Interpretation of phase structures using the program “DiffraCalc” and
EDX
‐spectrometry
Authors: Kuleshova Evgenia , Frolov Alexey , Krikun Ekaterina 
Published: December 2016

The micro‐diffraction method in transmission electron microscopy allows to determine the crystalline types of phase components in alloys, that is necessary to calculate the effect on mechanical properties of the materials.

The analysis of chemical composition of the selection has to be done to determine the crystalline type of phase inclusion within the micro‐diffraction method. The method of EDX‐spectrometry is usually used in case of steel researches, because this method is more sensitive to heavy elements which are defining in the formation of phase components in iron‐based alloys. When the elemental composition of the inclusion obtained from containing sample site is taken into account, decoding of the diffraction pattern can be made. This procedure helps to determine the crystallographic structure of the considered phase.

This paper presents a modified program for indexing electron diffraction patterns «DiffraCalc», which allows to decode the diffraction patterns obtained from the phase inclusions, considering the chemical composition. This work is based on a well‐known method of diffraction patterns indexing ‐ a method of paired reflexes [1]. Diffraction patterns indexing manually using this method has several disadvantages. Usage of this modified program has solved most of the indexing problems and has led to significant work acceleration and has improved the quality of indexing results.

The program includes: automatic indexing algorithm, the connection to the extensive database of crystalline substances, compatibility with standard CIF (Crystallografic information file) [2] files, the availability to choose three reflexes pairs at the same time and the ability to work simultaneously with several phases within one‐time setting scale. Indexing mode in the modified program allows to provide automatic calculations that are held simultaneously in several prospective phases, and moreover to analyze the phase matching for diffraction pattern without user intervention. The developed program has also implemented simulation mode of the diffraction pattern of a single crystal. This mode is intended to check the indexing results.

Verification of the diffraction patterns simulation mode was carried out under the program Single Crystall [3] for a number of substances (Fe, Fe
7
C
3
, Fe
3
C, etc.) with all types of crystal systems and has shown good compatibility between the calculated diffraction patterns of the Single Crystall results and the one obtained in the modified program «DiffraCalc». The examples of diffraction patterns simulation from a variety of substances is shown on Figure 1.

Analysis with the proposed program does not require multiple downloads of the experimental image of diffraction pattern, has the advantage of automatic conformity assessment estimated the experimental diffraction pattern based on an assessment of conformity of interplanar distances and angles between the reciprocal lattice vectors. Thus, the program «DiffraCalc» has broad capabilities to simulate the calculated diffraction patterns and indexing of experimental one.

Also, the paper contains investigations of low‐alloyed reactor pressure vessel (RPV) steels phase composition. Studies of the chemical composition of phase components were carried out by EDX‐method, the crystal structure of the phase components investigations were carried out by the micro‐diffraction method with subsequent analysis of the experimental diffraction patterns using the «DiffraCalc» program. An example of the automatic indexing of one of the electron diffraction patterns is shown on Figure 2, 3. This phase inclusion into the RPV steel is carbide of type Cr
21
Fe
2
C
6
. Experimental reflexes positions have shown good compatibility of reflexes positions, calculated in the program, with considering small deviation of the zone axis [111] from the direction of the electron beam.