701449 fuchs arthur 2018

Arthur Fuchs, BSc
Experimental determination of dislocation densities in martensitic
steels
to achieve the university degree of
MASTER'S THESIS
Master's degree programme: Advanced Materials Science
submitted to
Graz University of Technology
Univ.-Prof. Dipl.-Ing. Dr.techn. Bernhard Sonderegger
Institute of Materials Science, Joining and Forming
Diplom-Ingenieur
Supervisor
Graz, March 2018

AFFIDAVIT
I declare that I have authored this thesis independently, that I have not used other
than the declared sources/resources, and that I have explicitly indicated all ma-
terial which has been quoted either literally or by content from the sources used.
The text document uploaded to TUGRAZonline is identical to the present master‘s
thesis dissertation.
Date Signature

Diploma Thesis Arthur Fuchs
Acknowledgements
First of all, I would like to thank my supervisor Univ.-Prof. Dipl.-Ing. Dr.techn. Bernhard Sonderegger
for his guidance. I really appreciate giving me my space but immediately being there when I needed
directions.
In addition, I would like to thank all my colleagues from the IMAT for their support and their friendly
attitude.
Furthermore, great appreciation is directed towards the XRD group of the Institute of Solid State
Physics (TU Graz), especially Ao.Univ.-Prof. Dipl.-Ing. Dr.techn. Roland Resel, for the considerable
amount of measurement time granted and for teaching me how to operate the X-ray diffractometer in
the first place.
Special thanks go to the Institute of Materials Physics (TU Graz), especially Dipl.-Ing. BSc Robert
Enzinger, for doing the dilatometer measurements and redoing them under argon atmosphere, as soon
as we realized that evaporation of Cr is a problem under high vacuum.
I would also like to thank my family, especially my parents and my girlfriend, for their great support.
And finally to all my friends: Thanks to those of you who studied with me for exams, those of you
who prevented me from studying when I overdid it and those who did both!
Thank you very much, everyone!
Arthur Fuchs
Graz, March 2018.

Diploma Thesis Arthur Fuchs
1 Abstract
This work deals with the measurement of dislocation densities in complex martensitic alloys. In these
materials, dislocation motion is responsible for plastic deformation and thus determines properties
such as yield strength or creep resistance. Up to now, there has been no physically justified standard
procedure to determine this property with good statistical significance and reasonable experimental
effort. X-Ray diffraction (XRD) potentially is such a method, combining low experimental cost with
sufficient sample volume. Nevertheless, the complexity of the microstructure interferes with a straight-
forward application of standard methods usually applied in other, less complex, materials.
In this work, conventional XRD-methods are adapted to a martensitic 9% Cr steel (P91). The basic
physical concept is simple: An (ideally) monochromatic and parallel X-ray beam interacts with the
crystal structure of the sample, the diffraction angle(s) Θ is a measure of the interatomic distance(s)
according to Bragg’s law. The broadening of the diffraction peak results from (i) properties of the
experimental equipment and (ii) the microstructure of the sample. (i) is referred to as instrumental
broadening and includes properties of the beam, the slits and the lenses, (ii) comprises any effect
leading to local deviations from the ideal lattice constant, which is mostly dislocations and inner
boundaries of any type.
For the measurement, a PANalytical Empyrean XRD with Cu Kαradiation was used. The samples
were a NIST LaB6standard and seven P91 samples (austenitized (1), austenitized and heat treated
(2-7)).
In a first step, the instrumental broadening was characterized by measurement of the LaB6standard.
With this reference, the data from the steel samples could be de-convoluted into sample- and instru-
mental broadening. Finally, the sample broadening was split into microstrains and broadening due to
coherent domain size. For this procedure, some established methods like ”Williamson-Hall”, ”mod-
ified Williamson-Hall”, ”Warren-Averbach” and ”double-Voigt” are compared qualitatively to each
other. Furthermore, the frequently used “modified Warren Averbach” method and the most common
mistake with this method is discussed in detail. A workaround, by introducing the contrast factor of
dislocations into the double-Voigt method, is presented. In the modified Williamson-Hall and mod-
ified Warren-Averbach methods, the dislocation density is calculated assuming, that all microstrain-
inducing defects except dislocations are negligible.
Exemplarily, the results are compared to findings from high-precision dilatometry tests and the results
differ by less than a factor of two.
II

Diploma Thesis Arthur Fuchs
Contents
1 Abstract II
2 Parameter List V
3 Introduction 1
3.1 Task.............................................. 1
4 Basics 2
4.1 General information about X-ray diffraction . . . . . . . . . . . . . . . . . . . . . . . . 2
4.1.1 GenerationofX-rays................................. 2
4.1.2 Bragg’slaw...................................... 3
4.1.3 Setup of an X-ray diffractometer . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1.4 Peakcharacteristics ................................. 5
4.1.5 Peak-shapes in XRD-measurements . . . . . . . . . . . . . . . . . . . . . . . . 6
4.2 Instrumental broadening correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2.1 Cauchy-/Gauss deconvolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2.2 Voigtdeconvolution ................................. 7
4.2.3 Stokesdeconvolution................................. 8
4.3 General information about size-/strain analysis . . . . . . . . . . . . . . . . . . . . . . 8
4.4 Williamson-Hallmethod................................... 8
4.5 Warren-Averbachmethod .................................. 10
4.6 Modified Williamson-Hall and modified Warren-Averbach approach . . . . . . . . . . . 12
4.7 Groma’smethod ....................................... 12
4.8 Double-Voigt ......................................... 13
4.9 Dilatometry.......................................... 13
5 Experimental 14
5.1 Mathematical considerations for data processing . . . . . . . . . . . . . . . . . . . . . 14
5.1.1 Combination of modified Williamson-Hall and modified Warren-Averbach . . . 14
5.1.2 Combination of modified Warren-Averbach and Double-Voigt . . . . . . . . . . 16
5.2 Samplepreparation...................................... 19
5.3 X-raydiffraction ....................................... 19
5.4 Dilatometry.......................................... 20
5.5 Dataprocessing........................................ 20
5.5.1 Matlab programs for data processing . . . . . . . . . . . . . . . . . . . . . . . . 21
6 Results 25
6.1 X-raydiffraction ....................................... 25
6.2 Dilatometry.......................................... 30
7 Data Analysis and Discussion 31
7.1 FittingofXRDpeaks .................................... 31
7.2 Separation of size- and strainbroadening . . . . . . . . . . . . . . . . . . . . . . . . . . 34
7.2.1 Williamson-Hall method (also see chapter 4.4): . . . . . . . . . . . . . . . . . . 34
7.2.2 Modified Williamson-Hall technique (also see chapter 4.6): . . . . . . . . . . . . 35
7.2.3 Warren-Averbach method (also see chapter 4.5): . . . . . . . . . . . . . . . . . 36
7.3 Modified Warren-Averbach method (also see chapter 4.6): . . . . . . . . . . . . . . . . 37
7.3.1 Double-Voigt method (also see chapter 4.8): . . . . . . . . . . . . . . . . . . . . 37
III