quantity and/or particle dimensions) secondary phases (crystalline of course) may be evaluated. And the broadening does not matter here if the peak position and the peak-to-background ratio are distinct. Strange, similar applications to steels are practically nul. In reasonable  time we will demonstrate (publish) potentiality of this marvelous tool.  

Dear Linke Huang,

Given the quality of the above answers this response can only be approximative. Nevertheless I dare to make some remarks.

1. Your original question is rather general: different phases in nanocrystalline alloys. So, intermetallics and primary/secondary metal phases are included.

2. In your addendum you focus on nanocrystalline Fe-based alloys with bcc and fcc phases; these are rather simple lattices to be linked to ferrite and austenite; apparently you exclude the presence of intermetallics.

3. What do you mean with the word nanocrystalline? If you hint at the size of the crystalline particles I note that an individual particle with nanodimensions still contains hundreds of unit cells. So, in my opinion there is no such thing as nanocrystallinity.

4. Of course I agree that on applying diffraction methods the nanosize of the particles inevitably implies peak broadening, thereby at least phase recognition not making easier. Indeed: 'there is no such thing as a simple or straightforward identification method', as Dierk Raabe noted.

5. The basic aspect of nanocrystalline alloys seems to me the contribution of surface effects on the behaviour of these alloys: i. contribution to the free energy/enthalpy, ii. the vicinity of these surfaces for moving dislocations and/or solute atoms.

6. Ferrite and austenite can react differently on temperature changes; these can reveal magnetic, thermal and dillational effects. In combination with diffraction methods these effects might be used to study phases and phase transformations in nano-particle Fe-based alloys. 

Dear  P. Van Mourik,

So much thanks for kind answers; I would like to make a response to you.

1. Yeah, the first describtion of my question is pretty general. What I want to show is the grain size distributions of bcc and fcc phases (for example, ferrite and austenite) in nanosized Fe alloys with the absence of intermetallics.

2. The word, nanocrystalline, I used, means grain size (rather than particle size) is in nanoscale; also, nanosized is OK. Further, some other words,for example, nanoparticle, nanostructured should also be clarified. 

3. Diffraction method, such as XRD ,can be applied to estimate the mean grain size or grain size distribution, while due to the instrument resolution, XRD is oftentimes effective for the system with a small nanograin size.

4. Size effect , indeed, consists of two aspects, that is, thermodynamics and kinetics.

For easy and effective SAED calculations you can explore this link and explore this website. 

http://www.instanano.com/characterization/d-value-calculation/saed-d-value-calculation/