The large spots visible in the picture were indexed and are identified as gamma matrix (Ni,Cr). I'm rather interested in the small substructure spots. I think that they are caused by gamma double prime but I have to be sure of it. Furthermore, how can I check if gamma prime is present in the material if the precipitation may have a few nanometers of diameter?
You have clearly multiple diffraction in the diffraction pattern. Hence the [1 1 2] direction of incidence beam is not very suitable for simple indexing of a distorted specimen. Try dark field imaging with the diffraction spots of the precipitation if HRTEM does not work.
http://www.sciencedirect.com/science/article/pii/S0921509311013281
http://eng.global.cnki.net.caj-archiv.erf.sbb.spk-berlin.de/kns50/detail.aspx?QueryID=20&CurRec=10
Hi Piotr,
The intense spots come from the matrix (Gamma'=G') the space group of which is Fm-3m. In this condition, the diffraction pattern is recorded along [112]G'. The less intense spots originate from the (Gamma"=G"). This phase is an ordered phase, DO22, which crystallizes in the tetragonal system (Space group ?). If you have the lattice parameters of this G" phase you can index the zone axis pattern. This G" phase adopt an orientation relationship (OR) with the G' phase. This deduced OR can be written as: [112]G'//[uvw]G", (1-11) G'//(h1k1l1), (0-42)G'//(h2k2l2)G". I have inserted a.ppt file for the indexing. Do not hesitate to ask more inf not clear.
To observe the fine precipitates, you can record dark field images, by selecting reflections coming from the G" phases.
Best regads.
Abdelkrim
As far as I know, gamma is the non-ordered phase with space group F m-3m (A1), whereas gamma' is an ordered phase with space group P m-3m (L12 structure).
I have analyzed Your answers but I'm curious of the pattern in the background that is almost invisible (the spots are very blurred but they can be seen in magnification upon closer inspection). Firstly i thought that they are a the result of of error but then I've noticed that they form hexagons (see attached figure).
But you forgot the centered spot which is also a reciprocal lattice point of your matrix. In general, you don't need to define three lattice vectors as basis since the third can be always described as kinear combination of the formerly defined.
Hi Piotr,
In your Diffraction pattern, there are only 2 phases: the one corresponding to the intense reflections and the other one to the less intense reflections.
May be, we can have a look the corresponding bright field image!
On the other hand did you find the lattice parameters (tetragonal) of the supposed G" phase (The double prime G-phase)?
I have drawn the reflections lattice of phase the supposed G" in the attached .ppt file.
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