Dear Leonid,
sorry to come back to an old thread, but there is something to be know
that you didn't tell completely.
So finally I came across this morning to a picture of the fit by ddm of
this bbm48bis sample with planar defects (coming from Maud examples,
picture and example now also on the ddm web site). I didn't see it at that
time as it was not included in the zip of Leonid and I didn't have Windows
to install ddm and check it (nor the time).
Now that I saw in the picture that Leonid did not use a cubic cell to fit
the alpha-brass, I took the time to download ddm, and check the solution.
So in the end, your model for faulting as you describe: "A more general
model [J Appl Cryst (2000) 338] is included in DDM:" was to use a trigonal
cell with hexagonal axis to allow refining the anisotropic shift of the
peaks caused by the planar defects? That is a nice trick, you can try to
justify it by reasoning that one effect of the planar defect is to get
same stacking sequence of the hexagonal (we all know that), but what about
the FCC stacking now?
To make the audience aware, just changing the cell was not sufficient, you
have to reproduce the intensities. So in the structure a couple of Cu, Zn
was set in 0,0,0 and another in 0,0,1/3 and the occupancies refined (to
values as 1.10392 for the first position and 0.39608 for the second, but
look like the second is calculated from the first) to adjust the
intensities and the density. But actually this didn't work out completely
as the resulting quantitative phase analysis is completely wrong. Not to
mention a quite high B factor (2.17 compared to the near 0.6 value of Maud
and the other BCC phase) for the atoms in the alpha phase, probably to
compensate for the wrong structure and kill the two high peaks at high
angle resulting from the trigonal structure.
Well this kind of trick may be able in certain cases to fit the pattern
(here just because peaks are broad and you don't notice the split on the
first peak and others of the alpha), but the results you get speak for
themselves.
And where are the crystallite sizes? Planar defect densities? So what kind
of results did you get from the material science point of view?
You could have used just a full profile pattern fitting at this point, and
at least get the crystallite sizes. So now I am asking myself why I don't
use always a triclinic cell to describe my phases, I will have the
flexibility to fit everything without resorting to complex "physical"
models.
Best regards,
Luca
On Jun 28, 2013, at 6:34, Leonid Solovyov <l_solov...@yahoo.com> wrote:
Dear Luca,
My doubts regarding the applicability of the faulting model in Moud are
due to the absence of its description in the program and the
non-availability of options required for crystal systems where the faulted
atomic displacements and domain planes are not obviously pre-defined as in
the close-packed metals.
The model included in DDM is, indeed, simplified and phenomenological to
some extent (as well any other model) but it is equally applicable to any
structure with any crystallographic symmetry and any faulting type that
may be described by a lattice of unit cell domains differing in the atomic
arrangement. For your bbm48bis example it gives a similar profile fit as
that in Maud in both Rietveld and DDM mode with even fewer number of
variable parameters - 14 for DDM and 17 for Rietveld compared to 22 in
Maud:
http://sites.google.com/site/ddmsuite/tutorials/bbm48bis.zip
The profile R-factor output by DDM in the Rietveld mode is high (20.77%)
since it is calculated for the background-subtracted data. The equivalent
standard Rwp is 7.88% (and 7.59% in Maud). Some microstructural variable
parameters in the Maud model bbm48bis.par are, apparently, excessive as
their estimated errors exceed the refined values.
Concerning the FeAl.ddm example, it does contain antiphase domains whose
influence on the selective broadening is allowed for by the connection
between the pseudo-position occupancy and the size-ellipsoid (regulated by
the NDPAR field).
Best regards,
Leonid
*******************************************************
Leonid A. Solovyov
Institute of Chemistry and Chemical Technology
660049, K. Marx 42, Krasnoyarsk, Russia
http://sites.google.com/site/solovyovleonid
*******************************************************
From: Luca Lutterotti <luca.luttero...@ing.unitn.it>
To: "rietveld_l@ill.fr" <rietveld_l@ill.fr>
Sent: Wednesday, June 26, 2013 5:19 PM
Subject: Re: Stacking faults and antiphase boundary
I made a quick analysis on Leonid FeAl example.
Very simple, no stacking/twin faults, no antiphase domains, only
stoichiometry is 15% in favor of Fe. So you only fit the relative
occupancy of Al and the different sites accordingly.
So if you want to compare the two programs on that simple example, you can
download:
http://www.ing.unitn.it/~maud/FeAl.zip
there are the same files of the example of Leonid, plus I added:
FeAl_m.dat (reformatted data file to comply with Maud dat format)
FeAl_Maud.par (the analysis file you can load with Maud to see how
the fitting was done and what parameter I refined and occupancies
setting/binding/refinement)
FeAl_Maud_fit.pdf (figure with the final fitting, plotted in sqrt
root of the intensities).
The step I used:
- loaded the datafile
- imported FeAl structure from Maud database
- automatic quantitative analysis
- you recognize the first peak wrong intensity, so set the binding of the
occupancies and refined both Al1, Fe1 occupancies.
- you can get lower occupancy for Al1, so fixed the Fe1 to 1 and only
refined the Al1 occupancy
- refined also the instrument asymmetry as I started with the default
instrument resolution in Maud (that has a higher asymmetry)
Now I wait for the Leonid refinement of bbm48bis (beta-brass milled 48
hours sample bis) example of Maud.
Best regards,
Luca
On Jun 26, 2013, at 11:43 AM, Luca Lutterotti
<luca.luttero...@ing.unitn.it> wrote:
Well at this point I need to post a clarification,
what Leonid call a classical treatment (Warren) is still the best and more
comprehensive theory on antiphase domain and stacking/twin faults in close
packed alloys. I took the time to read the Leonid paper of 2000 and it
doesn't offer anything more than a phenomenological approach to
anisotropic broadening, even taking my old way of doing it in 1990 that I
am not using anymore after the much better Popa treatment.
But the Warren theory on ordered/disordered alloys is a "little" bit more
than that:
- antiphase domains cause additional broadening only on so-called
"superstructure" peaks. This is not well accounted for with just some
anisotropic broadening. In fact having worked a lot with such alloys I had
to implement the Warren theory to really have a good fit on them (and get
the interesting quantities).
- stacking and twin faults, depending on the symmetry (BCC, FCC, HCP,
etc.) cause not only anisotropic broadening but also:
- anisotropic peak shift and even splitting
- peak asymmetry and this can be in one side or the other and alternating
- the Warren theory gives you directly the antiphase, stacking faults
(intrinsic and extrinsic) and twin faults probabilities that are what a
metallurgist or a material scientist is interest in, not simply
phenomenological parameters to do the fit.
So if you really are interested in trying a pattern with some of these
effects (not very good from statistical point of view, but sufficient to
appreciate the "problems" you encounter with this kind of samples), take
the example bbm48bis.par of Maud (the datafile is bbm48bis.dat) and try to
do the fitting. You need to download Maud, run it at least once so it
extracts all the examples and needed files in a directory you choose and
there you will find that example. With Maud you can load as an analysis
file the bbm48bis.par file, and press the hammer button on the toolbar to
do the final fitting (the example is not at convergence). See if you can
fit it as well with another model/program. Editing either phases, under
the microstructure tabpanel you will find the microstructural model used
and the probabilities. In this case, being a brass alloy, by X-ray we
cannot fit the occupancies of the atoms as the contrast is only one
electron, otherwise in Maud you just duplicate the sites changing the atom
type and use the "equal to" binding of the parameters to set the rules
based on the alloy compositions and site exchange. Nothing difficult.
As an additional note, if you select the Warren model with the wrong
symmetry, Maud will disable it; so it enforces to use it only when it is
applicable.
Again, if you don't work with close packed alloys and you need to model
serious faultings (like turbostratic faultings) that the anisotropic
broadening model of Popa cannot fit (heavy asymmetry etc.), in Maud is
implemented the single layer model of Ufer (the same implemented in bgmn)
that works very well in such cases.
Here are the two papers detailing the Maud/Warren models for antiphase
domains and stacking/twin faults and Ufer model (so there you find the
original Ufer and bgmn references):
L. Lutterotti and S. Gialanella, "X-ray diffraction characterization of
heavily deformed metallic specimens", Acta Mater., 46[1], 101-110, (1998)
L. Lutterotti, M. Voltolini, H.-R. Wenk, K. Bandyopadhyay and T. Vanorio,
"Texture analysis of a turbostratically disordered Ca-montmorillonite",
Amer. Mineral. 95, 98-103, (2010)
Finally (in response to the original poster) the Maud site is not out of
date. There is a new version coming very soon. It is at the moment still
in beta but downloadable from the Maud site (updated nearly every week or
two). But the old stable version works quite well anyway. And sorry if I
don't have all that time to maintain a fully featured site always updated
to the last news.
Best regards,
Luca Lutterotti
On Jun 26, 2013, at 10:07 AM, Leonid Solovyov <l_solov...@yahoo.com> wrote:
Dear David,
As I may guess from the microstructural options in Maud, the Warren model
of faults and antiphase domains there seems to be applicable only to
simple classical cases of cubic close-packed metals and alloys such as
AuCu etc.
A more general model [J Appl Cryst (2000) 338] is included in DDM:
http://sites.google.com/site/ddmsuite
An example DDM-template for a hypothetical semi-ordered FeAl with
antiphase domains separated by (001) planes may be downloaded here:
http://sites.google.com/site/ddmsuite/tutorials/AlFe.zip
In this example, the variable (Fe,Al) site occupancy parameter number is
entered in the NDPAR field to be connected with the selective
(superstructure) peak broadening due to the antiphase domains. The domain
shape is approximated by the size-ellipsoid. The domains are considered to
divide the crystal by layers along [001] direction. The ellipsoid
parameters hh and kk are set to zero since the domain sizes along [100]
and [010] directions are equal to the crystal size. Note that for the
anisotropic domain size modeling in cubic system the Expanded hkl
generation option is chosen instead of the Symmetry-filtered one.
For particular sample and faulting model the respective parameters may be
adjusted and refined.
Other examples for faulted structures may be found in the
\EXAMPLES\Defects\ folder of DDMsuite.
Best regards,
Leonid
*******************************************************
Leonid A. Solovyov
Institute of Chemistry and Chemical Technology
660049, K. Marx 42, Krasnoyarsk, Russia
http://sites.google.com/site/solovyovleonid
*******************************************************
From: David Martínez Blanco <martinezbda...@uniovi.es>
To: rietveld_l@ill.fr
Sent: Tuesday, June 25, 2013 4:50 PM
Subject: Stacking faults and antiphase boundary
Dear Rietvelders,
I am currently analyzing X-ray powder pattern diffraction for the FeAl
binary system obtained with Cu anode in Bragg-Bretano geometry, especially
focus on the study of order-disorder transition. I usually do Rietveld
refinement by means FullProf Suite package but I do not know the specific
formulation to deal with stacking faults and antiphase boundaries, in
particular for body centred cubic structures. Surfing in the web I
discover the program Maud that seems to be capable for analyze these kinds
of defects but I can not find related information in the web page of the
Maud program (it looks like is out of date since 2011) to the following
topics that are specific to my samples:
Which are the expressions of the different models using to model the line
broadening due to antiphase boundaries and planar defects?
Has anyone checked that they function correctly?
Maud program includes and works properly with corrections for the
micro-absorption for Fe rich phases?
Any case, Is it possible to deal the line broadening ascribe to these
defects also in the FullProf program for these structures?
Best regards and thanks in advance,
David
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