EBSDEBSD and EDS characterization of high entropy alloys.Introduction

A high entropy alloy, Al8Co17Cr17Cu8Fe17Ni33, has been developed from an equiatomic AlCoCrCuFeNi alloy. Due to

its promising properties, such as high corrosion and oxidation resistance and high thermal stability, it is a candidate

for various applications at elevated temperature as, e.g., furnace parts, tools and moulds. The exploration of new

metallic systems for high temperature applications is an important challenge in todays materials science.

However, an increase of the strength of this alloy is desirable and requires further optimization. In order to improve

these mechanical properties, knowledge of the microstructure is necessary. Therefore, Al8Co17Cr17Cu8Fe17Ni33 high

entropy alloy has been studied by means of energy-dispersive X-ray spectroscopy (EDS) and electron backscatter

diffraction (EBSD).

High entropy alloysHigh entropy alloys have been defined as consisting of five or more elements, each one having a concentration between 5 and 35 at.% [1-3]. These alloys are crystalline materials which predominantly form simple solid solutions, mainly of face-centred or body-centred cubic structures. High entropy alloys promise an interesting combination of properties, such as oxidation resistance, thermal stability, high strength and soft magnetic behaviour [3, 6-9].

The most studied high entropy alloy is AlCoCrCuFeNi with equiatomic composition [4, 5, 8, 10, 11]. It solidifies dendritically within a large variation of casting conditions. The dendrites mainly consist of two phases, an Al/Ni-rich and a Fe/Cr-rich phase, whereas interdendritic regions are enriched in Cu [4, 10]. It has also been found that the Fe/Cr-rich phase decomposes into Fe-rich and Cr-rich domains. A length scale between these domains of just a few nanometres indicates spinodal decomposition as the formation process [8], which is often reported for Fe-Cr systems [12, 13].

The ductile Al8Co17Cr17Cu8Fe17Ni33 alloy shows mainly two fcc phases in transmission electron microscopy, an ordered phase of L12 structure and a disordered fcc matrix. The ordered phase has been found to be Ni- and Al-rich as measured by atom probe tomography. Very few Cu-rich precipitates are found at the grain boundaries [14].

In the present work, an Al8Co17Cr17Cu8Fe17Ni33 high entropy alloy which had undergone cold rolling was investigated in order to reveal the microstructural properties as well as elemental distributions within the specimen.

The Al8Co17Cr17Cu8Fe17Ni33 alloy was prepared in a vacuum induction furnace. The alloy constituents were of 99.99% purity. The ingots were remelted three times to achieve a better homogenisation. Specimens for analysis were mechanically ground and polished using an OP-U colloidal silica suspension.

The investigated samples underwent a cold rolling, with up to 70% decrease in thickness.

Elemental distribution maps by EDS and EBSD maps given in the present work, were obtained using Oxford Instruments X-Max 80 X-ray and NordlysNano EBSD detectors. The acceleration voltages and beam currents applied were 15 kV and 10nA. EDS and EBSD acquisitions, and analyses, were performed using the AZtec software package. Evaluation of stored EBSD patterns for strain/stress analysis within individual grains were conducted using CrossCourt 3 (BLG Productions).

Experimental procedure

EBSDEBSD and EDS characterization of high entropy alloys.

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ResultsThe scanning electron micrograph in Fig. 1 reveals a high density of extended structural defects within individual grains. The EBSD maps (using fcc Ni as reference) in Fig. 2 show that along linear features, the local orientation is changed when compared with the matrix of the

Al8Co17Cr17Cu8Fe17Ni33 specimen. The distributions of the 11, 22, and 33 strain tensor components, calculated from the EBSD data, indeed indicates that the strain is modulated across these linear features, with peak values of about 6 %

strain.

Fig. 2: EBSD band contrast as well as orientation-distribution maps of the region highlighted by a white frame in Fig. 1. The local orientations are given as colors (see legend) for the x, y, and z directions.

Fig. 3: Distributions of the 11, 22, and 33 strain tensor components, as calculated from the EBSD data (same identical position as in Figs. 1 and 2) by means of the CrossCourt software.

Fig. 1: Scanning electron micrograph of Al8Co17Cr17Cu8Fe17Ni33 alloy. On the region highlighted by a white frame, EBSD and EDS measurements (Figs. 2-4) were performed.

EBSDEBSD and EDS characterization of high entropy alloys.

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It is apparent from the elemental distribution maps acquired (Fig. 4)

that the Al8Co17Cr17Cu8Fe17Ni33 alloy decomposes after casting into dendrites mainly consisting of Al/Ni/Cu-rich and Cr/Fe/Co-rich regions, which agrees well to previous reports [4,10]. In addition, elemental distribution maps acquired in cross-section show the layered structure of the Al/Ni/Cu-rich dentrites. Together, the distribution of the Al/Ni/Cu-rich dendrites appears similar to the distributions of the 11, 22, and 33 strain tensor components.

Fig. 4: EDS elemental distribution maps calculated, using the Al-K, Ni-L, Cr-L, Fe-L, Cu-L, and Co-L signals, acquired on the same identical position as in Figs. 1-3.

Fig. 5: Fig. 4: EDS elemental distribution maps calculated using the Al-K, Ni-L, Cr-L, Fe-L, Cu-L, and Co-L signals, acquired on a cross-section specimen (view perpendicular to the maps in Fig. 4).

EBSDEBSD and EDS characterization of high entropy alloys.

The materials presented here are summary in nature, subject to change, and intended for general information only. Oxford Instruments NanoAnalysis is certified to ISO9001, ISO14001 and OHSAS 18001. AZtec is a Registered Trademark of Oxford Instruments plc. Oxford Instruments plc, 2013. All rights reserved. Document reference: OINA/EDS/AN114/0113.

www.oxford-instruments.com

The materials presented here are summary in nature, subject to change, and intended for general information only.Performances are configuration dependent. Additional details are available. Oxford Instruments NanoAnalysisis certified to ISO9001, ISO14001 and OHSAS 18001. AZtec and Tru-I are Registered Trademarks of Oxford Instruments plc,all other trademarks acknowledged. Oxford Instruments plc, 2013. All rights reserved. Document reference: OINA/ENSD/AN01/1113

www.oxford-instruments.com/ebsd

ConclusionThe present work shows how correlative imaging as well as EBSD and EDS analysis in scanning electron microscopy on identical

specimen positions may reveal the microstructural properties and the elemental distributions in an Al8Co17Cr17Cu8Fe17Ni33 alloy. The distributions of Al/Ni/Cu-rich and Cr/Fe/Co-rich were detected and correlated to the distributions of the 11, 22, and 33 strain tensor components.

AcknowledgementsA. Manzoni, N. Wanderka, N. Schfer, D. Abou-Ras (all Helmholtz-Zentrum Berlin fr Materialien und Energie, Germany) for providing the Ni high entropy sample and the EDS/EBSD analyses.

Special thanks are due to A. Bakai for carrying out the cold rolling experiments (Kharkov Institute of Experimental and Theoretical Physics, Ukraine).

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